diff options
Diffstat (limited to 'Documentation/RCU')
37 files changed, 2311 insertions, 1783 deletions
diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.rst b/Documentation/RCU/Design/Data-Structures/Data-Structures.rst index 4a48e20a46f2..b34990c7c377 100644 --- a/Documentation/RCU/Design/Data-Structures/Data-Structures.rst +++ b/Documentation/RCU/Design/Data-Structures/Data-Structures.rst @@ -963,7 +963,7 @@ exit and perhaps also vice versa. Therefore, whenever the ``->dynticks_nesting`` field is incremented up from zero, the ``->dynticks_nmi_nesting`` field is set to a large positive number, and whenever the ``->dynticks_nesting`` field is decremented down to zero, -the the ``->dynticks_nmi_nesting`` field is set to zero. Assuming that +the ``->dynticks_nmi_nesting`` field is set to zero. Assuming that the number of misnested interrupts is not sufficient to overflow the counter, this approach corrects the ``->dynticks_nmi_nesting`` field every time the corresponding CPU enters the idle loop from process @@ -973,7 +973,7 @@ The ``->dynticks`` field counts the corresponding CPU's transitions to and from either dyntick-idle or user mode, so that this counter has an even value when the CPU is in dyntick-idle mode or user mode and an odd value otherwise. The transitions to/from user mode need to be counted -for user mode adaptive-ticks support (see timers/NO_HZ.txt). +for user mode adaptive-ticks support (see Documentation/timers/no_hz.rst). The ``->rcu_need_heavy_qs`` field is used to record the fact that the RCU core code would really like to see a quiescent state from the diff --git a/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.rst b/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.rst index 72f0f6fbd53c..414f8a2012d6 100644 --- a/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.rst +++ b/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.rst @@ -38,7 +38,7 @@ sections. RCU-preempt Expedited Grace Periods =================================== -``CONFIG_PREEMPT=y`` kernels implement RCU-preempt. +``CONFIG_PREEMPTION=y`` kernels implement RCU-preempt. The overall flow of the handling of a given CPU by an RCU-preempt expedited grace period is shown in the following diagram: @@ -112,7 +112,7 @@ things. RCU-sched Expedited Grace Periods --------------------------------- -``CONFIG_PREEMPT=n`` kernels implement RCU-sched. The overall flow of +``CONFIG_PREEMPTION=n`` kernels implement RCU-sched. The overall flow of the handling of a given CPU by an RCU-sched expedited grace period is shown in the following diagram: @@ -181,7 +181,7 @@ operations is carried out at several levels: of this wait (or series of waits, as the case may be) is to permit a concurrent CPU-hotplug operation to complete. #. In the case of RCU-sched, one of the last acts of an outgoing CPU is - to invoke ``rcu_report_dead()``, which reports a quiescent state for + to invoke ``rcutree_report_cpu_dead()``, which reports a quiescent state for that CPU. However, this is likely paranoia-induced redundancy. +-----------------------------------------------------------------------+ @@ -277,7 +277,7 @@ the following access functions: Again, only one request in a given batch need actually carry out a grace-period operation, which means there must be an efficient way to -identify which of many concurrent reqeusts will initiate the grace +identify which of many concurrent requests will initiate the grace period, and that there be an efficient way for the remaining requests to wait for that grace period to complete. However, that is the topic of the next section. @@ -405,8 +405,8 @@ Use of Workqueues In earlier implementations, the task requesting the expedited grace period also drove it to completion. This straightforward approach had the disadvantage of needing to account for POSIX signals sent to user -tasks, so more recent implemementations use the Linux kernel's -`workqueues <https://www.kernel.org/doc/Documentation/core-api/workqueue.rst>`__. +tasks, so more recent implementations use the Linux kernel's +workqueues (see Documentation/core-api/workqueue.rst). The requesting task still does counter snapshotting and funnel-lock processing, but the task reaching the top of the funnel lock does a @@ -465,7 +465,7 @@ corresponding disadvantage that workqueues cannot be used until they are initialized, which does not happen until some time after the scheduler spawns the first task. Given that there are parts of the kernel that really do want to execute grace periods during this mid-boot “dead -zone”, expedited grace periods must do something else during thie time. +zone”, expedited grace periods must do something else during this time. What they do is to fall back to the old practice of requiring that the requesting task drive the expedited grace period, as was the case before diff --git a/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel0.svg b/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel0.svg index 98af66557908..16b1ff0ad38c 100644 --- a/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel0.svg +++ b/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel0.svg @@ -116,7 +116,7 @@ <flowRoot xml:space="preserve" id="flowRoot2985" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"><flowRegion + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"><flowRegion id="flowRegion2987"><rect id="rect2989" width="82.85714" @@ -125,7 +125,7 @@ y="492.36218" /></flowRegion><flowPara id="flowPara2991" /></flowRoot> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="362.371" y="262.51819" id="text4441" diff --git a/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel1.svg b/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel1.svg index e0184a37aec7..684a4b969725 100644 --- a/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel1.svg +++ b/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel1.svg @@ -116,7 +116,7 @@ <flowRoot xml:space="preserve" id="flowRoot2985" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"><flowRegion + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"><flowRegion id="flowRegion2987"><rect id="rect2989" width="82.85714" @@ -125,7 +125,7 @@ y="492.36218" /></flowRegion><flowPara id="flowPara2991" /></flowRoot> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="362.371" y="262.51819" id="text4441" diff --git a/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel2.svg b/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel2.svg index 1bc3fed54d58..8fb2454d9544 100644 --- a/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel2.svg +++ b/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel2.svg @@ -116,7 +116,7 @@ <flowRoot xml:space="preserve" id="flowRoot2985" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"><flowRegion + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"><flowRegion id="flowRegion2987"><rect id="rect2989" width="82.85714" @@ -125,7 +125,7 @@ y="492.36218" /></flowRegion><flowPara id="flowPara2991" /></flowRoot> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="362.371" y="262.51819" id="text4441" diff --git a/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel3.svg b/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel3.svg index 6d8a1bffb3e4..5d4f22d5662c 100644 --- a/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel3.svg +++ b/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel3.svg @@ -116,7 +116,7 @@ <flowRoot xml:space="preserve" id="flowRoot2985" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"><flowRegion + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"><flowRegion id="flowRegion2987"><rect id="rect2989" width="82.85714" @@ -125,7 +125,7 @@ y="492.36218" /></flowRegion><flowPara id="flowPara2991" /></flowRoot> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="362.371" y="262.51819" id="text4441" diff --git a/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel4.svg b/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel4.svg index 44018fd6342b..b89b02869914 100644 --- a/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel4.svg +++ b/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel4.svg @@ -116,7 +116,7 @@ <flowRoot xml:space="preserve" id="flowRoot2985" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"><flowRegion + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"><flowRegion id="flowRegion2987"><rect id="rect2989" width="82.85714" @@ -125,7 +125,7 @@ y="492.36218" /></flowRegion><flowPara id="flowPara2991" /></flowRoot> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="362.371" y="262.51819" id="text4441" diff --git a/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel5.svg b/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel5.svg index e5eef50454fb..90f1c77bea2f 100644 --- a/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel5.svg +++ b/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel5.svg @@ -116,7 +116,7 @@ <flowRoot xml:space="preserve" id="flowRoot2985" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"><flowRegion + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"><flowRegion id="flowRegion2987"><rect id="rect2989" width="82.85714" @@ -125,7 +125,7 @@ y="492.36218" /></flowRegion><flowPara id="flowPara2991" /></flowRoot> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="362.371" y="262.51819" id="text4441" diff --git a/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel6.svg b/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel6.svg index fbd2c1892886..3e5651da031a 100644 --- a/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel6.svg +++ b/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel6.svg @@ -116,7 +116,7 @@ <flowRoot xml:space="preserve" id="flowRoot2985" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"><flowRegion + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"><flowRegion id="flowRegion2987"><rect id="rect2989" width="82.85714" @@ -125,7 +125,7 @@ y="492.36218" /></flowRegion><flowPara id="flowPara2991" /></flowRoot> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="362.371" y="262.51819" id="text4441" diff --git a/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel7.svg b/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel7.svg index 502e159ed278..9483f08d345e 100644 --- a/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel7.svg +++ b/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel7.svg @@ -116,7 +116,7 @@ <flowRoot xml:space="preserve" id="flowRoot2985" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"><flowRegion + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"><flowRegion id="flowRegion2987"><rect id="rect2989" width="82.85714" @@ -125,7 +125,7 @@ y="492.36218" /></flowRegion><flowPara id="flowPara2991" /></flowRoot> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="362.371" y="262.51819" id="text4441" diff --git a/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel8.svg b/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel8.svg index 677401551c7d..1101ec30e604 100644 --- a/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel8.svg +++ b/Documentation/RCU/Design/Expedited-Grace-Periods/Funnel8.svg @@ -116,7 +116,7 @@ <flowRoot xml:space="preserve" id="flowRoot2985" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"><flowRegion + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"><flowRegion id="flowRegion2987"><rect id="rect2989" width="82.85714" @@ -125,7 +125,7 @@ y="492.36218" /></flowRegion><flowPara id="flowPara2991" /></flowRoot> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="362.371" y="262.51819" id="text4441" diff --git a/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst index 83ae3b79a643..5750f125361b 100644 --- a/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst +++ b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.rst @@ -21,7 +21,7 @@ Any code that happens after the end of a given RCU grace period is guaranteed to see the effects of all accesses prior to the beginning of that grace period that are within RCU read-side critical sections. Similarly, any code that happens before the beginning of a given RCU grace -period is guaranteed to see the effects of all accesses following the end +period is guaranteed to not see the effects of all accesses following the end of that grace period that are within RCU read-side critical sections. Note well that RCU-sched read-side critical sections include any region @@ -112,6 +112,35 @@ on PowerPC. The ``smp_mb__after_unlock_lock()`` invocations prevent this ``WARN_ON()`` from triggering. ++-----------------------------------------------------------------------+ +| **Quick Quiz**: | ++-----------------------------------------------------------------------+ +| But the chain of rcu_node-structure lock acquisitions guarantees | +| that new readers will see all of the updater's pre-grace-period | +| accesses and also guarantees that the updater's post-grace-period | +| accesses will see all of the old reader's accesses. So why do we | +| need all of those calls to smp_mb__after_unlock_lock()? | ++-----------------------------------------------------------------------+ +| **Answer**: | ++-----------------------------------------------------------------------+ +| Because we must provide ordering for RCU's polling grace-period | +| primitives, for example, get_state_synchronize_rcu() and | +| poll_state_synchronize_rcu(). Consider this code:: | +| | +| CPU 0 CPU 1 | +| ---- ---- | +| WRITE_ONCE(X, 1) WRITE_ONCE(Y, 1) | +| g = get_state_synchronize_rcu() smp_mb() | +| while (!poll_state_synchronize_rcu(g)) r1 = READ_ONCE(X) | +| continue; | +| r0 = READ_ONCE(Y) | +| | +| RCU guarantees that the outcome r0 == 0 && r1 == 0 will not | +| happen, even if CPU 1 is in an RCU extended quiescent state | +| (idle or offline) and thus won't interact directly with the RCU | +| core processing at all. | ++-----------------------------------------------------------------------+ + This approach must be extended to include idle CPUs, which need RCU's grace-period memory ordering guarantee to extend to any RCU read-side critical sections preceding and following the current @@ -139,7 +168,7 @@ an ``atomic_add_return()`` of zero) to detect idle CPUs. +-----------------------------------------------------------------------+ The approach must be extended to handle one final case, that of waking a -task blocked in ``synchronize_rcu()``. This task might be affinitied to +task blocked in ``synchronize_rcu()``. This task might be affined to a CPU that is not yet aware that the grace period has ended, and thus might not yet be subject to the grace period's memory ordering. Therefore, there is an ``smp_mb()`` after the return from @@ -173,49 +202,44 @@ newly arrived RCU callbacks against future grace periods: 1 static void rcu_prepare_for_idle(void) 2 { 3 bool needwake; - 4 struct rcu_data *rdp; - 5 struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks); - 6 struct rcu_node *rnp; - 7 struct rcu_state *rsp; - 8 int tne; - 9 - 10 if (IS_ENABLED(CONFIG_RCU_NOCB_CPU_ALL) || - 11 rcu_is_nocb_cpu(smp_processor_id())) - 12 return; + 4 struct rcu_data *rdp = this_cpu_ptr(&rcu_data); + 5 struct rcu_node *rnp; + 6 int tne; + 7 + 8 lockdep_assert_irqs_disabled(); + 9 if (rcu_rdp_is_offloaded(rdp)) + 10 return; + 11 + 12 /* Handle nohz enablement switches conservatively. */ 13 tne = READ_ONCE(tick_nohz_active); - 14 if (tne != rdtp->tick_nohz_enabled_snap) { - 15 if (rcu_cpu_has_callbacks(NULL)) - 16 invoke_rcu_core(); - 17 rdtp->tick_nohz_enabled_snap = tne; + 14 if (tne != rdp->tick_nohz_enabled_snap) { + 15 if (!rcu_segcblist_empty(&rdp->cblist)) + 16 invoke_rcu_core(); /* force nohz to see update. */ + 17 rdp->tick_nohz_enabled_snap = tne; 18 return; - 19 } + 19 } 20 if (!tne) 21 return; - 22 if (rdtp->all_lazy && - 23 rdtp->nonlazy_posted != rdtp->nonlazy_posted_snap) { - 24 rdtp->all_lazy = false; - 25 rdtp->nonlazy_posted_snap = rdtp->nonlazy_posted; - 26 invoke_rcu_core(); - 27 return; - 28 } - 29 if (rdtp->last_accelerate == jiffies) - 30 return; - 31 rdtp->last_accelerate = jiffies; - 32 for_each_rcu_flavor(rsp) { - 33 rdp = this_cpu_ptr(rsp->rda); - 34 if (rcu_segcblist_pend_cbs(&rdp->cblist)) - 35 continue; - 36 rnp = rdp->mynode; - 37 raw_spin_lock_rcu_node(rnp); - 38 needwake = rcu_accelerate_cbs(rsp, rnp, rdp); - 39 raw_spin_unlock_rcu_node(rnp); - 40 if (needwake) - 41 rcu_gp_kthread_wake(rsp); - 42 } - 43 } + 22 + 23 /* + 24 * If we have not yet accelerated this jiffy, accelerate all + 25 * callbacks on this CPU. + 26 */ + 27 if (rdp->last_accelerate == jiffies) + 28 return; + 29 rdp->last_accelerate = jiffies; + 30 if (rcu_segcblist_pend_cbs(&rdp->cblist)) { + 31 rnp = rdp->mynode; + 32 raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */ + 33 needwake = rcu_accelerate_cbs(rnp, rdp); + 34 raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */ + 35 if (needwake) + 36 rcu_gp_kthread_wake(); + 37 } + 38 } But the only part of ``rcu_prepare_for_idle()`` that really matters for -this discussion are lines 37–39. We will therefore abbreviate this +this discussion are lines 32–34. We will therefore abbreviate this function as follows: .. kernel-figure:: rcu_node-lock.svg @@ -339,14 +363,14 @@ The diagram below shows the path of ordering if the leftmost leftmost ``rcu_node`` structure offlines its last CPU and if the next ``rcu_node`` structure has no online CPUs). -.. kernel-figure:: TreeRCU-gp-init-1.svg +.. kernel-figure:: TreeRCU-gp-init-2.svg The final ``rcu_gp_init()`` pass through the ``rcu_node`` tree traverses breadth-first, setting each ``rcu_node`` structure's ``->gp_seq`` field to the newly advanced value from the ``rcu_state`` structure, as shown in the following diagram. -.. kernel-figure:: TreeRCU-gp-init-1.svg +.. kernel-figure:: TreeRCU-gp-init-3.svg This change will also cause each CPU's next call to ``__note_gp_changes()`` to notice that a new grace period has started, @@ -473,7 +497,7 @@ read-side critical sections that follow the idle period (the oval near the bottom of the diagram above). Plumbing this into the full grace-period execution is described -`below <#Forcing%20Quiescent%20States>`__. +`below <Forcing Quiescent States_>`__. CPU-Hotplug Interface ^^^^^^^^^^^^^^^^^^^^^ @@ -494,7 +518,7 @@ mask to detect CPUs having gone offline since the beginning of this grace period. Plumbing this into the full grace-period execution is described -`below <#Forcing%20Quiescent%20States>`__. +`below <Forcing Quiescent States_>`__. Forcing Quiescent States ^^^^^^^^^^^^^^^^^^^^^^^^ @@ -532,7 +556,7 @@ from other CPUs. | RCU. But this diagram is complex enough as it is, so simplicity | | overrode accuracy. You can think of it as poetic license, or you can | | think of it as misdirection that is resolved in the | -| `stitched-together diagram <#Putting%20It%20All%20Together>`__. | +| `stitched-together diagram <Putting It All Together_>`__. | +-----------------------------------------------------------------------+ Grace-Period Cleanup @@ -596,7 +620,7 @@ maintain ordering. For example, if the callback function wakes up a task that runs on some other CPU, proper ordering must in place in both the callback function and the task being awakened. To see why this is important, consider the top half of the `grace-period -cleanup <#Grace-Period%20Cleanup>`__ diagram. The callback might be +cleanup`_ diagram. The callback might be running on a CPU corresponding to the leftmost leaf ``rcu_node`` structure, and awaken a task that is to run on a CPU corresponding to the rightmost leaf ``rcu_node`` structure, and the grace-period kernel diff --git a/Documentation/RCU/Design/Memory-Ordering/TreeRCU-callback-registry.svg b/Documentation/RCU/Design/Memory-Ordering/TreeRCU-callback-registry.svg index 7ac6f9269806..63eff867175a 100644 --- a/Documentation/RCU/Design/Memory-Ordering/TreeRCU-callback-registry.svg +++ b/Documentation/RCU/Design/Memory-Ordering/TreeRCU-callback-registry.svg @@ -566,15 +566,6 @@ style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">rcutree_migrate_callbacks()</text> <text xml:space="preserve" - x="8335.4873" - y="5357.1006" - font-style="normal" - font-weight="bold" - font-size="192" - id="text202-7-9-6-0" - style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">rcu_migrate_callbacks()</text> - <text - xml:space="preserve" x="8768.4678" y="6224.9038" font-style="normal" diff --git a/Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp-fqs.svg b/Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp-fqs.svg index 7ddc094d7f28..d82a77d03d8c 100644 --- a/Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp-fqs.svg +++ b/Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp-fqs.svg @@ -1135,7 +1135,7 @@ font-weight="bold" font-size="192" id="text202-7-5-3-27-6-5" - style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">rcu_report_dead()</text> + style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">rcutree_report_cpu_dead()</text> <text xml:space="preserve" x="3745.7725" @@ -1256,7 +1256,7 @@ font-style="normal" y="3679.27" x="-3804.9949" - xml:space="preserve">rcu_cpu_starting()</text> + xml:space="preserve">rcutree_report_cpu_starting()</text> <g style="fill:none;stroke-width:0.025in" id="g3107-7-5-0" diff --git a/Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp.svg b/Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp.svg index 069f6f8371c2..53e0dc2a2c79 100644 --- a/Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp.svg +++ b/Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp.svg @@ -1448,15 +1448,6 @@ style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">rcutree_migrate_callbacks()</text> <text xml:space="preserve" - x="8335.4873" - y="5357.1006" - font-style="normal" - font-weight="bold" - font-size="192" - id="text202-7-9-6-0" - style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">rcu_migrate_callbacks()</text> - <text - xml:space="preserve" x="8768.4678" y="6224.9038" font-style="normal" @@ -3274,7 +3265,7 @@ font-weight="bold" font-size="192" id="text202-7-5-3-27-6-5" - style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">rcu_report_dead()</text> + style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">rcutree_report_cpu_dead()</text> <text xml:space="preserve" x="3745.7725" @@ -3395,7 +3386,7 @@ font-style="normal" y="3679.27" x="-3804.9949" - xml:space="preserve">rcu_cpu_starting()</text> + xml:space="preserve">rcutree_report_cpu_starting()</text> <g style="fill:none;stroke-width:0.025in" id="g3107-7-5-0" diff --git a/Documentation/RCU/Design/Memory-Ordering/TreeRCU-hotplug.svg b/Documentation/RCU/Design/Memory-Ordering/TreeRCU-hotplug.svg index 2c9310ba29ba..4fa7506082bf 100644 --- a/Documentation/RCU/Design/Memory-Ordering/TreeRCU-hotplug.svg +++ b/Documentation/RCU/Design/Memory-Ordering/TreeRCU-hotplug.svg @@ -607,7 +607,7 @@ font-weight="bold" font-size="192" id="text202-7-5-3-27-6" - style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">rcu_report_dead()</text> + style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">rcutree_report_cpu_dead()</text> <text xml:space="preserve" x="3745.7725" @@ -728,7 +728,7 @@ font-style="normal" y="3679.27" x="-3804.9949" - xml:space="preserve">rcu_cpu_starting()</text> + xml:space="preserve">rcutree_report_cpu_starting()</text> <g style="fill:none;stroke-width:0.025in" id="g3107-7-5-0" diff --git a/Documentation/RCU/Design/Requirements/GPpartitionReaders1.svg b/Documentation/RCU/Design/Requirements/GPpartitionReaders1.svg index 4b4014fda770..87851a8fac1e 100644 --- a/Documentation/RCU/Design/Requirements/GPpartitionReaders1.svg +++ b/Documentation/RCU/Design/Requirements/GPpartitionReaders1.svg @@ -88,7 +88,7 @@ <flowRoot xml:space="preserve" id="flowRoot2985" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"><flowRegion + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"><flowRegion id="flowRegion2987"><rect id="rect2989" width="82.85714" @@ -103,7 +103,7 @@ id="text2993" y="-261.66608" x="412.12299" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" xml:space="preserve" transform="matrix(0,1,-1,0,0,0)"><tspan y="-261.66608" @@ -135,7 +135,7 @@ </g> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="112.04738" y="268.18076" id="text4429" @@ -146,7 +146,7 @@ y="268.18076">WRITE_ONCE(a, 1);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="112.04738" y="439.13766" id="text4441" @@ -157,7 +157,7 @@ y="439.13766">WRITE_ONCE(b, 1);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="255.60869" y="309.29346" id="text4445" @@ -168,7 +168,7 @@ y="309.29346">r1 = READ_ONCE(a);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="255.14423" y="520.61786" id="text4449" @@ -179,7 +179,7 @@ y="520.61786">WRITE_ONCE(c, 1);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="396.10254" y="384.71124" id="text4453" @@ -190,7 +190,7 @@ y="384.71124">r2 = READ_ONCE(b);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="396.10254" y="582.13617" id="text4457" @@ -201,7 +201,7 @@ y="582.13617">r3 = READ_ONCE(c);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="112.08231" y="213.91006" id="text4461" @@ -212,7 +212,7 @@ y="213.91006">thread0()</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="252.34512" y="213.91006" id="text4461-6" @@ -223,7 +223,7 @@ y="213.91006">thread1()</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="396.42557" y="213.91006" id="text4461-2" @@ -251,7 +251,7 @@ inkscape:connector-curvature="0" /> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="111.75929" y="251.53981" id="text4429-8" @@ -262,7 +262,7 @@ y="251.53981">rcu_read_lock();</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="396.10254" y="367.91556" id="text4429-8-9" @@ -273,7 +273,7 @@ y="367.91556">rcu_read_lock();</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="396.10254" y="597.40289" id="text4429-8-9-3" @@ -284,7 +284,7 @@ y="597.40289">rcu_read_unlock();</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="111.75929" y="453.15311" id="text4429-8-9-3-1" @@ -300,7 +300,7 @@ inkscape:connector-curvature="0" /> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="394.94427" y="345.66351" id="text4648" @@ -324,7 +324,7 @@ sodipodi:open="true" /> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="112.11968" y="475.77856" id="text4648-4" @@ -361,7 +361,7 @@ sodipodi:open="true" /> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="254.85066" y="348.96619" id="text4648-4-3" diff --git a/Documentation/RCU/Design/Requirements/ReadersPartitionGP1.svg b/Documentation/RCU/Design/Requirements/ReadersPartitionGP1.svg index 48cd1623d4d4..e2a8af592bab 100644 --- a/Documentation/RCU/Design/Requirements/ReadersPartitionGP1.svg +++ b/Documentation/RCU/Design/Requirements/ReadersPartitionGP1.svg @@ -116,7 +116,7 @@ <flowRoot xml:space="preserve" id="flowRoot2985" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"><flowRegion + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace"><flowRegion id="flowRegion2987"><rect id="rect2989" width="82.85714" @@ -131,7 +131,7 @@ id="text2993" y="-261.66608" x="436.12299" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" xml:space="preserve" transform="matrix(0,1,-1,0,0,0)"><tspan y="-261.66608" @@ -163,7 +163,7 @@ </g> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="112.04738" y="268.18076" id="text4429" @@ -174,7 +174,7 @@ y="268.18076">WRITE_ONCE(a, 1);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="112.04738" y="487.13766" id="text4441" @@ -185,7 +185,7 @@ y="487.13766">WRITE_ONCE(b, 1);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="255.60869" y="297.29346" id="text4445" @@ -196,7 +196,7 @@ y="297.29346">r1 = READ_ONCE(a);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="255.14423" y="554.61786" id="text4449" @@ -207,7 +207,7 @@ y="554.61786">WRITE_ONCE(c, 1);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="396.10254" y="370.71124" id="text4453" @@ -218,7 +218,7 @@ y="370.71124">WRITE_ONCE(d, 1);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="396.10254" y="572.13617" id="text4457" @@ -229,7 +229,7 @@ y="572.13617">r2 = READ_ONCE(c);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="112.08231" y="213.91006" id="text4461" @@ -240,7 +240,7 @@ y="213.91006">thread0()</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="252.34512" y="213.91006" id="text4461-6" @@ -251,7 +251,7 @@ y="213.91006">thread1()</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="396.42557" y="213.91006" id="text4461-2" @@ -281,7 +281,7 @@ sodipodi:nodetypes="cc" /> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="111.75929" y="251.53981" id="text4429-8" @@ -292,7 +292,7 @@ y="251.53981">rcu_read_lock();</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="396.10254" y="353.91556" id="text4429-8-9" @@ -303,7 +303,7 @@ y="353.91556">rcu_read_lock();</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="396.10254" y="587.40289" id="text4429-8-9-3" @@ -314,7 +314,7 @@ y="587.40289">rcu_read_unlock();</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="111.75929" y="501.15311" id="text4429-8-9-3-1" @@ -331,7 +331,7 @@ sodipodi:nodetypes="cc" /> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="394.94427" y="331.66351" id="text4648" @@ -355,7 +355,7 @@ sodipodi:open="true" /> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="112.11968" y="523.77856" id="text4648-4" @@ -392,7 +392,7 @@ sodipodi:open="true" /> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="254.85066" y="336.96619" id="text4648-4-3" @@ -421,7 +421,7 @@ id="text2993-7" y="-261.66608" x="440.12299" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" xml:space="preserve" transform="matrix(0,1,-1,0,0,0)"><tspan y="-261.66608" @@ -453,7 +453,7 @@ </g> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="541.70508" y="387.6217" id="text4445-0" @@ -464,7 +464,7 @@ y="387.6217">r3 = READ_ONCE(d);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="541.2406" y="646.94611" id="text4449-6" @@ -488,7 +488,7 @@ sodipodi:open="true" /> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="540.94702" y="427.29443" id="text4648-4-3-1" @@ -499,7 +499,7 @@ y="427.29443">QS</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="686.27747" y="461.83929" id="text4453-7" @@ -510,7 +510,7 @@ y="461.83929">r4 = READ_ONCE(b);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="686.27747" y="669.26422" id="text4457-9" @@ -521,7 +521,7 @@ y="669.26422">r5 = READ_ONCE(e);</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="686.27747" y="445.04358" id="text4429-8-9-33" @@ -532,7 +532,7 @@ y="445.04358">rcu_read_lock();</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="686.27747" y="684.53094" id="text4429-8-9-3-8" @@ -543,7 +543,7 @@ y="684.53094">rcu_read_unlock();</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="685.11914" y="422.79153" id="text4648-9" @@ -567,7 +567,7 @@ sodipodi:open="true" /> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="397.85934" y="609.59003" id="text4648-5" @@ -591,7 +591,7 @@ sodipodi:open="true" /> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="256.75986" y="586.99133" id="text4648-5-2" @@ -615,7 +615,7 @@ sodipodi:open="true" /> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="546.22791" y="213.91006" id="text4461-2-5" @@ -626,7 +626,7 @@ y="213.91006">thread3()</tspan></text> <text xml:space="preserve" - style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol" + style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:monospace;-inkscape-font-specification:monospace" x="684.00067" y="213.91006" id="text4461-2-1" diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst index fd5e2cbc4935..cccafdaa1f84 100644 --- a/Documentation/RCU/Design/Requirements/Requirements.rst +++ b/Documentation/RCU/Design/Requirements/Requirements.rst @@ -45,7 +45,7 @@ requirements: #. `Other RCU Flavors`_ #. `Possible Future Changes`_ -This is followed by a `summary <#Summary>`__, however, the answers to +This is followed by a summary_, however, the answers to each quick quiz immediately follows the quiz. Select the big white space with your mouse to see the answer. @@ -72,13 +72,13 @@ understanding of this guarantee. RCU's grace-period guarantee allows updaters to wait for the completion of all pre-existing RCU read-side critical sections. An RCU read-side -critical section begins with the marker ``rcu_read_lock()`` and ends -with the marker ``rcu_read_unlock()``. These markers may be nested, and +critical section begins with the marker rcu_read_lock() and ends +with the marker rcu_read_unlock(). These markers may be nested, and RCU treats a nested set as one big RCU read-side critical section. -Production-quality implementations of ``rcu_read_lock()`` and -``rcu_read_unlock()`` are extremely lightweight, and in fact have +Production-quality implementations of rcu_read_lock() and +rcu_read_unlock() are extremely lightweight, and in fact have exactly zero overhead in Linux kernels built for production use with -``CONFIG_PREEMPT=n``. +``CONFIG_PREEMPTION=n``. This guarantee allows ordering to be enforced with extremely low overhead to readers, for example: @@ -102,12 +102,12 @@ overhead to readers, for example: 15 WRITE_ONCE(y, 1); 16 } -Because the ``synchronize_rcu()`` on line 14 waits for all pre-existing -readers, any instance of ``thread0()`` that loads a value of zero from -``x`` must complete before ``thread1()`` stores to ``y``, so that +Because the synchronize_rcu() on line 14 waits for all pre-existing +readers, any instance of thread0() that loads a value of zero from +``x`` must complete before thread1() stores to ``y``, so that instance must also load a value of zero from ``y``. Similarly, any -instance of ``thread0()`` that loads a value of one from ``y`` must have -started after the ``synchronize_rcu()`` started, and must therefore also +instance of thread0() that loads a value of one from ``y`` must have +started after the synchronize_rcu() started, and must therefore also load a value of one from ``x``. Therefore, the outcome: :: @@ -121,14 +121,14 @@ cannot happen. +-----------------------------------------------------------------------+ | Wait a minute! You said that updaters can make useful forward | | progress concurrently with readers, but pre-existing readers will | -| block ``synchronize_rcu()``!!! | +| block synchronize_rcu()!!! | | Just who are you trying to fool??? | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ | First, if updaters do not wish to be blocked by readers, they can use | -| ``call_rcu()`` or ``kfree_rcu()``, which will be discussed later. | -| Second, even when using ``synchronize_rcu()``, the other update-side | +| call_rcu() or kfree_rcu(), which will be discussed later. | +| Second, even when using synchronize_rcu(), the other update-side | | code does run concurrently with readers, whether pre-existing or not. | +-----------------------------------------------------------------------+ @@ -170,34 +170,34 @@ recovery from node failure, more or less as follows: 29 WRITE_ONCE(state, STATE_NORMAL); 30 } -The RCU read-side critical section in ``do_something_dlm()`` works with -the ``synchronize_rcu()`` in ``start_recovery()`` to guarantee that -``do_something()`` never runs concurrently with ``recovery()``, but with -little or no synchronization overhead in ``do_something_dlm()``. +The RCU read-side critical section in do_something_dlm() works with +the synchronize_rcu() in start_recovery() to guarantee that +do_something() never runs concurrently with recovery(), but with +little or no synchronization overhead in do_something_dlm(). +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ -| Why is the ``synchronize_rcu()`` on line 28 needed? | +| Why is the synchronize_rcu() on line 28 needed? | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ | Without that extra grace period, memory reordering could result in | -| ``do_something_dlm()`` executing ``do_something()`` concurrently with | -| the last bits of ``recovery()``. | +| do_something_dlm() executing do_something() concurrently with | +| the last bits of recovery(). | +-----------------------------------------------------------------------+ In order to avoid fatal problems such as deadlocks, an RCU read-side -critical section must not contain calls to ``synchronize_rcu()``. +critical section must not contain calls to synchronize_rcu(). Similarly, an RCU read-side critical section must not contain anything that waits, directly or indirectly, on completion of an invocation of -``synchronize_rcu()``. +synchronize_rcu(). Although RCU's grace-period guarantee is useful in and of itself, with `quite a few use cases <https://lwn.net/Articles/573497/>`__, it would be good to be able to use RCU to coordinate read-side access to linked data structures. For this, the grace-period guarantee is not sufficient, -as can be seen in function ``add_gp_buggy()`` below. We will look at the +as can be seen in function add_gp_buggy() below. We will look at the reader's code later, but in the meantime, just think of the reader as locklessly picking up the ``gp`` pointer, and, if the value loaded is non-\ ``NULL``, locklessly accessing the ``->a`` and ``->b`` fields. @@ -256,8 +256,8 @@ Publish/Subscribe Guarantee RCU's publish-subscribe guarantee allows data to be inserted into a linked data structure without disrupting RCU readers. The updater uses -``rcu_assign_pointer()`` to insert the new data, and readers use -``rcu_dereference()`` to access data, whether new or old. The following +rcu_assign_pointer() to insert the new data, and readers use +rcu_dereference() to access data, whether new or old. The following shows an example of insertion: :: @@ -279,7 +279,7 @@ shows an example of insertion: 15 return true; 16 } -The ``rcu_assign_pointer()`` on line 13 is conceptually equivalent to a +The rcu_assign_pointer() on line 13 is conceptually equivalent to a simple assignment statement, but also guarantees that its assignment will happen after the two assignments in lines 11 and 12, similar to the C11 ``memory_order_release`` store operation. It also prevents any @@ -289,7 +289,7 @@ number of “interesting” compiler optimizations, for example, the use of +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ -| But ``rcu_assign_pointer()`` does nothing to prevent the two | +| But rcu_assign_pointer() does nothing to prevent the two | | assignments to ``p->a`` and ``p->b`` from being reordered. Can't that | | also cause problems? | +-----------------------------------------------------------------------+ @@ -303,7 +303,7 @@ number of “interesting” compiler optimizations, for example, the use of It is tempting to assume that the reader need not do anything special to control its accesses to the RCU-protected data, as shown in -``do_something_gp_buggy()`` below: +do_something_gp_buggy() below: :: @@ -321,11 +321,10 @@ control its accesses to the RCU-protected data, as shown in 12 } However, this temptation must be resisted because there are a -surprisingly large number of ways that the compiler (to say nothing of -`DEC Alpha CPUs <https://h71000.www7.hp.com/wizard/wiz_2637.html>`__) -can trip this code up. For but one example, if the compiler were short -of registers, it might choose to refetch from ``gp`` rather than keeping -a separate copy in ``p`` as follows: +surprisingly large number of ways that the compiler (or weak ordering +CPUs like the DEC Alpha) can trip this code up. For but one example, if +the compiler were short of registers, it might choose to refetch from +``gp`` rather than keeping a separate copy in ``p`` as follows: :: @@ -345,7 +344,7 @@ If this function ran concurrently with a series of updates that replaced the current structure with a new one, the fetches of ``gp->a`` and ``gp->b`` might well come from two different structures, which could cause serious confusion. To prevent this (and much else besides), -``do_something_gp()`` uses ``rcu_dereference()`` to fetch from ``gp``: +do_something_gp() uses rcu_dereference() to fetch from ``gp``: :: @@ -362,21 +361,23 @@ cause serious confusion. To prevent this (and much else besides), 11 return false; 12 } -The ``rcu_dereference()`` uses volatile casts and (for DEC Alpha) memory -barriers in the Linux kernel. Should a `high-quality implementation of -C11 ``memory_order_consume`` -[PDF] <http://www.rdrop.com/users/paulmck/RCU/consume.2015.07.13a.pdf>`__ -ever appear, then ``rcu_dereference()`` could be implemented as a +The rcu_dereference() uses volatile casts and (for DEC Alpha) memory +barriers in the Linux kernel. Should a |high-quality implementation of +C11 memory_order_consume [PDF]|_ +ever appear, then rcu_dereference() could be implemented as a ``memory_order_consume`` load. Regardless of the exact implementation, a -pointer fetched by ``rcu_dereference()`` may not be used outside of the +pointer fetched by rcu_dereference() may not be used outside of the outermost RCU read-side critical section containing that -``rcu_dereference()``, unless protection of the corresponding data +rcu_dereference(), unless protection of the corresponding data element has been passed from RCU to some other synchronization -mechanism, most commonly locking or `reference -counting <https://www.kernel.org/doc/Documentation/RCU/rcuref.txt>`__. +mechanism, most commonly locking or reference counting +(see ../../rcuref.rst). -In short, updaters use ``rcu_assign_pointer()`` and readers use -``rcu_dereference()``, and these two RCU API elements work together to +.. |high-quality implementation of C11 memory_order_consume [PDF]| replace:: high-quality implementation of C11 ``memory_order_consume`` [PDF] +.. _high-quality implementation of C11 memory_order_consume [PDF]: http://www.rdrop.com/users/paulmck/RCU/consume.2015.07.13a.pdf + +In short, updaters use rcu_assign_pointer() and readers use +rcu_dereference(), and these two RCU API elements work together to ensure that readers have a consistent view of newly added data elements. Of course, it is also necessary to remove elements from RCU-protected @@ -388,9 +389,9 @@ data structures, for example, using the following process: the newly removed data element). #. At this point, only the updater has a reference to the newly removed data element, so it can safely reclaim the data element, for example, - by passing it to ``kfree()``. + by passing it to kfree(). -This process is implemented by ``remove_gp_synchronous()``: +This process is implemented by remove_gp_synchronous(): :: @@ -413,16 +414,16 @@ This process is implemented by ``remove_gp_synchronous()``: This function is straightforward, with line 13 waiting for a grace period before line 14 frees the old data element. This waiting ensures -that readers will reach line 7 of ``do_something_gp()`` before the data -element referenced by ``p`` is freed. The ``rcu_access_pointer()`` on -line 6 is similar to ``rcu_dereference()``, except that: +that readers will reach line 7 of do_something_gp() before the data +element referenced by ``p`` is freed. The rcu_access_pointer() on +line 6 is similar to rcu_dereference(), except that: -#. The value returned by ``rcu_access_pointer()`` cannot be +#. The value returned by rcu_access_pointer() cannot be dereferenced. If you want to access the value pointed to as well as - the pointer itself, use ``rcu_dereference()`` instead of - ``rcu_access_pointer()``. -#. The call to ``rcu_access_pointer()`` need not be protected. In - contrast, ``rcu_dereference()`` must either be within an RCU + the pointer itself, use rcu_dereference() instead of + rcu_access_pointer(). +#. The call to rcu_access_pointer() need not be protected. In + contrast, rcu_dereference() must either be within an RCU read-side critical section or in a code segment where the pointer cannot change, for example, in code protected by the corresponding update-side lock. @@ -430,13 +431,13 @@ line 6 is similar to ``rcu_dereference()``, except that: +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ -| Without the ``rcu_dereference()`` or the ``rcu_access_pointer()``, | +| Without the rcu_dereference() or the rcu_access_pointer(), | | what destructive optimizations might the compiler make use of? | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ -| Let's start with what happens to ``do_something_gp()`` if it fails to | -| use ``rcu_dereference()``. It could reuse a value formerly fetched | +| Let's start with what happens to do_something_gp() if it fails to | +| use rcu_dereference(). It could reuse a value formerly fetched | | from this same pointer. It could also fetch the pointer from ``gp`` | | in a byte-at-a-time manner, resulting in *load tearing*, in turn | | resulting a bytewise mash-up of two distinct pointer values. It might | @@ -445,15 +446,15 @@ line 6 is similar to ``rcu_dereference()``, except that: | update has changed the pointer to match the wrong guess. Too bad | | about any dereferences that returned pre-initialization garbage in | | the meantime! | -| For ``remove_gp_synchronous()``, as long as all modifications to | +| For remove_gp_synchronous(), as long as all modifications to | | ``gp`` are carried out while holding ``gp_lock``, the above | | optimizations are harmless. However, ``sparse`` will complain if you | | define ``gp`` with ``__rcu`` and then access it without using either | -| ``rcu_access_pointer()`` or ``rcu_dereference()``. | +| rcu_access_pointer() or rcu_dereference(). | +-----------------------------------------------------------------------+ In short, RCU's publish-subscribe guarantee is provided by the -combination of ``rcu_assign_pointer()`` and ``rcu_dereference()``. This +combination of rcu_assign_pointer() and rcu_dereference(). This guarantee allows data elements to be safely added to RCU-protected linked data structures without disrupting RCU readers. This guarantee can be used in combination with the grace-period guarantee to also allow @@ -462,9 +463,9 @@ again without disrupting RCU readers. This guarantee was only partially premeditated. DYNIX/ptx used an explicit memory barrier for publication, but had nothing resembling -``rcu_dereference()`` for subscription, nor did it have anything -resembling the ``smp_read_barrier_depends()`` that was later subsumed -into ``rcu_dereference()`` and later still into ``READ_ONCE()``. The +rcu_dereference() for subscription, nor did it have anything +resembling the dependency-ordering barrier that was later subsumed +into rcu_dereference() and later still into READ_ONCE(). The need for these operations made itself known quite suddenly at a late-1990s meeting with the DEC Alpha architects, back in the days when DEC was still a free-standing company. It took the Alpha architects a @@ -474,7 +475,7 @@ documentation did not make this point clear. More recent work with the C and C++ standards committees have provided much education on tricks and traps from the compiler. In short, compilers were much less tricky in the early 1990s, but in 2015, don't even think about omitting -``rcu_dereference()``! +rcu_dereference()! Memory-Barrier Guarantees ~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -484,31 +485,31 @@ demonstrates the need for RCU's stringent memory-ordering guarantees on systems with more than one CPU: #. Each CPU that has an RCU read-side critical section that begins - before ``synchronize_rcu()`` starts is guaranteed to execute a full + before synchronize_rcu() starts is guaranteed to execute a full memory barrier between the time that the RCU read-side critical - section ends and the time that ``synchronize_rcu()`` returns. Without + section ends and the time that synchronize_rcu() returns. Without this guarantee, a pre-existing RCU read-side critical section might hold a reference to the newly removed ``struct foo`` after the - ``kfree()`` on line 14 of ``remove_gp_synchronous()``. + kfree() on line 14 of remove_gp_synchronous(). #. Each CPU that has an RCU read-side critical section that ends after - ``synchronize_rcu()`` returns is guaranteed to execute a full memory - barrier between the time that ``synchronize_rcu()`` begins and the + synchronize_rcu() returns is guaranteed to execute a full memory + barrier between the time that synchronize_rcu() begins and the time that the RCU read-side critical section begins. Without this guarantee, a later RCU read-side critical section running after the - ``kfree()`` on line 14 of ``remove_gp_synchronous()`` might later run - ``do_something_gp()`` and find the newly deleted ``struct foo``. -#. If the task invoking ``synchronize_rcu()`` remains on a given CPU, + kfree() on line 14 of remove_gp_synchronous() might later run + do_something_gp() and find the newly deleted ``struct foo``. +#. If the task invoking synchronize_rcu() remains on a given CPU, then that CPU is guaranteed to execute a full memory barrier sometime - during the execution of ``synchronize_rcu()``. This guarantee ensures - that the ``kfree()`` on line 14 of ``remove_gp_synchronous()`` really + during the execution of synchronize_rcu(). This guarantee ensures + that the kfree() on line 14 of remove_gp_synchronous() really does execute after the removal on line 11. -#. If the task invoking ``synchronize_rcu()`` migrates among a group of +#. If the task invoking synchronize_rcu() migrates among a group of CPUs during that invocation, then each of the CPUs in that group is guaranteed to execute a full memory barrier sometime during the - execution of ``synchronize_rcu()``. This guarantee also ensures that - the ``kfree()`` on line 14 of ``remove_gp_synchronous()`` really does + execution of synchronize_rcu(). This guarantee also ensures that + the kfree() on line 14 of remove_gp_synchronous() really does execute after the removal on line 11, but also in the case where the - thread executing the ``synchronize_rcu()`` migrates in the meantime. + thread executing the synchronize_rcu() migrates in the meantime. +-----------------------------------------------------------------------+ | **Quick Quiz**: | @@ -516,19 +517,19 @@ systems with more than one CPU: | Given that multiple CPUs can start RCU read-side critical sections at | | any time without any ordering whatsoever, how can RCU possibly tell | | whether or not a given RCU read-side critical section starts before a | -| given instance of ``synchronize_rcu()``? | +| given instance of synchronize_rcu()? | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ | If RCU cannot tell whether or not a given RCU read-side critical | -| section starts before a given instance of ``synchronize_rcu()``, then | +| section starts before a given instance of synchronize_rcu(), then | | it must assume that the RCU read-side critical section started first. | -| In other words, a given instance of ``synchronize_rcu()`` can avoid | +| In other words, a given instance of synchronize_rcu() can avoid | | waiting on a given RCU read-side critical section only if it can | -| prove that ``synchronize_rcu()`` started first. | -| A related question is “When ``rcu_read_lock()`` doesn't generate any | +| prove that synchronize_rcu() started first. | +| A related question is “When rcu_read_lock() doesn't generate any | | code, why does it matter how it relates to a grace period?” The | -| answer is that it is not the relationship of ``rcu_read_lock()`` | +| answer is that it is not the relationship of rcu_read_lock() | | itself that is important, but rather the relationship of the code | | within the enclosed RCU read-side critical section to the code | | preceding and following the grace period. If we take this viewpoint, | @@ -556,14 +557,14 @@ systems with more than one CPU: | Yes, they really are required. To see why the first guarantee is | | required, consider the following sequence of events: | | | -| #. CPU 1: ``rcu_read_lock()`` | +| #. CPU 1: rcu_read_lock() | | #. CPU 1: ``q = rcu_dereference(gp); /* Very likely to return p. */`` | | #. CPU 0: ``list_del_rcu(p);`` | -| #. CPU 0: ``synchronize_rcu()`` starts. | +| #. CPU 0: synchronize_rcu() starts. | | #. CPU 1: ``do_something_with(q->a);`` | | ``/* No smp_mb(), so might happen after kfree(). */`` | -| #. CPU 1: ``rcu_read_unlock()`` | -| #. CPU 0: ``synchronize_rcu()`` returns. | +| #. CPU 1: rcu_read_unlock() | +| #. CPU 0: synchronize_rcu() returns. | | #. CPU 0: ``kfree(p);`` | | | | Therefore, there absolutely must be a full memory barrier between the | @@ -574,14 +575,14 @@ systems with more than one CPU: | is roughly similar: | | | | #. CPU 0: ``list_del_rcu(p);`` | -| #. CPU 0: ``synchronize_rcu()`` starts. | -| #. CPU 1: ``rcu_read_lock()`` | +| #. CPU 0: synchronize_rcu() starts. | +| #. CPU 1: rcu_read_lock() | | #. CPU 1: ``q = rcu_dereference(gp);`` | | ``/* Might return p if no memory barrier. */`` | -| #. CPU 0: ``synchronize_rcu()`` returns. | +| #. CPU 0: synchronize_rcu() returns. | | #. CPU 0: ``kfree(p);`` | | #. CPU 1: ``do_something_with(q->a); /* Boom!!! */`` | -| #. CPU 1: ``rcu_read_unlock()`` | +| #. CPU 1: rcu_read_unlock() | | | | And similarly, without a memory barrier between the beginning of the | | grace period and the beginning of the RCU read-side critical section, | @@ -597,7 +598,7 @@ systems with more than one CPU: +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ -| You claim that ``rcu_read_lock()`` and ``rcu_read_unlock()`` generate | +| You claim that rcu_read_lock() and rcu_read_unlock() generate | | absolutely no code in some kernel builds. This means that the | | compiler might arbitrarily rearrange consecutive RCU read-side | | critical sections. Given such rearrangement, if a given RCU read-side | @@ -607,11 +608,11 @@ systems with more than one CPU: +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ -| In cases where ``rcu_read_lock()`` and ``rcu_read_unlock()`` generate | +| In cases where rcu_read_lock() and rcu_read_unlock() generate | | absolutely no code, RCU infers quiescent states only at special | | locations, for example, within the scheduler. Because calls to | -| ``schedule()`` had better prevent calling-code accesses to shared | -| variables from being rearranged across the call to ``schedule()``, if | +| schedule() had better prevent calling-code accesses to shared | +| variables from being rearranged across the call to schedule(), if | | RCU detects the end of a given RCU read-side critical section, it | | will necessarily detect the end of all prior RCU read-side critical | | sections, no matter how aggressively the compiler scrambles the code. | @@ -655,8 +656,8 @@ read-side critical section might search for a given data element, and then might acquire the update-side spinlock in order to update that element, all while remaining in that RCU read-side critical section. Of course, it is necessary to exit the RCU read-side critical section -before invoking ``synchronize_rcu()``, however, this inconvenience can -be avoided through use of the ``call_rcu()`` and ``kfree_rcu()`` API +before invoking synchronize_rcu(), however, this inconvenience can +be avoided through use of the call_rcu() and kfree_rcu() API members described later in this document. +-----------------------------------------------------------------------+ @@ -694,10 +695,10 @@ these non-guarantees were premeditated. Readers Impose Minimal Ordering ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -Reader-side markers such as ``rcu_read_lock()`` and -``rcu_read_unlock()`` provide absolutely no ordering guarantees except +Reader-side markers such as rcu_read_lock() and +rcu_read_unlock() provide absolutely no ordering guarantees except through their interaction with the grace-period APIs such as -``synchronize_rcu()``. To see this, consider the following pair of +synchronize_rcu(). To see this, consider the following pair of threads: :: @@ -722,7 +723,7 @@ threads: 18 rcu_read_unlock(); 19 } -After ``thread0()`` and ``thread1()`` execute concurrently, it is quite +After thread0() and thread1() execute concurrently, it is quite possible to have :: @@ -730,7 +731,7 @@ possible to have (r1 == 1 && r2 == 0) (that is, ``y`` appears to have been assigned before ``x``), which would -not be possible if ``rcu_read_lock()`` and ``rcu_read_unlock()`` had +not be possible if rcu_read_lock() and rcu_read_unlock() had much in the way of ordering properties. But they do not, so the CPU is within its rights to do significant reordering. This is by design: Any significant ordering constraints would slow down these fast-path APIs. @@ -742,14 +743,14 @@ significant ordering constraints would slow down these fast-path APIs. +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ -| No, the volatile casts in ``READ_ONCE()`` and ``WRITE_ONCE()`` | +| No, the volatile casts in READ_ONCE() and WRITE_ONCE() | | prevent the compiler from reordering in this particular case. | +-----------------------------------------------------------------------+ Readers Do Not Exclude Updaters ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -Neither ``rcu_read_lock()`` nor ``rcu_read_unlock()`` exclude updates. +Neither rcu_read_lock() nor rcu_read_unlock() exclude updates. All they do is to prevent grace periods from ending. The following example illustrates this: @@ -775,19 +776,19 @@ example illustrates this: 18 spin_unlock(&my_lock); 19 } -If the ``thread0()`` function's ``rcu_read_lock()`` excluded the -``thread1()`` function's update, the ``WARN_ON()`` could never fire. But -the fact is that ``rcu_read_lock()`` does not exclude much of anything -aside from subsequent grace periods, of which ``thread1()`` has none, so -the ``WARN_ON()`` can and does fire. +If the thread0() function's rcu_read_lock() excluded the +thread1() function's update, the WARN_ON() could never fire. But +the fact is that rcu_read_lock() does not exclude much of anything +aside from subsequent grace periods, of which thread1() has none, so +the WARN_ON() can and does fire. Updaters Only Wait For Old Readers ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -It might be tempting to assume that after ``synchronize_rcu()`` +It might be tempting to assume that after synchronize_rcu() completes, there are no readers executing. This temptation must be avoided because new readers can start immediately after -``synchronize_rcu()`` starts, and ``synchronize_rcu()`` is under no +synchronize_rcu() starts, and synchronize_rcu() is under no obligation to wait for these new readers. +-----------------------------------------------------------------------+ @@ -799,10 +800,10 @@ obligation to wait for these new readers. +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ -| For no time at all. Even if ``synchronize_rcu()`` were to wait until | +| For no time at all. Even if synchronize_rcu() were to wait until | | all readers had completed, a new reader might start immediately after | -| ``synchronize_rcu()`` completed. Therefore, the code following | -| ``synchronize_rcu()`` can *never* rely on there being no readers. | +| synchronize_rcu() completed. Therefore, the code following | +| synchronize_rcu() can *never* rely on there being no readers. | +-----------------------------------------------------------------------+ Grace Periods Don't Partition Read-Side Critical Sections @@ -892,12 +893,12 @@ period is known to end before the second grace period starts: 28 rcu_read_unlock(); 29 } -Here, if ``(r1 == 1)``, then ``thread0()``'s write to ``b`` must happen -before the end of ``thread1()``'s grace period. If in addition -``(r4 == 1)``, then ``thread3()``'s read from ``b`` must happen after -the beginning of ``thread2()``'s grace period. If it is also the case -that ``(r2 == 1)``, then the end of ``thread1()``'s grace period must -precede the beginning of ``thread2()``'s grace period. This mean that +Here, if ``(r1 == 1)``, then thread0()'s write to ``b`` must happen +before the end of thread1()'s grace period. If in addition +``(r4 == 1)``, then thread3()'s read from ``b`` must happen after +the beginning of thread2()'s grace period. If it is also the case +that ``(r2 == 1)``, then the end of thread1()'s grace period must +precede the beginning of thread2()'s grace period. This mean that the two RCU read-side critical sections cannot overlap, guaranteeing that ``(r3 == 1)``. As a result, the outcome: @@ -1076,8 +1077,8 @@ is captured by the following list of situations: b. Wait-free read-side primitives for real-time use. This focus on read-mostly situations means that RCU must interoperate -with other synchronization primitives. For example, the ``add_gp()`` and -``remove_gp_synchronous()`` examples discussed earlier use RCU to +with other synchronization primitives. For example, the add_gp() and +remove_gp_synchronous() examples discussed earlier use RCU to protect readers and locking to coordinate updaters. However, the need extends much farther, requiring that a variety of synchronization primitives be legal within RCU read-side critical sections, including @@ -1096,7 +1097,7 @@ memory barriers. | case, voluntary context switch) within an RCU read-side critical | | section. However, sleeping locks may be used within userspace RCU | | read-side critical sections, and also within Linux-kernel sleepable | -| RCU `(SRCU) <#Sleepable%20RCU>`__ read-side critical sections. In | +| RCU `(SRCU) <Sleepable RCU_>`__ read-side critical sections. In | | addition, the -rt patchset turns spinlocks into a sleeping locks so | | that the corresponding critical sections can be preempted, which also | | means that these sleeplockified spinlocks (but not other sleeping | @@ -1104,11 +1105,11 @@ memory barriers. | sections. | | Note that it *is* legal for a normal RCU read-side critical section | | to conditionally acquire a sleeping locks (as in | -| ``mutex_trylock()``), but only as long as it does not loop | +| mutex_trylock()), but only as long as it does not loop | | indefinitely attempting to conditionally acquire that sleeping locks. | -| The key point is that things like ``mutex_trylock()`` either return | +| The key point is that things like mutex_trylock() either return | | with the mutex held, or return an error indication if the mutex was | -| not immediately available. Either way, ``mutex_trylock()`` returns | +| not immediately available. Either way, mutex_trylock() returns | | immediately without sleeping. | +-----------------------------------------------------------------------+ @@ -1182,66 +1183,66 @@ and has become decreasingly so as memory sizes have expanded and memory costs have plummeted. However, as I learned from Matt Mackall's `bloatwatch <http://elinux.org/Linux_Tiny-FAQ>`__ efforts, memory footprint is critically important on single-CPU systems with -non-preemptible (``CONFIG_PREEMPT=n``) kernels, and thus `tiny -RCU <https://lkml.kernel.org/g/20090113221724.GA15307@linux.vnet.ibm.com>`__ +non-preemptible (``CONFIG_PREEMPTION=n``) kernels, and thus `tiny +RCU <https://lore.kernel.org/r/20090113221724.GA15307@linux.vnet.ibm.com>`__ was born. Josh Triplett has since taken over the small-memory banner with his `Linux kernel tinification <https://tiny.wiki.kernel.org/>`__ -project, which resulted in `SRCU <#Sleepable%20RCU>`__ becoming optional +project, which resulted in `SRCU <Sleepable RCU_>`__ becoming optional for those kernels not needing it. The remaining performance requirements are, for the most part, unsurprising. For example, in keeping with RCU's read-side -specialization, ``rcu_dereference()`` should have negligible overhead +specialization, rcu_dereference() should have negligible overhead (for example, suppression of a few minor compiler optimizations). -Similarly, in non-preemptible environments, ``rcu_read_lock()`` and -``rcu_read_unlock()`` should have exactly zero overhead. +Similarly, in non-preemptible environments, rcu_read_lock() and +rcu_read_unlock() should have exactly zero overhead. In preemptible environments, in the case where the RCU read-side critical section was not preempted (as will be the case for the -highest-priority real-time process), ``rcu_read_lock()`` and -``rcu_read_unlock()`` should have minimal overhead. In particular, they +highest-priority real-time process), rcu_read_lock() and +rcu_read_unlock() should have minimal overhead. In particular, they should not contain atomic read-modify-write operations, memory-barrier instructions, preemption disabling, interrupt disabling, or backwards branches. However, in the case where the RCU read-side critical section -was preempted, ``rcu_read_unlock()`` may acquire spinlocks and disable +was preempted, rcu_read_unlock() may acquire spinlocks and disable interrupts. This is why it is better to nest an RCU read-side critical section within a preempt-disable region than vice versa, at least in cases where that critical section is short enough to avoid unduly degrading real-time latencies. -The ``synchronize_rcu()`` grace-period-wait primitive is optimized for +The synchronize_rcu() grace-period-wait primitive is optimized for throughput. It may therefore incur several milliseconds of latency in addition to the duration of the longest RCU read-side critical section. On the other hand, multiple concurrent invocations of -``synchronize_rcu()`` are required to use batching optimizations so that +synchronize_rcu() are required to use batching optimizations so that they can be satisfied by a single underlying grace-period-wait operation. For example, in the Linux kernel, it is not unusual for a single grace-period-wait operation to serve more than `1,000 separate invocations <https://www.usenix.org/conference/2004-usenix-annual-technical-conference/making-rcu-safe-deep-sub-millisecond-response>`__ -of ``synchronize_rcu()``, thus amortizing the per-invocation overhead +of synchronize_rcu(), thus amortizing the per-invocation overhead down to nearly zero. However, the grace-period optimization is also required to avoid measurable degradation of real-time scheduling and interrupt latencies. -In some cases, the multi-millisecond ``synchronize_rcu()`` latencies are -unacceptable. In these cases, ``synchronize_rcu_expedited()`` may be +In some cases, the multi-millisecond synchronize_rcu() latencies are +unacceptable. In these cases, synchronize_rcu_expedited() may be used instead, reducing the grace-period latency down to a few tens of microseconds on small systems, at least in cases where the RCU read-side critical sections are short. There are currently no special latency -requirements for ``synchronize_rcu_expedited()`` on large systems, but, +requirements for synchronize_rcu_expedited() on large systems, but, consistent with the empirical nature of the RCU specification, that is subject to change. However, there most definitely are scalability -requirements: A storm of ``synchronize_rcu_expedited()`` invocations on +requirements: A storm of synchronize_rcu_expedited() invocations on 4096 CPUs should at least make reasonable forward progress. In return -for its shorter latencies, ``synchronize_rcu_expedited()`` is permitted +for its shorter latencies, synchronize_rcu_expedited() is permitted to impose modest degradation of real-time latency on non-idle online CPUs. Here, “modest” means roughly the same latency degradation as a scheduling-clock interrupt. There are a number of situations where even -``synchronize_rcu_expedited()``'s reduced grace-period latency is -unacceptable. In these situations, the asynchronous ``call_rcu()`` can -be used in place of ``synchronize_rcu()`` as follows: +synchronize_rcu_expedited()'s reduced grace-period latency is +unacceptable. In these situations, the asynchronous call_rcu() can +be used in place of synchronize_rcu() as follows: :: @@ -1275,19 +1276,19 @@ be used in place of ``synchronize_rcu()`` as follows: 28 } A definition of ``struct foo`` is finally needed, and appears on -lines 1-5. The function ``remove_gp_cb()`` is passed to ``call_rcu()`` +lines 1-5. The function remove_gp_cb() is passed to call_rcu() on line 25, and will be invoked after the end of a subsequent grace -period. This gets the same effect as ``remove_gp_synchronous()``, but +period. This gets the same effect as remove_gp_synchronous(), but without forcing the updater to wait for a grace period to elapse. The -``call_rcu()`` function may be used in a number of situations where -neither ``synchronize_rcu()`` nor ``synchronize_rcu_expedited()`` would -be legal, including within preempt-disable code, ``local_bh_disable()`` +call_rcu() function may be used in a number of situations where +neither synchronize_rcu() nor synchronize_rcu_expedited() would +be legal, including within preempt-disable code, local_bh_disable() code, interrupt-disable code, and interrupt handlers. However, even -``call_rcu()`` is illegal within NMI handlers and from idle and offline -CPUs. The callback function (``remove_gp_cb()`` in this case) will be +call_rcu() is illegal within NMI handlers and from idle and offline +CPUs. The callback function (remove_gp_cb() in this case) will be executed within softirq (software interrupt) environment within the Linux kernel, either within a real softirq handler or under the -protection of ``local_bh_disable()``. In both the Linux kernel and in +protection of local_bh_disable(). In both the Linux kernel and in userspace, it is bad practice to write an RCU callback function that takes too long. Long-running operations should be relegated to separate threads or (in the Linux kernel) workqueues. @@ -1295,23 +1296,23 @@ threads or (in the Linux kernel) workqueues. +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ -| Why does line 19 use ``rcu_access_pointer()``? After all, | -| ``call_rcu()`` on line 25 stores into the structure, which would | +| Why does line 19 use rcu_access_pointer()? After all, | +| call_rcu() on line 25 stores into the structure, which would | | interact badly with concurrent insertions. Doesn't this mean that | -| ``rcu_dereference()`` is required? | +| rcu_dereference() is required? | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ | Presumably the ``->gp_lock`` acquired on line 18 excludes any | -| changes, including any insertions that ``rcu_dereference()`` would | +| changes, including any insertions that rcu_dereference() would | | protect against. Therefore, any insertions will be delayed until | | after ``->gp_lock`` is released on line 25, which in turn means that | -| ``rcu_access_pointer()`` suffices. | +| rcu_access_pointer() suffices. | +-----------------------------------------------------------------------+ -However, all that ``remove_gp_cb()`` is doing is invoking ``kfree()`` on +However, all that remove_gp_cb() is doing is invoking kfree() on the data element. This is a common idiom, and is supported by -``kfree_rcu()``, which allows “fire and forget” operation as shown +kfree_rcu(), which allows “fire and forget” operation as shown below: :: @@ -1338,20 +1339,20 @@ below: 20 return true; 21 } -Note that ``remove_gp_faf()`` simply invokes ``kfree_rcu()`` and +Note that remove_gp_faf() simply invokes kfree_rcu() and proceeds, without any need to pay any further attention to the -subsequent grace period and ``kfree()``. It is permissible to invoke -``kfree_rcu()`` from the same environments as for ``call_rcu()``. -Interestingly enough, DYNIX/ptx had the equivalents of ``call_rcu()`` -and ``kfree_rcu()``, but not ``synchronize_rcu()``. This was due to the +subsequent grace period and kfree(). It is permissible to invoke +kfree_rcu() from the same environments as for call_rcu(). +Interestingly enough, DYNIX/ptx had the equivalents of call_rcu() +and kfree_rcu(), but not synchronize_rcu(). This was due to the fact that RCU was not heavily used within DYNIX/ptx, so the very few -places that needed something like ``synchronize_rcu()`` simply +places that needed something like synchronize_rcu() simply open-coded it. +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ -| Earlier it was claimed that ``call_rcu()`` and ``kfree_rcu()`` | +| Earlier it was claimed that call_rcu() and kfree_rcu() | | allowed updaters to avoid being blocked by readers. But how can that | | be correct, given that the invocation of the callback and the freeing | | of the memory (respectively) must still wait for a grace period to | @@ -1363,16 +1364,16 @@ open-coded it. | definition would say that updates in garbage-collected languages | | cannot complete until the next time the garbage collector runs, which | | does not seem at all reasonable. The key point is that in most cases, | -| an updater using either ``call_rcu()`` or ``kfree_rcu()`` can proceed | -| to the next update as soon as it has invoked ``call_rcu()`` or | -| ``kfree_rcu()``, without having to wait for a subsequent grace | +| an updater using either call_rcu() or kfree_rcu() can proceed | +| to the next update as soon as it has invoked call_rcu() or | +| kfree_rcu(), without having to wait for a subsequent grace | | period. | +-----------------------------------------------------------------------+ But what if the updater must wait for the completion of code to be executed after the end of the grace period, but has other tasks that can be carried out in the meantime? The polling-style -``get_state_synchronize_rcu()`` and ``cond_synchronize_rcu()`` functions +get_state_synchronize_rcu() and cond_synchronize_rcu() functions may be used for this purpose, as shown below: :: @@ -1397,11 +1398,11 @@ may be used for this purpose, as shown below: 18 return true; 19 } -On line 14, ``get_state_synchronize_rcu()`` obtains a “cookie” from RCU, +On line 14, get_state_synchronize_rcu() obtains a “cookie” from RCU, then line 15 carries out other tasks, and finally, line 16 returns immediately if a grace period has elapsed in the meantime, but otherwise waits as required. The need for ``get_state_synchronize_rcu`` and -``cond_synchronize_rcu()`` has appeared quite recently, so it is too +cond_synchronize_rcu() has appeared quite recently, so it is too early to tell whether they will stand the test of time. RCU thus provides a range of tools to allow updaters to strike the @@ -1421,8 +1422,8 @@ example, an infinite loop in an RCU read-side critical section must by definition prevent later grace periods from ever completing. For a more involved example, consider a 64-CPU system built with ``CONFIG_RCU_NOCB_CPU=y`` and booted with ``rcu_nocbs=1-63``, where -CPUs 1 through 63 spin in tight loops that invoke ``call_rcu()``. Even -if these tight loops also contain calls to ``cond_resched()`` (thus +CPUs 1 through 63 spin in tight loops that invoke call_rcu(). Even +if these tight loops also contain calls to cond_resched() (thus allowing grace periods to complete), CPU 0 simply will not be able to invoke callbacks as fast as the other 63 CPUs can register them, at least not until the system runs out of memory. In both of these @@ -1435,21 +1436,21 @@ RCU takes the following steps to encourage timely completion of grace periods: #. If a grace period fails to complete within 100 milliseconds, RCU - causes future invocations of ``cond_resched()`` on the holdout CPUs + causes future invocations of cond_resched() on the holdout CPUs to provide an RCU quiescent state. RCU also causes those CPUs' - ``need_resched()`` invocations to return ``true``, but only after the + need_resched() invocations to return ``true``, but only after the corresponding CPU's next scheduling-clock. #. CPUs mentioned in the ``nohz_full`` kernel boot parameter can run indefinitely in the kernel without scheduling-clock interrupts, which - defeats the above ``need_resched()`` strategem. RCU will therefore - invoke ``resched_cpu()`` on any ``nohz_full`` CPUs still holding out + defeats the above need_resched() strategem. RCU will therefore + invoke resched_cpu() on any ``nohz_full`` CPUs still holding out after 109 milliseconds. #. In kernels built with ``CONFIG_RCU_BOOST=y``, if a given task that has been preempted within an RCU read-side critical section is holding out for more than 500 milliseconds, RCU will resort to priority boosting. #. If a CPU is still holding out 10 seconds into the grace period, RCU - will invoke ``resched_cpu()`` on it regardless of its ``nohz_full`` + will invoke resched_cpu() on it regardless of its ``nohz_full`` state. The above values are defaults for systems running with ``HZ=1000``. They @@ -1457,10 +1458,10 @@ will vary as the value of ``HZ`` varies, and can also be changed using the relevant Kconfig options and kernel boot parameters. RCU currently does not do much sanity checking of these parameters, so please use caution when changing them. Note that these forward-progress measures -are provided only for RCU, not for `SRCU <#Sleepable%20RCU>`__ or `Tasks -RCU <#Tasks%20RCU>`__. +are provided only for RCU, not for `SRCU <Sleepable RCU_>`__ or `Tasks +RCU`_. -RCU takes the following steps in ``call_rcu()`` to encourage timely +RCU takes the following steps in call_rcu() to encourage timely invocation of callbacks when any given non-\ ``rcu_nocbs`` CPU has 10,000 callbacks, or has 10,000 more callbacks than it had the last time encouragement was provided: @@ -1477,12 +1478,12 @@ encouragement was provided: Again, these are default values when running at ``HZ=1000``, and can be overridden. Again, these forward-progress measures are provided only for -RCU, not for `SRCU <#Sleepable%20RCU>`__ or `Tasks -RCU <#Tasks%20RCU>`__. Even for RCU, callback-invocation forward +RCU, not for `SRCU <Sleepable RCU_>`__ or `Tasks +RCU`_. Even for RCU, callback-invocation forward progress for ``rcu_nocbs`` CPUs is much less well-developed, in part because workloads benefiting from ``rcu_nocbs`` CPUs tend to invoke -``call_rcu()`` relatively infrequently. If workloads emerge that need -both ``rcu_nocbs`` CPUs and high ``call_rcu()`` invocation rates, then +call_rcu() relatively infrequently. If workloads emerge that need +both ``rcu_nocbs`` CPUs and high call_rcu() invocation rates, then additional forward-progress work will be required. Composability @@ -1496,11 +1497,11 @@ in fact may be nested arbitrarily deeply. In practice, as with all real-world implementations of composable constructs, there are limitations. -Implementations of RCU for which ``rcu_read_lock()`` and -``rcu_read_unlock()`` generate no code, such as Linux-kernel RCU when -``CONFIG_PREEMPT=n``, can be nested arbitrarily deeply. After all, there +Implementations of RCU for which rcu_read_lock() and +rcu_read_unlock() generate no code, such as Linux-kernel RCU when +``CONFIG_PREEMPTION=n``, can be nested arbitrarily deeply. After all, there is no overhead. Except that if all these instances of -``rcu_read_lock()`` and ``rcu_read_unlock()`` are visible to the +rcu_read_lock() and rcu_read_unlock() are visible to the compiler, compilation will eventually fail due to exhausting memory, mass storage, or user patience, whichever comes first. If the nesting is not visible to the compiler, as is the case with mutually recursive @@ -1558,11 +1559,11 @@ argue that such workloads should instead use something other than RCU, the fact remains that RCU must handle such workloads gracefully. This requirement is another factor driving batching of grace periods, but it is also the driving force behind the checks for large numbers of queued -RCU callbacks in the ``call_rcu()`` code path. Finally, high update +RCU callbacks in the call_rcu() code path. Finally, high update rates should not delay RCU read-side critical sections, although some small read-side delays can occur when using -``synchronize_rcu_expedited()``, courtesy of this function's use of -``smp_call_function_single()``. +synchronize_rcu_expedited(), courtesy of this function's use of +smp_call_function_single(). Although all three of these corner cases were understood in the early 1990s, a simple user-level test consisting of ``close(open(path))`` in a @@ -1583,48 +1584,48 @@ Software-Engineering Requirements Between Murphy's Law and “To err is human”, it is necessary to guard against mishaps and misuse: -#. It is all too easy to forget to use ``rcu_read_lock()`` everywhere +#. It is all too easy to forget to use rcu_read_lock() everywhere that it is needed, so kernels built with ``CONFIG_PROVE_RCU=y`` will - splat if ``rcu_dereference()`` is used outside of an RCU read-side + splat if rcu_dereference() is used outside of an RCU read-side critical section. Update-side code can use - ``rcu_dereference_protected()``, which takes a `lockdep + rcu_dereference_protected(), which takes a `lockdep expression <https://lwn.net/Articles/371986/>`__ to indicate what is providing the protection. If the indicated protection is not provided, a lockdep splat is emitted. Code shared between readers and updaters can use - ``rcu_dereference_check()``, which also takes a lockdep expression, - and emits a lockdep splat if neither ``rcu_read_lock()`` nor the + rcu_dereference_check(), which also takes a lockdep expression, + and emits a lockdep splat if neither rcu_read_lock() nor the indicated protection is in place. In addition, - ``rcu_dereference_raw()`` is used in those (hopefully rare) cases + rcu_dereference_raw() is used in those (hopefully rare) cases where the required protection cannot be easily described. Finally, - ``rcu_read_lock_held()`` is provided to allow a function to verify + rcu_read_lock_held() is provided to allow a function to verify that it has been invoked within an RCU read-side critical section. I was made aware of this set of requirements shortly after Thomas Gleixner audited a number of RCU uses. #. A given function might wish to check for RCU-related preconditions upon entry, before using any other RCU API. The - ``rcu_lockdep_assert()`` does this job, asserting the expression in + rcu_lockdep_assert() does this job, asserting the expression in kernels having lockdep enabled and doing nothing otherwise. -#. It is also easy to forget to use ``rcu_assign_pointer()`` and - ``rcu_dereference()``, perhaps (incorrectly) substituting a simple +#. It is also easy to forget to use rcu_assign_pointer() and + rcu_dereference(), perhaps (incorrectly) substituting a simple assignment. To catch this sort of error, a given RCU-protected pointer may be tagged with ``__rcu``, after which sparse will complain about simple-assignment accesses to that pointer. Arnd Bergmann made me aware of this requirement, and also supplied the needed `patch series <https://lwn.net/Articles/376011/>`__. #. Kernels built with ``CONFIG_DEBUG_OBJECTS_RCU_HEAD=y`` will splat if - a data element is passed to ``call_rcu()`` twice in a row, without a + a data element is passed to call_rcu() twice in a row, without a grace period in between. (This error is similar to a double free.) The corresponding ``rcu_head`` structures that are dynamically allocated are automatically tracked, but ``rcu_head`` structures allocated on the stack must be initialized with - ``init_rcu_head_on_stack()`` and cleaned up with - ``destroy_rcu_head_on_stack()``. Similarly, statically allocated + init_rcu_head_on_stack() and cleaned up with + destroy_rcu_head_on_stack(). Similarly, statically allocated non-stack ``rcu_head`` structures must be initialized with - ``init_rcu_head()`` and cleaned up with ``destroy_rcu_head()``. + init_rcu_head() and cleaned up with destroy_rcu_head(). Mathieu Desnoyers made me aware of this requirement, and also supplied the needed - `patch <https://lkml.kernel.org/g/20100319013024.GA28456@Krystal>`__. + `patch <https://lore.kernel.org/r/20100319013024.GA28456@Krystal>`__. #. An infinite loop in an RCU read-side critical section will eventually trigger an RCU CPU stall warning splat, with the duration of “eventually” being controlled by the ``RCU_CPU_STALL_TIMEOUT`` @@ -1638,9 +1639,9 @@ against mishaps and misuse: ``rcupdate.rcu_cpu_stall_suppress`` to suppress the splats. This kernel parameter may also be set via ``sysfs``. Furthermore, RCU CPU stall warnings are counter-productive during sysrq dumps and during - panics. RCU therefore supplies the ``rcu_sysrq_start()`` and - ``rcu_sysrq_end()`` API members to be called before and after long - sysrq dumps. RCU also supplies the ``rcu_panic()`` notifier that is + panics. RCU therefore supplies the rcu_sysrq_start() and + rcu_sysrq_end() API members to be called before and after long + sysrq dumps. RCU also supplies the rcu_panic() notifier that is automatically invoked at the beginning of a panic to suppress further RCU CPU stall warnings. @@ -1656,7 +1657,7 @@ against mishaps and misuse: synchronization mechanism, for example, reference counting. #. In kernels built with ``CONFIG_RCU_TRACE=y``, RCU-related information is provided via event tracing. -#. Open-coded use of ``rcu_assign_pointer()`` and ``rcu_dereference()`` +#. Open-coded use of rcu_assign_pointer() and rcu_dereference() to create typical linked data structures can be surprisingly error-prone. Therefore, RCU-protected `linked lists <https://lwn.net/Articles/609973/#RCU%20List%20APIs>`__ and, @@ -1665,12 +1666,11 @@ against mishaps and misuse: other special-purpose RCU-protected data structures are available in the Linux kernel and the userspace RCU library. #. Some linked structures are created at compile time, but still require - ``__rcu`` checking. The ``RCU_POINTER_INITIALIZER()`` macro serves + ``__rcu`` checking. The RCU_POINTER_INITIALIZER() macro serves this purpose. -#. It is not necessary to use ``rcu_assign_pointer()`` when creating +#. It is not necessary to use rcu_assign_pointer() when creating linked structures that are to be published via a single external - pointer. The ``RCU_INIT_POINTER()`` macro is provided for this task - and also for assigning ``NULL`` pointers at runtime. + pointer. The RCU_INIT_POINTER() macro is provided for this task. This not a hard-and-fast list: RCU's diagnostic capabilities will continue to be guided by the number and type of usage bugs found in @@ -1716,7 +1716,7 @@ requires almost all of them be hidden behind a ``CONFIG_RCU_EXPERT`` This all should be quite obvious, but the fact remains that Linus Torvalds recently had to -`remind <https://lkml.kernel.org/g/CA+55aFy4wcCwaL4okTs8wXhGZ5h-ibecy_Meg9C4MNQrUnwMcg@mail.gmail.com>`__ +`remind <https://lore.kernel.org/r/CA+55aFy4wcCwaL4okTs8wXhGZ5h-ibecy_Meg9C4MNQrUnwMcg@mail.gmail.com>`__ me of this requirement. Firmware Interface @@ -1743,17 +1743,17 @@ Early Boot ~~~~~~~~~~ The Linux kernel's boot sequence is an interesting process, and RCU is -used early, even before ``rcu_init()`` is invoked. In fact, a number of +used early, even before rcu_init() is invoked. In fact, a number of RCU's primitives can be used as soon as the initial task's ``task_struct`` is available and the boot CPU's per-CPU variables are -set up. The read-side primitives (``rcu_read_lock()``, -``rcu_read_unlock()``, ``rcu_dereference()``, and -``rcu_access_pointer()``) will operate normally very early on, as will -``rcu_assign_pointer()``. +set up. The read-side primitives (rcu_read_lock(), +rcu_read_unlock(), rcu_dereference(), and +rcu_access_pointer()) will operate normally very early on, as will +rcu_assign_pointer(). -Although ``call_rcu()`` may be invoked at any time during boot, +Although call_rcu() may be invoked at any time during boot, callbacks are not guaranteed to be invoked until after all of RCU's -kthreads have been spawned, which occurs at ``early_initcall()`` time. +kthreads have been spawned, which occurs at early_initcall() time. This delay in callback invocation is due to the fact that RCU does not invoke callbacks until it is fully initialized, and this full initialization cannot occur until after the scheduler has initialized @@ -1762,22 +1762,22 @@ it would be possible to invoke callbacks earlier, however, this is not a panacea because there would be severe restrictions on what operations those callbacks could invoke. -Perhaps surprisingly, ``synchronize_rcu()`` and -``synchronize_rcu_expedited()``, will operate normally during very early +Perhaps surprisingly, synchronize_rcu() and +synchronize_rcu_expedited(), will operate normally during very early boot, the reason being that there is only one CPU and preemption is -disabled. This means that the call ``synchronize_rcu()`` (or friends) +disabled. This means that the call synchronize_rcu() (or friends) itself is a quiescent state and thus a grace period, so the early-boot implementation can be a no-op. However, once the scheduler has spawned its first kthread, this early -boot trick fails for ``synchronize_rcu()`` (as well as for -``synchronize_rcu_expedited()``) in ``CONFIG_PREEMPT=y`` kernels. The +boot trick fails for synchronize_rcu() (as well as for +synchronize_rcu_expedited()) in ``CONFIG_PREEMPTION=y`` kernels. The reason is that an RCU read-side critical section might be preempted, -which means that a subsequent ``synchronize_rcu()`` really does have to +which means that a subsequent synchronize_rcu() really does have to wait for something, as opposed to simply returning immediately. -Unfortunately, ``synchronize_rcu()`` can't do this until all of its +Unfortunately, synchronize_rcu() can't do this until all of its kthreads are spawned, which doesn't happen until some time during -``early_initcalls()`` time. But this is no excuse: RCU is nevertheless +early_initcalls() time. But this is no excuse: RCU is nevertheless required to correctly handle synchronous grace periods during this time period. Once all of its kthreads are up and running, RCU starts running normally. @@ -1820,7 +1820,7 @@ Interrupts and NMIs The Linux kernel has interrupts, and RCU read-side critical sections are legal within interrupt handlers and within interrupt-disabled regions of -code, as are invocations of ``call_rcu()``. +code, as are invocations of call_rcu(). Some Linux-kernel architectures can enter an interrupt handler from non-idle process context, and then just never leave it, instead @@ -1832,22 +1832,22 @@ way during a rewrite of RCU's dyntick-idle code. The Linux kernel has non-maskable interrupts (NMIs), and RCU read-side critical sections are legal within NMI handlers. Thankfully, RCU -update-side primitives, including ``call_rcu()``, are prohibited within +update-side primitives, including call_rcu(), are prohibited within NMI handlers. The name notwithstanding, some Linux-kernel architectures can have nested NMIs, which RCU must handle correctly. Andy Lutomirski `surprised -me <https://lkml.kernel.org/r/CALCETrXLq1y7e_dKFPgou-FKHB6Pu-r8+t-6Ds+8=va7anBWDA@mail.gmail.com>`__ +me <https://lore.kernel.org/r/CALCETrXLq1y7e_dKFPgou-FKHB6Pu-r8+t-6Ds+8=va7anBWDA@mail.gmail.com>`__ with this requirement; he also kindly surprised me with `an -algorithm <https://lkml.kernel.org/r/CALCETrXSY9JpW3uE6H8WYk81sg56qasA2aqmjMPsq5dOtzso=g@mail.gmail.com>`__ +algorithm <https://lore.kernel.org/r/CALCETrXSY9JpW3uE6H8WYk81sg56qasA2aqmjMPsq5dOtzso=g@mail.gmail.com>`__ that meets this requirement. Furthermore, NMI handlers can be interrupted by what appear to RCU to be normal interrupts. One way that this can happen is for code that -directly invokes ``rcu_irq_enter()`` and ``rcu_irq_exit()`` to be called +directly invokes ct_irq_enter() and ct_irq_exit() to be called from an NMI handler. This astonishing fact of life prompted the current -code structure, which has ``rcu_irq_enter()`` invoking -``rcu_nmi_enter()`` and ``rcu_irq_exit()`` invoking ``rcu_nmi_exit()``. +code structure, which has ct_irq_enter() invoking +ct_nmi_enter() and ct_irq_exit() invoking ct_nmi_exit(). And yes, I also learned of this requirement the hard way. Loadable Modules @@ -1857,45 +1857,45 @@ The Linux kernel has loadable modules, and these modules can also be unloaded. After a given module has been unloaded, any attempt to call one of its functions results in a segmentation fault. The module-unload functions must therefore cancel any delayed calls to loadable-module -functions, for example, any outstanding ``mod_timer()`` must be dealt -with via ``del_timer_sync()`` or similar. +functions, for example, any outstanding mod_timer() must be dealt +with via timer_shutdown_sync() or similar. Unfortunately, there is no way to cancel an RCU callback; once you -invoke ``call_rcu()``, the callback function is eventually going to be +invoke call_rcu(), the callback function is eventually going to be invoked, unless the system goes down first. Because it is normally considered socially irresponsible to crash the system in response to a module unload request, we need some other way to deal with in-flight RCU callbacks. -RCU therefore provides ``rcu_barrier()``, which waits until all +RCU therefore provides rcu_barrier(), which waits until all in-flight RCU callbacks have been invoked. If a module uses -``call_rcu()``, its exit function should therefore prevent any future -invocation of ``call_rcu()``, then invoke ``rcu_barrier()``. In theory, -the underlying module-unload code could invoke ``rcu_barrier()`` +call_rcu(), its exit function should therefore prevent any future +invocation of call_rcu(), then invoke rcu_barrier(). In theory, +the underlying module-unload code could invoke rcu_barrier() unconditionally, but in practice this would incur unacceptable latencies. Nikita Danilov noted this requirement for an analogous filesystem-unmount situation, and Dipankar Sarma incorporated -``rcu_barrier()`` into RCU. The need for ``rcu_barrier()`` for module +rcu_barrier() into RCU. The need for rcu_barrier() for module unloading became apparent later. .. important:: - The ``rcu_barrier()`` function is not, repeat, + The rcu_barrier() function is not, repeat, *not*, obligated to wait for a grace period. It is instead only required to wait for RCU callbacks that have already been posted. Therefore, if there are no RCU callbacks posted anywhere in the system, - ``rcu_barrier()`` is within its rights to return immediately. Even if - there are callbacks posted, ``rcu_barrier()`` does not necessarily need + rcu_barrier() is within its rights to return immediately. Even if + there are callbacks posted, rcu_barrier() does not necessarily need to wait for a grace period. +-----------------------------------------------------------------------+ | **Quick Quiz**: | +-----------------------------------------------------------------------+ | Wait a minute! Each RCU callbacks must wait for a grace period to | -| complete, and ``rcu_barrier()`` must wait for each pre-existing | -| callback to be invoked. Doesn't ``rcu_barrier()`` therefore need to | +| complete, and rcu_barrier() must wait for each pre-existing | +| callback to be invoked. Doesn't rcu_barrier() therefore need to | | wait for a full grace period if there is even one callback posted | | anywhere in the system? | +-----------------------------------------------------------------------+ @@ -1904,14 +1904,14 @@ unloading became apparent later. | Absolutely not!!! | | Yes, each RCU callbacks must wait for a grace period to complete, but | | it might well be partly (or even completely) finished waiting by the | -| time ``rcu_barrier()`` is invoked. In that case, ``rcu_barrier()`` | +| time rcu_barrier() is invoked. In that case, rcu_barrier() | | need only wait for the remaining portion of the grace period to | | elapse. So even if there are quite a few callbacks posted, | -| ``rcu_barrier()`` might well return quite quickly. | +| rcu_barrier() might well return quite quickly. | | | | So if you need to wait for a grace period as well as for all | | pre-existing callbacks, you will need to invoke both | -| ``synchronize_rcu()`` and ``rcu_barrier()``. If latency is a concern, | +| synchronize_rcu() and rcu_barrier(). If latency is a concern, | | you can always use workqueues to invoke them concurrently. | +-----------------------------------------------------------------------+ @@ -1920,7 +1920,7 @@ Hotplug CPU The Linux kernel supports CPU hotplug, which means that CPUs can come and go. It is of course illegal to use any RCU API member from an -offline CPU, with the exception of `SRCU <#Sleepable%20RCU>`__ read-side +offline CPU, with the exception of `SRCU <Sleepable RCU_>`__ read-side critical sections. This requirement was present from day one in DYNIX/ptx, but on the other hand, the Linux kernel's CPU-hotplug implementation is “interesting.” @@ -1929,76 +1929,77 @@ The Linux-kernel CPU-hotplug implementation has notifiers that are used to allow the various kernel subsystems (including RCU) to respond appropriately to a given CPU-hotplug operation. Most RCU operations may be invoked from CPU-hotplug notifiers, including even synchronous -grace-period operations such as ``synchronize_rcu()`` and -``synchronize_rcu_expedited()``. - -However, all-callback-wait operations such as ``rcu_barrier()`` are also -not supported, due to the fact that there are phases of CPU-hotplug -operations where the outgoing CPU's callbacks will not be invoked until -after the CPU-hotplug operation ends, which could also result in -deadlock. Furthermore, ``rcu_barrier()`` blocks CPU-hotplug operations -during its execution, which results in another type of deadlock when -invoked from a CPU-hotplug notifier. +grace-period operations such as (synchronize_rcu() and +synchronize_rcu_expedited()). However, these synchronous operations +do block and therefore cannot be invoked from notifiers that execute via +stop_machine(), specifically those between the ``CPUHP_AP_OFFLINE`` +and ``CPUHP_AP_ONLINE`` states. + +In addition, all-callback-wait operations such as rcu_barrier() may +not be invoked from any CPU-hotplug notifier. This restriction is due +to the fact that there are phases of CPU-hotplug operations where the +outgoing CPU's callbacks will not be invoked until after the CPU-hotplug +operation ends, which could also result in deadlock. Furthermore, +rcu_barrier() blocks CPU-hotplug operations during its execution, +which results in another type of deadlock when invoked from a CPU-hotplug +notifier. + +Finally, RCU must avoid deadlocks due to interaction between hotplug, +timers and grace period processing. It does so by maintaining its own set +of books that duplicate the centrally maintained ``cpu_online_mask``, +and also by reporting quiescent states explicitly when a CPU goes +offline. This explicit reporting of quiescent states avoids any need +for the force-quiescent-state loop (FQS) to report quiescent states for +offline CPUs. However, as a debugging measure, the FQS loop does splat +if offline CPUs block an RCU grace period for too long. + +An offline CPU's quiescent state will be reported either: + +1. As the CPU goes offline using RCU's hotplug notifier (rcutree_report_cpu_dead()). +2. When grace period initialization (rcu_gp_init()) detects a + race either with CPU offlining or with a task unblocking on a leaf + ``rcu_node`` structure whose CPUs are all offline. + +The CPU-online path (rcutree_report_cpu_starting()) should never need to report +a quiescent state for an offline CPU. However, as a debugging measure, +it does emit a warning if a quiescent state was not already reported +for that CPU. + +During the checking/modification of RCU's hotplug bookkeeping, the +corresponding CPU's leaf node lock is held. This avoids race conditions +between RCU's hotplug notifier hooks, the grace period initialization +code, and the FQS loop, all of which refer to or modify this bookkeeping. Scheduler and RCU ~~~~~~~~~~~~~~~~~ -RCU depends on the scheduler, and the scheduler uses RCU to protect some -of its data structures. The preemptible-RCU ``rcu_read_unlock()`` -implementation must therefore be written carefully to avoid deadlocks -involving the scheduler's runqueue and priority-inheritance locks. In -particular, ``rcu_read_unlock()`` must tolerate an interrupt where the -interrupt handler invokes both ``rcu_read_lock()`` and -``rcu_read_unlock()``. This possibility requires ``rcu_read_unlock()`` -to use negative nesting levels to avoid destructive recursion via -interrupt handler's use of RCU. - -This scheduler-RCU requirement came as a `complete -surprise <https://lwn.net/Articles/453002/>`__. - -As noted above, RCU makes use of kthreads, and it is necessary to avoid -excessive CPU-time accumulation by these kthreads. This requirement was -no surprise, but RCU's violation of it when running context-switch-heavy -workloads when built with ``CONFIG_NO_HZ_FULL=y`` `did come as a -surprise +RCU makes use of kthreads, and it is necessary to avoid excessive CPU-time +accumulation by these kthreads. This requirement was no surprise, but +RCU's violation of it when running context-switch-heavy workloads when +built with ``CONFIG_NO_HZ_FULL=y`` `did come as a surprise [PDF] <http://www.rdrop.com/users/paulmck/scalability/paper/BareMetal.2015.01.15b.pdf>`__. RCU has made good progress towards meeting this requirement, even for context-switch-heavy ``CONFIG_NO_HZ_FULL=y`` workloads, but there is room for further improvement. -It is forbidden to hold any of scheduler's runqueue or -priority-inheritance spinlocks across an ``rcu_read_unlock()`` unless -interrupts have been disabled across the entire RCU read-side critical -section, that is, up to and including the matching ``rcu_read_lock()``. -Violating this restriction can result in deadlocks involving these -scheduler spinlocks. There was hope that this restriction might be -lifted when interrupt-disabled calls to ``rcu_read_unlock()`` started -deferring the reporting of the resulting RCU-preempt quiescent state -until the end of the corresponding interrupts-disabled region. -Unfortunately, timely reporting of the corresponding quiescent state to -expedited grace periods requires a call to ``raise_softirq()``, which -can acquire these scheduler spinlocks. In addition, real-time systems -using RCU priority boosting need this restriction to remain in effect -because deferred quiescent-state reporting would also defer deboosting, -which in turn would degrade real-time latencies. - -In theory, if a given RCU read-side critical section could be guaranteed -to be less than one second in duration, holding a scheduler spinlock -across that critical section's ``rcu_read_unlock()`` would require only -that preemption be disabled across the entire RCU read-side critical -section, not interrupts. Unfortunately, given the possibility of vCPU -preemption, long-running interrupts, and so on, it is not possible in -practice to guarantee that a given RCU read-side critical section will -complete in less than one second. Therefore, as noted above, if -scheduler spinlocks are held across a given call to -``rcu_read_unlock()``, interrupts must be disabled across the entire RCU -read-side critical section. +There is no longer any prohibition against holding any of +scheduler's runqueue or priority-inheritance spinlocks across an +rcu_read_unlock(), even if interrupts and preemption were enabled +somewhere within the corresponding RCU read-side critical section. +Therefore, it is now perfectly legal to execute rcu_read_lock() +with preemption enabled, acquire one of the scheduler locks, and hold +that lock across the matching rcu_read_unlock(). + +Similarly, the RCU flavor consolidation has removed the need for negative +nesting. The fact that interrupt-disabled regions of code act as RCU +read-side critical sections implicitly avoids earlier issues that used +to result in destructive recursion via interrupt handler's use of RCU. Tracing and RCU ~~~~~~~~~~~~~~~ It is possible to use tracing on RCU code, but tracing itself uses RCU. -For this reason, ``rcu_dereference_raw_check()`` is provided for use +For this reason, rcu_dereference_raw_check() is provided for use by tracing, which avoids the destructive recursion that could otherwise ensue. This API is also used by virtualization in some architectures, where RCU readers execute in environments in which tracing cannot be @@ -2009,12 +2010,12 @@ Accesses to User Memory and RCU ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The kernel needs to access user-space memory, for example, to access data -referenced by system-call parameters. The ``get_user()`` macro does this job. +referenced by system-call parameters. The get_user() macro does this job. However, user-space memory might well be paged out, which means that -``get_user()`` might well page-fault and thus block while waiting for the +get_user() might well page-fault and thus block while waiting for the resulting I/O to complete. It would be a very bad thing for the compiler to -reorder a ``get_user()`` invocation into an RCU read-side critical section. +reorder a get_user() invocation into an RCU read-side critical section. For example, suppose that the source code looked like this: @@ -2039,23 +2040,23 @@ the following: 5 rcu_read_unlock(); 6 do_something_with(v, user_v); -If the compiler did make this transformation in a ``CONFIG_PREEMPT=n`` kernel -build, and if ``get_user()`` did page fault, the result would be a quiescent +If the compiler did make this transformation in a ``CONFIG_PREEMPTION=n`` kernel +build, and if get_user() did page fault, the result would be a quiescent state in the middle of an RCU read-side critical section. This misplaced quiescent state could result in line 4 being a use-after-free access, which could be bad for your kernel's actuarial statistics. Similar examples -can be constructed with the call to ``get_user()`` preceding the -``rcu_read_lock()``. +can be constructed with the call to get_user() preceding the +rcu_read_lock(). -Unfortunately, ``get_user()`` doesn't have any particular ordering properties, +Unfortunately, get_user() doesn't have any particular ordering properties, and in some architectures the underlying ``asm`` isn't even marked ``volatile``. And even if it was marked ``volatile``, the above access to ``p->value`` is not volatile, so the compiler would not have any reason to keep those two accesses in order. -Therefore, the Linux-kernel definitions of ``rcu_read_lock()`` and -``rcu_read_unlock()`` must act as compiler barriers, at least for outermost -instances of ``rcu_read_lock()`` and ``rcu_read_unlock()`` within a nested set +Therefore, the Linux-kernel definitions of rcu_read_lock() and +rcu_read_unlock() must act as compiler barriers, at least for outermost +instances of rcu_read_lock() and rcu_read_unlock() within a nested set of RCU read-side critical sections. Energy Efficiency @@ -2070,41 +2071,7 @@ call. Because RCU avoids interrupting idle CPUs, it is illegal to execute an RCU read-side critical section on an idle CPU. (Kernels built with -``CONFIG_PROVE_RCU=y`` will splat if you try it.) The ``RCU_NONIDLE()`` -macro and ``_rcuidle`` event tracing is provided to work around this -restriction. In addition, ``rcu_is_watching()`` may be used to test -whether or not it is currently legal to run RCU read-side critical -sections on this CPU. I learned of the need for diagnostics on the one -hand and ``RCU_NONIDLE()`` on the other while inspecting idle-loop code. -Steven Rostedt supplied ``_rcuidle`` event tracing, which is used quite -heavily in the idle loop. However, there are some restrictions on the -code placed within ``RCU_NONIDLE()``: - -#. Blocking is prohibited. In practice, this is not a serious - restriction given that idle tasks are prohibited from blocking to - begin with. -#. Although nesting ``RCU_NONIDLE()`` is permitted, they cannot nest - indefinitely deeply. However, given that they can be nested on the - order of a million deep, even on 32-bit systems, this should not be a - serious restriction. This nesting limit would probably be reached - long after the compiler OOMed or the stack overflowed. -#. Any code path that enters ``RCU_NONIDLE()`` must sequence out of that - same ``RCU_NONIDLE()``. For example, the following is grossly - illegal: - - :: - - 1 RCU_NONIDLE({ - 2 do_something(); - 3 goto bad_idea; /* BUG!!! */ - 4 do_something_else();}); - 5 bad_idea: - - - It is just as illegal to transfer control into the middle of - ``RCU_NONIDLE()``'s argument. Yes, in theory, you could transfer in - as long as you also transferred out, but in practice you could also - expect to get sharply worded review comments. +``CONFIG_PROVE_RCU=y`` will splat if you try it.) It is similarly socially unacceptable to interrupt an ``nohz_full`` CPU running in userspace. RCU must therefore track ``nohz_full`` userspace @@ -2176,7 +2143,7 @@ handles these states differently: However, RCU must be reliably informed as to whether any given CPU is currently in the idle loop, and, for ``NO_HZ_FULL``, also whether that CPU is executing in usermode, as discussed -`earlier <#Energy%20Efficiency>`__. It also requires that the +`earlier <Energy Efficiency_>`__. It also requires that the scheduling-clock interrupt be enabled when RCU needs it to be: #. If a CPU is either idle or executing in usermode, and RCU believes it @@ -2191,12 +2158,12 @@ scheduling-clock interrupt be enabled when RCU needs it to be: this sort of thing. #. If a CPU is in a portion of the kernel that is absolutely positively no-joking guaranteed to never execute any RCU read-side critical - sections, and RCU believes this CPU to to be idle, no problem. This + sections, and RCU believes this CPU to be idle, no problem. This sort of thing is used by some architectures for light-weight exception handlers, which can then avoid the overhead of - ``rcu_irq_enter()`` and ``rcu_irq_exit()`` at exception entry and + ct_irq_enter() and ct_irq_exit() at exception entry and exit, respectively. Some go further and avoid the entireties of - ``irq_enter()`` and ``irq_exit()``. + irq_enter() and irq_exit(). Just make very sure you are running some of your tests with ``CONFIG_PROVE_RCU=y``, just in case one of your code paths was in fact joking about not doing RCU read-side critical sections. @@ -2220,12 +2187,12 @@ scheduling-clock interrupt be enabled when RCU needs it to be: | **Quick Quiz**: | +-----------------------------------------------------------------------+ | But what if my driver has a hardware interrupt handler that can run | -| for many seconds? I cannot invoke ``schedule()`` from an hardware | +| for many seconds? I cannot invoke schedule() from an hardware | | interrupt handler, after all! | +-----------------------------------------------------------------------+ | **Answer**: | +-----------------------------------------------------------------------+ -| One approach is to do ``rcu_irq_exit();rcu_irq_enter();`` every so | +| One approach is to do ``ct_irq_exit();ct_irq_enter();`` every so | | often. But given that long-running interrupt handlers can cause other | | problems, not least for response time, shouldn't you work to keep | | your interrupt handler's runtime within reasonable bounds? | @@ -2242,8 +2209,8 @@ Memory Efficiency Although small-memory non-realtime systems can simply use Tiny RCU, code size is only one aspect of memory efficiency. Another aspect is the size -of the ``rcu_head`` structure used by ``call_rcu()`` and -``kfree_rcu()``. Although this structure contains nothing more than a +of the ``rcu_head`` structure used by call_rcu() and +kfree_rcu(). Although this structure contains nothing more than a pair of pointers, it does appear in many RCU-protected data structures, including some that are size critical. The ``page`` structure is a case in point, as evidenced by the many occurrences of the ``union`` keyword @@ -2253,7 +2220,7 @@ This need for memory efficiency is one reason that RCU uses hand-crafted singly linked lists to track the ``rcu_head`` structures that are waiting for a grace period to elapse. It is also the reason why ``rcu_head`` structures do not contain debug information, such as fields -tracking the file and line of the ``call_rcu()`` or ``kfree_rcu()`` that +tracking the file and line of the call_rcu() or kfree_rcu() that posted them. Although this information might appear in debug-only kernel builds at some point, in the meantime, the ``->func`` field will often provide the needed debug information. @@ -2263,18 +2230,18 @@ more extreme measures. Returning to the ``page`` structure, the ``rcu_head`` field shares storage with a great many other structures that are used at various points in the corresponding page's lifetime. In order to correctly resolve certain `race -conditions <https://lkml.kernel.org/g/1439976106-137226-1-git-send-email-kirill.shutemov@linux.intel.com>`__, +conditions <https://lore.kernel.org/r/1439976106-137226-1-git-send-email-kirill.shutemov@linux.intel.com>`__, the Linux kernel's memory-management subsystem needs a particular bit to remain zero during all phases of grace-period processing, and that bit happens to map to the bottom bit of the ``rcu_head`` structure's -``->next`` field. RCU makes this guarantee as long as ``call_rcu()`` is -used to post the callback, as opposed to ``kfree_rcu()`` or some future -“lazy” variant of ``call_rcu()`` that might one day be created for +``->next`` field. RCU makes this guarantee as long as call_rcu() is +used to post the callback, as opposed to kfree_rcu() or some future +“lazy” variant of call_rcu() that might one day be created for energy-efficiency purposes. That said, there are limits. RCU requires that the ``rcu_head`` structure be aligned to a two-byte boundary, and passing a misaligned -``rcu_head`` structure to one of the ``call_rcu()`` family of functions +``rcu_head`` structure to one of the call_rcu() family of functions will result in a splat. It is therefore necessary to exercise caution when packing structures containing fields of type ``rcu_head``. Why not a four-byte or even eight-byte alignment requirement? Because the m68k @@ -2293,12 +2260,12 @@ Performance, Scalability, Response Time, and Reliability ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Expanding on the `earlier -discussion <#Performance%20and%20Scalability>`__, RCU is used heavily by +discussion <Performance and Scalability_>`__, RCU is used heavily by hot code paths in performance-critical portions of the Linux kernel's networking, security, virtualization, and scheduling code paths. RCU must therefore use efficient implementations, especially in its read-side primitives. To that end, it would be good if preemptible RCU's -implementation of ``rcu_read_lock()`` could be inlined, however, doing +implementation of rcu_read_lock() could be inlined, however, doing this requires resolving ``#include`` issues with the ``task_struct`` structure. @@ -2311,23 +2278,23 @@ on the ``rcu_node`` structure. RCU is required to tolerate all CPUs continuously invoking any combination of RCU's runtime primitives with minimal per-operation overhead. In fact, in many cases, increasing load must *decrease* the per-operation overhead, witness the batching -optimizations for ``synchronize_rcu()``, ``call_rcu()``, -``synchronize_rcu_expedited()``, and ``rcu_barrier()``. As a general +optimizations for synchronize_rcu(), call_rcu(), +synchronize_rcu_expedited(), and rcu_barrier(). As a general rule, RCU must cheerfully accept whatever the rest of the Linux kernel decides to throw at it. The Linux kernel is used for real-time workloads, especially in conjunction with the `-rt -patchset <https://rt.wiki.kernel.org/index.php/Main_Page>`__. The +patchset <https://wiki.linuxfoundation.org/realtime/>`__. The real-time-latency response requirements are such that the traditional approach of disabling preemption across RCU read-side critical sections -is inappropriate. Kernels built with ``CONFIG_PREEMPT=y`` therefore use +is inappropriate. Kernels built with ``CONFIG_PREEMPTION=y`` therefore use an RCU implementation that allows RCU read-side critical sections to be preempted. This requirement made its presence known after users made it clear that an earlier `real-time patch <https://lwn.net/Articles/107930/>`__ did not meet their needs, in conjunction with some `RCU -issues <https://lkml.kernel.org/g/20050318002026.GA2693@us.ibm.com>`__ +issues <https://lore.kernel.org/r/20050318002026.GA2693@us.ibm.com>`__ encountered by a very early version of the -rt patchset. In addition, RCU must make do with a sub-100-microsecond real-time @@ -2345,7 +2312,7 @@ number of race conditions. RCU must avoid degrading real-time response for CPU-bound threads, whether executing in usermode (which is one use case for ``CONFIG_NO_HZ_FULL=y``) or in the kernel. That said, CPU-bound loops in -the kernel must execute ``cond_resched()`` at least once per few tens of +the kernel must execute cond_resched() at least once per few tens of milliseconds in order to avoid receiving an IPI from RCU. Finally, RCU's status as a synchronization primitive means that any RCU @@ -2411,7 +2378,7 @@ grace periods from ever ending. The result was an out-of-memory condition and a system hang. The solution was the creation of RCU-bh, which does -``local_bh_disable()`` across its read-side critical sections, and which +local_bh_disable() across its read-side critical sections, and which uses the transition from one type of softirq processing to another as a quiescent state in addition to context switch, idle, user mode, and offline. This means that RCU-bh grace periods can complete even when @@ -2419,31 +2386,31 @@ some of the CPUs execute in softirq indefinitely, thus allowing algorithms based on RCU-bh to withstand network-based denial-of-service attacks. -Because ``rcu_read_lock_bh()`` and ``rcu_read_unlock_bh()`` disable and +Because rcu_read_lock_bh() and rcu_read_unlock_bh() disable and re-enable softirq handlers, any attempt to start a softirq handlers during the RCU-bh read-side critical section will be deferred. In this -case, ``rcu_read_unlock_bh()`` will invoke softirq processing, which can +case, rcu_read_unlock_bh() will invoke softirq processing, which can take considerable time. One can of course argue that this softirq overhead should be associated with the code following the RCU-bh -read-side critical section rather than ``rcu_read_unlock_bh()``, but the +read-side critical section rather than rcu_read_unlock_bh(), but the fact is that most profiling tools cannot be expected to make this sort of fine distinction. For example, suppose that a three-millisecond-long RCU-bh read-side critical section executes during a time of heavy networking load. There will very likely be an attempt to invoke at least one softirq handler during that three milliseconds, but any such invocation will be delayed until the time of the -``rcu_read_unlock_bh()``. This can of course make it appear at first -glance as if ``rcu_read_unlock_bh()`` was executing very slowly. +rcu_read_unlock_bh(). This can of course make it appear at first +glance as if rcu_read_unlock_bh() was executing very slowly. The `RCU-bh API <https://lwn.net/Articles/609973/#RCU%20Per-Flavor%20API%20Table>`__ -includes ``rcu_read_lock_bh()``, ``rcu_read_unlock_bh()``, -``rcu_dereference_bh()``, ``rcu_dereference_bh_check()``, -``synchronize_rcu_bh()``, ``synchronize_rcu_bh_expedited()``, -``call_rcu_bh()``, ``rcu_barrier_bh()``, and -``rcu_read_lock_bh_held()``. However, the update-side APIs are now -simple wrappers for other RCU flavors, namely RCU-sched in -CONFIG_PREEMPT=n kernels and RCU-preempt otherwise. +includes rcu_read_lock_bh(), rcu_read_unlock_bh(), rcu_dereference_bh(), +rcu_dereference_bh_check(), and rcu_read_lock_bh_held(). However, the +old RCU-bh update-side APIs are now gone, replaced by synchronize_rcu(), +synchronize_rcu_expedited(), call_rcu(), and rcu_barrier(). In addition, +anything that disables bottom halves also marks an RCU-bh read-side +critical section, including local_bh_disable() and local_bh_enable(), +local_irq_save() and local_irq_restore(), and so on. Sched Flavor (Historical) ~~~~~~~~~~~~~~~~~~~~~~~~~ @@ -2460,33 +2427,33 @@ However, there are legitimate preemptible-RCU implementations that do not have this property, given that any point in the code outside of an RCU read-side critical section can be a quiescent state. Therefore, *RCU-sched* was created, which follows “classic” RCU in that an -RCU-sched grace period waits for for pre-existing interrupt and NMI -handlers. In kernels built with ``CONFIG_PREEMPT=n``, the RCU and +RCU-sched grace period waits for pre-existing interrupt and NMI +handlers. In kernels built with ``CONFIG_PREEMPTION=n``, the RCU and RCU-sched APIs have identical implementations, while kernels built with -``CONFIG_PREEMPT=y`` provide a separate implementation for each. +``CONFIG_PREEMPTION=y`` provide a separate implementation for each. -Note well that in ``CONFIG_PREEMPT=y`` kernels, -``rcu_read_lock_sched()`` and ``rcu_read_unlock_sched()`` disable and +Note well that in ``CONFIG_PREEMPTION=y`` kernels, +rcu_read_lock_sched() and rcu_read_unlock_sched() disable and re-enable preemption, respectively. This means that if there was a preemption attempt during the RCU-sched read-side critical section, -``rcu_read_unlock_sched()`` will enter the scheduler, with all the -latency and overhead entailed. Just as with ``rcu_read_unlock_bh()``, -this can make it look as if ``rcu_read_unlock_sched()`` was executing +rcu_read_unlock_sched() will enter the scheduler, with all the +latency and overhead entailed. Just as with rcu_read_unlock_bh(), +this can make it look as if rcu_read_unlock_sched() was executing very slowly. However, the highest-priority task won't be preempted, so -that task will enjoy low-overhead ``rcu_read_unlock_sched()`` +that task will enjoy low-overhead rcu_read_unlock_sched() invocations. The `RCU-sched API <https://lwn.net/Articles/609973/#RCU%20Per-Flavor%20API%20Table>`__ -includes ``rcu_read_lock_sched()``, ``rcu_read_unlock_sched()``, -``rcu_read_lock_sched_notrace()``, ``rcu_read_unlock_sched_notrace()``, -``rcu_dereference_sched()``, ``rcu_dereference_sched_check()``, -``synchronize_sched()``, ``synchronize_rcu_sched_expedited()``, -``call_rcu_sched()``, ``rcu_barrier_sched()``, and -``rcu_read_lock_sched_held()``. However, anything that disables -preemption also marks an RCU-sched read-side critical section, including -``preempt_disable()`` and ``preempt_enable()``, ``local_irq_save()`` and -``local_irq_restore()``, and so on. +includes rcu_read_lock_sched(), rcu_read_unlock_sched(), +rcu_read_lock_sched_notrace(), rcu_read_unlock_sched_notrace(), +rcu_dereference_sched(), rcu_dereference_sched_check(), and +rcu_read_lock_sched_held(). However, the old RCU-sched update-side APIs +are now gone, replaced by synchronize_rcu(), synchronize_rcu_expedited(), +call_rcu(), and rcu_barrier(). In addition, anything that disables +preemption also marks an RCU-sched read-side critical section, +including preempt_disable() and preempt_enable(), local_irq_save() +and local_irq_restore(), and so on. Sleepable RCU ~~~~~~~~~~~~~ @@ -2508,7 +2475,7 @@ this structure must be passed in to each SRCU function, for example, structure. The key benefit of these domains is that a slow SRCU reader in one domain does not delay an SRCU grace period in some other domain. That said, one consequence of these domains is that read-side code must -pass a “cookie” from ``srcu_read_lock()`` to ``srcu_read_unlock()``, for +pass a “cookie” from srcu_read_lock() to srcu_read_unlock(), for example, as follows: :: @@ -2538,24 +2505,24 @@ period to elapse. For example, this results in a self-deadlock: 6 srcu_read_unlock(&ss, idx); However, if line 5 acquired a mutex that was held across a -``synchronize_srcu()`` for domain ``ss``, deadlock would still be +synchronize_srcu() for domain ``ss``, deadlock would still be possible. Furthermore, if line 5 acquired a mutex that was held across a -``synchronize_srcu()`` for some other domain ``ss1``, and if an +synchronize_srcu() for some other domain ``ss1``, and if an ``ss1``-domain SRCU read-side critical section acquired another mutex -that was held across as ``ss``-domain ``synchronize_srcu()``, deadlock +that was held across as ``ss``-domain synchronize_srcu(), deadlock would again be possible. Such a deadlock cycle could extend across an arbitrarily large number of different SRCU domains. Again, with great power comes great responsibility. Unlike the other RCU flavors, SRCU read-side critical sections can run on idle and even offline CPUs. This ability requires that -``srcu_read_lock()`` and ``srcu_read_unlock()`` contain memory barriers, +srcu_read_lock() and srcu_read_unlock() contain memory barriers, which means that SRCU readers will run a bit slower than would RCU -readers. It also motivates the ``smp_mb__after_srcu_read_unlock()`` API, -which, in combination with ``srcu_read_unlock()``, guarantees a full +readers. It also motivates the smp_mb__after_srcu_read_unlock() API, +which, in combination with srcu_read_unlock(), guarantees a full memory barrier. -Also unlike other RCU flavors, ``synchronize_srcu()`` may **not** be +Also unlike other RCU flavors, synchronize_srcu() may **not** be invoked from CPU-hotplug notifiers, due to the fact that SRCU grace periods make use of timers and the possibility of timers being temporarily “stranded” on the outgoing CPU. This stranding of timers @@ -2564,7 +2531,7 @@ the CPU-hotplug process. The problem is that if a notifier is waiting on an SRCU grace period, that grace period is waiting on a timer, and that timer is stranded on the outgoing CPU, then the notifier will never be awakened, in other words, deadlock has occurred. This same situation of -course also prohibits ``srcu_barrier()`` from being invoked from +course also prohibits srcu_barrier() from being invoked from CPU-hotplug notifiers. SRCU also differs from other RCU flavors in that SRCU's expedited and @@ -2575,12 +2542,12 @@ have not yet completed. (But please note that this is a property of the current implementation, not necessarily of future implementations.) In addition, if SRCU has been idle for longer than the interval specified by the ``srcutree.exp_holdoff`` kernel boot parameter (25 microseconds -by default), and if a ``synchronize_srcu()`` invocation ends this idle +by default), and if a synchronize_srcu() invocation ends this idle period, that invocation will be automatically expedited. As of v4.12, SRCU's callbacks are maintained per-CPU, eliminating a locking bottleneck present in prior kernel versions. Although this will -allow users to put much heavier stress on ``call_srcu()``, it is +allow users to put much heavier stress on call_srcu(), it is important to note that SRCU does not yet take any special steps to deal with callback flooding. So if you are posting (say) 10,000 SRCU callbacks per second per CPU, you are probably totally OK, but if you @@ -2591,14 +2558,32 @@ of your CPUs and the size of your memory. The `SRCU API <https://lwn.net/Articles/609973/#RCU%20Per-Flavor%20API%20Table>`__ -includes ``srcu_read_lock()``, ``srcu_read_unlock()``, -``srcu_dereference()``, ``srcu_dereference_check()``, -``synchronize_srcu()``, ``synchronize_srcu_expedited()``, -``call_srcu()``, ``srcu_barrier()``, and ``srcu_read_lock_held()``. It -also includes ``DEFINE_SRCU()``, ``DEFINE_STATIC_SRCU()``, and -``init_srcu_struct()`` APIs for defining and initializing +includes srcu_read_lock(), srcu_read_unlock(), +srcu_dereference(), srcu_dereference_check(), +synchronize_srcu(), synchronize_srcu_expedited(), +call_srcu(), srcu_barrier(), and srcu_read_lock_held(). It +also includes DEFINE_SRCU(), DEFINE_STATIC_SRCU(), and +init_srcu_struct() APIs for defining and initializing ``srcu_struct`` structures. +More recently, the SRCU API has added polling interfaces: + +#. start_poll_synchronize_srcu() returns a cookie identifying + the completion of a future SRCU grace period and ensures + that this grace period will be started. +#. poll_state_synchronize_srcu() returns ``true`` iff the + specified cookie corresponds to an already-completed + SRCU grace period. +#. get_state_synchronize_srcu() returns a cookie just like + start_poll_synchronize_srcu() does, but differs in that + it does nothing to ensure that any future SRCU grace period + will be started. + +These functions are used to avoid unnecessary SRCU grace periods in +certain types of buffer-cache algorithms having multi-stage age-out +mechanisms. The idea is that by the time the block has aged completely +from the cache, an SRCU grace period will be very likely to have elapsed. + Tasks RCU ~~~~~~~~~ @@ -2607,29 +2592,66 @@ required to install different types of probes. It would be good to be able to free old trampolines, which sounds like a job for some form of RCU. However, because it is necessary to be able to install a trace anywhere in the code, it is not possible to use read-side markers such -as ``rcu_read_lock()`` and ``rcu_read_unlock()``. In addition, it does +as rcu_read_lock() and rcu_read_unlock(). In addition, it does not work to have these markers in the trampoline itself, because there -would need to be instructions following ``rcu_read_unlock()``. Although -``synchronize_rcu()`` would guarantee that execution reached the -``rcu_read_unlock()``, it would not be able to guarantee that execution -had completely left the trampoline. +would need to be instructions following rcu_read_unlock(). Although +synchronize_rcu() would guarantee that execution reached the +rcu_read_unlock(), it would not be able to guarantee that execution +had completely left the trampoline. Worse yet, in some situations +the trampoline's protection must extend a few instructions *prior* to +execution reaching the trampoline. For example, these few instructions +might calculate the address of the trampoline, so that entering the +trampoline would be pre-ordained a surprisingly long time before execution +actually reached the trampoline itself. The solution, in the form of `Tasks RCU <https://lwn.net/Articles/607117/>`__, is to have implicit read-side critical sections that are delimited by voluntary context switches, that -is, calls to ``schedule()``, ``cond_resched()``, and -``synchronize_rcu_tasks()``. In addition, transitions to and from +is, calls to schedule(), cond_resched(), and +synchronize_rcu_tasks(). In addition, transitions to and from userspace execution also delimit tasks-RCU read-side critical sections. The tasks-RCU API is quite compact, consisting only of -``call_rcu_tasks()``, ``synchronize_rcu_tasks()``, and -``rcu_barrier_tasks()``. In ``CONFIG_PREEMPT=n`` kernels, trampolines -cannot be preempted, so these APIs map to ``call_rcu()``, -``synchronize_rcu()``, and ``rcu_barrier()``, respectively. In -``CONFIG_PREEMPT=y`` kernels, trampolines can be preempted, and these +call_rcu_tasks(), synchronize_rcu_tasks(), and +rcu_barrier_tasks(). In ``CONFIG_PREEMPTION=n`` kernels, trampolines +cannot be preempted, so these APIs map to call_rcu(), +synchronize_rcu(), and rcu_barrier(), respectively. In +``CONFIG_PREEMPTION=y`` kernels, trampolines can be preempted, and these three APIs are therefore implemented by separate functions that check for voluntary context switches. +Tasks Rude RCU +~~~~~~~~~~~~~~ + +Some forms of tracing need to wait for all preemption-disabled regions +of code running on any online CPU, including those executed when RCU is +not watching. This means that synchronize_rcu() is insufficient, and +Tasks Rude RCU must be used instead. This flavor of RCU does its work by +forcing a workqueue to be scheduled on each online CPU, hence the "Rude" +moniker. And this operation is considered to be quite rude by real-time +workloads that don't want their ``nohz_full`` CPUs receiving IPIs and +by battery-powered systems that don't want their idle CPUs to be awakened. + +The tasks-rude-RCU API is also reader-marking-free and thus quite compact, +consisting of call_rcu_tasks_rude(), synchronize_rcu_tasks_rude(), +and rcu_barrier_tasks_rude(). + +Tasks Trace RCU +~~~~~~~~~~~~~~~ + +Some forms of tracing need to sleep in readers, but cannot tolerate +SRCU's read-side overhead, which includes a full memory barrier in both +srcu_read_lock() and srcu_read_unlock(). This need is handled by a +Tasks Trace RCU that uses scheduler locking and IPIs to synchronize with +readers. Real-time systems that cannot tolerate IPIs may build their +kernels with ``CONFIG_TASKS_TRACE_RCU_READ_MB=y``, which avoids the IPIs at +the expense of adding full memory barriers to the read-side primitives. + +The tasks-trace-RCU API is also reasonably compact, +consisting of rcu_read_lock_trace(), rcu_read_unlock_trace(), +rcu_read_lock_trace_held(), call_rcu_tasks_trace(), +synchronize_rcu_tasks_trace(), and rcu_barrier_tasks_trace(). + Possible Future Changes ----------------------- @@ -2640,8 +2662,8 @@ grace-period state machine so as to avoid the need for the additional latency. RCU disables CPU hotplug in a few places, perhaps most notably in the -``rcu_barrier()`` operations. If there is a strong reason to use -``rcu_barrier()`` in CPU-hotplug notifiers, it will be necessary to +rcu_barrier() operations. If there is a strong reason to use +rcu_barrier() in CPU-hotplug notifiers, it will be necessary to avoid disabling CPU hotplug. This would introduce some complexity, so there had better be a *very* good reason. @@ -2658,7 +2680,7 @@ However, this combining tree does not spread its memory across NUMA nodes nor does it align the CPU groups with hardware features such as sockets or cores. Such spreading and alignment is currently believed to be unnecessary because the hotpath read-side primitives do not access -the combining tree, nor does ``call_rcu()`` in the common case. If you +the combining tree, nor does call_rcu() in the common case. If you believe that your architecture needs such spreading and alignment, then your architecture should also benefit from the ``rcutree.rcu_fanout_leaf`` boot parameter, which can be set to the @@ -2679,7 +2701,7 @@ likely that adjustments will be required to more gracefully handle extreme loads. It might also be necessary to be able to relate CPU utilization by RCU's kthreads and softirq handlers to the code that instigated this CPU utilization. For example, RCU callback overhead -might be charged back to the originating ``call_rcu()`` instance, though +might be charged back to the originating call_rcu() instance, though probably not in production kernels. Additional work may be required to provide reasonable forward-progress diff --git a/Documentation/RCU/NMI-RCU.rst b/Documentation/RCU/NMI-RCU.rst index 180958388ff9..dff60a80b386 100644 --- a/Documentation/RCU/NMI-RCU.rst +++ b/Documentation/RCU/NMI-RCU.rst @@ -8,8 +8,7 @@ Although RCU is usually used to protect read-mostly data structures, it is possible to use RCU to provide dynamic non-maskable interrupt handlers, as well as dynamic irq handlers. This document describes how to do this, drawing loosely from Zwane Mwaikambo's NMI-timer -work in "arch/x86/oprofile/nmi_timer_int.c" and in -"arch/x86/kernel/traps.c". +work in an old version of "arch/x86/kernel/traps.c". The relevant pieces of code are listed below, each followed by a brief explanation:: @@ -117,7 +116,7 @@ Answer to Quick Quiz: This same sad story can happen on other CPUs when using a compiler with aggressive pointer-value speculation - optimizations. + optimizations. (But please don't!) More important, the rcu_dereference_sched() makes it clear to someone reading the code that the pointer is diff --git a/Documentation/RCU/RTFP.txt b/Documentation/RCU/RTFP.txt index 9bccf16736f7..db8f16b392aa 100644 --- a/Documentation/RCU/RTFP.txt +++ b/Documentation/RCU/RTFP.txt @@ -201,7 +201,7 @@ work looked at debugging uses of RCU [Seyster:2011:RFA:2075416.2075425]. In 2012, Josh Triplett received his Ph.D. with his dissertation covering RCU-protected resizable hash tables and the relationship between memory barriers and read-side traversal order: If the updater -is making changes in the opposite direction from the read-side traveral +is making changes in the opposite direction from the read-side traversal order, the updater need only execute a memory-barrier instruction, but if in the same direction, the updater needs to wait for a grace period between the individual updates [JoshTriplettPhD]. Also in 2012, @@ -683,7 +683,7 @@ Orran Krieger and Rusty Russell and Dipankar Sarma and Maneesh Soni" ,month="October" ,year="2001" ,note="Available: -\url{http://lkml.org/lkml/2001/10/13/105} +\url{https://lore.kernel.org/r/Pine.LNX.4.33.0110131015410.8707-100000@penguin.transmeta.com} [Viewed August 21, 2004]" ,annotation={ } @@ -826,7 +826,7 @@ Symposium on Distributed Computing} ,month="October" ,year="2002" ,note="Available: -\url{https://lkml.org/lkml/2002/10/24/262} +\url{https://lore.kernel.org/r/3DB86B05.447E7410@us.ibm.com} [Viewed February 15, 2014]" ,annotation={ Mingming Cao's patch to introduce RCU to SysV IPC. @@ -839,7 +839,7 @@ Symposium on Distributed Computing} ,month="March" ,year="2003" ,note="Available: -\url{http://lkml.org/lkml/2003/3/9/205} +\url{https://lore.kernel.org/r/Pine.LNX.4.44.0303091831560.2129-100000@home.transmeta.com} [Viewed March 13, 2006]" ,annotation={ Linus suggests replacing brlock with RCU and/or seqlocks: @@ -847,7 +847,7 @@ Symposium on Distributed Computing} 'It's entirely possible that the current user could be replaced by RCU and/or seqlocks, and we could get rid of brlocks entirely.' . - Steve Hemminger responds by replacing them with RCU. + Stephen Hemminger responds by replacing them with RCU. } } @@ -1036,15 +1036,15 @@ Add per-cpu batch counter" ,annotation={ RCU runs reasonably on a 512-CPU SGI using Manfred Spraul's patches, which may be found at: - https://lkml.org/lkml/2004/5/20/49 (split vars into cachelines) - https://lkml.org/lkml/2004/5/22/114 (cpu_quiet() patch) - https://lkml.org/lkml/2004/5/25/24 (0/5) - https://lkml.org/lkml/2004/5/25/23 (1/5) - https://lkml.org/lkml/2004/5/25/265 (works for Jack) - https://lkml.org/lkml/2004/5/25/20 (2/5) - https://lkml.org/lkml/2004/5/25/22 (3/5) - https://lkml.org/lkml/2004/5/25/19 (4/5) - https://lkml.org/lkml/2004/5/25/21 (5/5) + https://lore.kernel.org/r/40AC9823.6020709@colorfullife.com (split vars into cachelines) + https://lore.kernel.org/r/Pine.LNX.4.44.0405222141260.11106-100000@dbl.q-ag.de (cpu_quiet() patch) + https://lore.kernel.org/r/200405250535.i4P5ZJo8017583@dbl.q-ag.de (0/5) + https://lore.kernel.org/r/200405250535.i4P5ZKAQ017591@dbl.q-ag.de (1/5) + https://lore.kernel.org/r/20040525203215.GB5127@sgi.com (works for Jack) + https://lore.kernel.org/r/200405250535.i4P5ZLiR017599@dbl.q-ag.de (2/5) + https://lore.kernel.org/r/200405250535.i4P5ZMFt017607@dbl.q-ag.de (3/5) + https://lore.kernel.org/r/200405250535.i4P5ZN6g017615@dbl.q-ag.de (4/5) + https://lore.kernel.org/r/200405250535.i4P5ZO7I017623@dbl.q-ag.de (5/5) } } @@ -1106,7 +1106,7 @@ Oregon Health and Sciences University" ,month="August" ,year="2004" ,note="Available: -\url{http://lkml.org/lkml/2004/8/6/237} +\url{https://lore.kernel.org/r/20040807192424.GF3936@in.ibm.com} [Viewed June 8, 2010]" ,annotation={ Introduce rcu_dereference(). @@ -1119,7 +1119,7 @@ Oregon Health and Sciences University" ,month="August" ,year="2004" ,note="Available: -\url{http://lkml.org/lkml/2004/8/30/87} +\url{https://lore.kernel.org/r/1093873222.984.12.camel@new.localdomain} [Viewed February 17, 2005]" ,annotation={ Uses active code in rcu_read_lock() and rcu_read_unlock() to @@ -1186,7 +1186,7 @@ Oregon Health and Sciences University" ,month="October" ,year="2004" ,note="Available: -\url{http://lkml.org/lkml/2004/10/23/241} +\url{https://lore.kernel.org/r/20041023202723.GA1930@us.ibm.com} [Viewed June 8, 2010]" ,annotation={ Introduce rcu_assign_pointer(). @@ -1203,7 +1203,7 @@ Oregon Health and Sciences University" ,annotation={ James Morris posts Kaigai Kohei's patch to LKML. [Viewed December 10, 2004] - Kaigai's patch is at https://lkml.org/lkml/2004/9/27/52 + Kaigai's patch is at https://lore.kernel.org/r/200409271057.i8RAvcA1007873@mailsv.bs1.fc.nec.co.jp } } @@ -1241,11 +1241,11 @@ Oregon Health and Sciences University" ,year="2005" ,day="17" ,note="Available: -\url{http://lkml.org/lkml/2005/3/17/199} +\url{https://lore.kernel.org/r/20050318002026.GA2693@us.ibm.com} [Viewed September 5, 2005]" ,annotation={ First posting showing how RCU can be safely adapted for - preemptable RCU read side critical sections. + preemptible RCU read side critical sections. } } @@ -1256,7 +1256,7 @@ Oregon Health and Sciences University" ,year="2005" ,day="18" ,note="Available: -\url{http://lkml.org/lkml/2005/3/18/122} +\url{https://lore.kernel.org/r/Pine.OSF.4.05.10503181336310.2466-100000@da410.phys.au.dk} [Viewed March 30, 2006]" ,annotation={ Esben Neilsen suggests read-side suppression of grace-period @@ -1302,7 +1302,7 @@ Data Structures" ,month="May" ,year="2005" ,note="Available: -\url{http://lkml.org/lkml/2005/5/9/185} +\url{https://lore.kernel.org/r/20050510012444.GA3011@us.ibm.com} [Viewed May 13, 2005]" ,annotation={ First publication of working lock-based deferred free patches @@ -1385,7 +1385,7 @@ Data Structures" ,day="1" ,year="2005" ,note="Available: -\url{http://lkml.org/lkml/2005/8/1/155} +\url{https://lore.kernel.org/r/20050801171137.GA1754@us.ibm.com} [Viewed March 14, 2006]" ,annotation={ First operating counter-based realtime RCU patch posted to LKML. @@ -1399,7 +1399,7 @@ Data Structures" ,day="8" ,year="2005" ,note="Available: -\url{http://lkml.org/lkml/2005/8/8/108} +\url{https://lore.kernel.org/r/20050808144216.GA1307@us.ibm.com} [Viewed March 14, 2006]" ,annotation={ First operating counter-based realtime RCU patch posted to LKML, @@ -1415,7 +1415,7 @@ Data Structures" ,day="1" ,year="2005" ,note="Available: -\url{http://lkml.org/lkml/2005/10/1/70} +\url{https://lore.kernel.org/r/20051001182056.GA1613@us.ibm.com} [Viewed March 14, 2006]" ,annotation={ First rcutorture patch. @@ -1429,7 +1429,7 @@ Data Structures" ,day="6" ,year="2006" ,note="Available: -\url{https://lkml.org/lkml/2006/1/7/22} +\url{https://lore.kernel.org/r/20060106.231054.43576567.davem@davemloft.net} [Viewed February 29, 2012]" ,annotation={ David Miller's view on hashed arrays of locks: used to really @@ -1464,7 +1464,7 @@ Distributed Processing Symposium" ,day="20" ,year="2006" ,note="Available: -\url{http://lkml.org/lkml/2006/6/20/238} +\url{https://lore.kernel.org/r/20060408134707.22479.33814.sendpatchset@linux.site} [Viewed March 25, 2008]" ,annotation={ RCU-protected radix tree. @@ -1554,7 +1554,7 @@ Revised: ,day="28" ,year="2006" ,note="Available: -\url{http://lkml.org/lkml/2006/9/28/160} +\url{https://lore.kernel.org/r/20060928142616.GA20185@infradead.org} [Viewed March 27, 2008]" } @@ -1593,7 +1593,7 @@ Revised: ,year="2006" ,day=26 ,note="Available: -\url{http://lkml.org/lkml/2006/10/26/73} +\url{https://lore.kernel.org/r/20061026105731.GE11803@in.ibm.com} [Viewed January 26, 2009]" ,annotation={ RCU-based reader-writer lock that allows readers to proceed with @@ -1612,12 +1612,12 @@ Revised: ,year="2006" ,day=17 ,note="Available: -\url{http://lkml.org/lkml/2006/11/17/56} +\url{https://lore.kernel.org/r/20061117092925.GT7164@kernel.dk} [Viewed May 28, 2007]" ,annotation={ SRCU's grace periods are too slow for Jens, even after a factor-of-three speedup. - Sped-up version of SRCU at http://lkml.org/lkml/2006/11/17/359. + Sped-up version of SRCU at https://lore.kernel.org/r/20061118002845.GF2632@us.ibm.com. } } @@ -1629,7 +1629,7 @@ Revised: ,year="2006" ,day=19 ,note="Available: -\url{http://lkml.org/lkml/2006/11/19/69} +\url{https://lore.kernel.org/r/20061119190027.GA3676@oleg} [Viewed May 28, 2007]" ,annotation={ First cut of QRCU. Expanded/corrected versions followed. @@ -1644,7 +1644,7 @@ Revised: ,year="2006" ,day=30 ,note="Available: -\url{http://lkml.org/lkml/2006/11/29/330} +\url{https://lore.kernel.org/r/20061130015714.GC1350@oleg} [Viewed November 26, 2008]" ,annotation={ Expanded/corrected version of QRCU. @@ -1709,7 +1709,7 @@ Revised: ,year="2007" ,day=3 ,note="Available: -\url{http://lkml.org/lkml/2007/1/3/112} +\url{https://lore.kernel.org/r/20070103152738.GA16063@localdomain} [Viewed May 28, 2007]" ,annotation={ Patch for list_splice_rcu(). @@ -1737,7 +1737,7 @@ Revised: ,year="2007" ,day=28 ,note="Available: -\url{http://lkml.org/lkml/2007/1/28/34} +\url{https://lore.kernel.org/r/20070128120509.719287000@programming.kicks-ass.net} [Viewed March 27, 2008]" ,annotation={ RCU-like implementation for frequent updaters and rare readers(!). @@ -1767,7 +1767,7 @@ Revised: ,year="2007" ,day=24 ,note="Available: -\url{http://lkml.org/lkml/2007/2/25/18} +\url{https://lore.kernel.org/r/20070225062349.GA17468@linux.vnet.ibm.com} [Viewed March 27, 2008]" ,annotation={ Patch for QRCU supplying lock-free fast path. @@ -1846,7 +1846,7 @@ Revised: ,annotation={ LWN article describing Promela and spin, and also using Oleg Nesterov's QRCU as an example (with Paul McKenney's fastpath). - Merged patch at: http://lkml.org/lkml/2007/2/25/18 + Merged patch at: https://lore.kernel.org/r/20070225062349.GA17468@linux.vnet.ibm.com } } @@ -1885,10 +1885,10 @@ Revised: ,day="10" ,year="2007" ,note="Available: -\url{http://lkml.org/lkml/2007/9/10/213} +\url{https://lore.kernel.org/r/20070910183004.GA3299@linux.vnet.ibm.com} [Viewed October 25, 2007]" ,annotation={ - Final patch for preemptable RCU to -rt. (Later patches were + Final patch for preemptible RCU to -rt. (Later patches were to mainline, eventually incorporated.) } } @@ -1933,7 +1933,7 @@ Revised: ,day="20" ,year="2007" ,note="Available: -\url{http://lkml.org/lkml/2007/12/20/244} +\url{https://lore.kernel.org/r/20071220142540.GB22523@Krystal} [Viewed March 27, 2008]" ,annotation={ Request for call_rcu_sched() and rcu_barrier_sched(). @@ -2013,7 +2013,7 @@ Revised: ,day="29" ,year="2008" ,note="Available: -\url{http://lkml.org/lkml/2008/1/29/208} +\url{https://lore.kernel.org/r/Pine.LNX.4.58.0801291113350.20371@gandalf.stny.rr.com} [Viewed March 27, 2008]" ,annotation={ Patch that prevents preemptible RCU from unnecessarily waking @@ -2028,7 +2028,7 @@ Revised: ,day="1" ,year="2008" ,note="Available: -\url{http://lkml.org/lkml/2008/2/2/255} +\url{https://lore.kernel.org/r/20080202214124.GA28612@linux.vnet.ibm.com} [Viewed October 18, 2008]" ,annotation={ Explanation of compilers violating dependency ordering. @@ -2088,7 +2088,7 @@ lot of {Linux} into your technology!!!" ,day="3" ,year="2008" ,note="Available: -\url{http://lkml.org/lkml/2008/6/2/539} +\url{https://lore.kernel.org/r/4844BE83.5010401@cn.fujitsu.com} [Viewed December 10, 2008]" ,annotation={ Updated RCU classic algorithm. Introduced multi-tailed list @@ -2122,7 +2122,7 @@ lot of {Linux} into your technology!!!" ,day="21" ,year="2008" ,note="Available: -\url{http://lkml.org/lkml/2008/8/21/336} +\url{https://lore.kernel.org/r/48AD8969.7060900@colorfullife.com} [Viewed December 8, 2008]" ,annotation={ State-based RCU. One key thing that this patch does is to @@ -2137,7 +2137,7 @@ lot of {Linux} into your technology!!!" ,day="6" ,year="2008" ,note="Available: -\url{http://lkml.org/lkml/2008/9/6/86} +\url{https://lore.kernel.org/r/48C2B1D2.5070801@colorfullife.com} [Viewed December 8, 2008]" ,annotation={ Manfred notes a fix required to my attempt to separate irq @@ -2183,7 +2183,7 @@ lot of {Linux} into your technology!!!" ,day="14" ,year="2009" ,note="Available: -\url{http://lkml.org/lkml/2009/1/14/449} +\url{https://lore.kernel.org/r/20090114202044.GJ6734@linux.vnet.ibm.com} [Viewed January 15, 2009]" ,annotation={ Small-footprint implementation of RCU for uniprocessor @@ -2218,7 +2218,7 @@ lot of {Linux} into your technology!!!" git://lttng.org/userspace-rcu.git http://lttng.org/cgi-bin/gitweb.cgi?p=userspace-rcu.git http://lttng.org/urcu - http://lkml.org/lkml/2009/2/5/572 + https://lore.kernel.org/r/20090206030543.GB8560@Krystal } } @@ -2258,7 +2258,7 @@ lot of {Linux} into your technology!!!" ,day="25" ,year="2009" ,note="Available: -\url{http://lkml.org/lkml/2009/6/25/306} +\url{https://lore.kernel.org/r/20090625160706.GA9467@linux.vnet.ibm.com} [Viewed August 16, 2009]" ,annotation={ First posting of expedited RCU to be accepted into -tip. @@ -2272,10 +2272,10 @@ lot of {Linux} into your technology!!!" ,day="23" ,year="2009" ,note="Available: -\url{http://lkml.org/lkml/2009/7/23/294} +\url{https://lore.kernel.org/r/20090724001429.GA17374@linux.vnet.ibm.com} [Viewed August 15, 2009]" ,annotation={ - First posting of simple and fast preemptable RCU. + First posting of simple and fast preemptible RCU. } } @@ -2350,7 +2350,7 @@ lot of {Linux} into your technology!!!" ,month="December" ,year="2009" ,note="Available: -\url{http://lkml.org/lkml/2009/10/18/129} +\url{https://lore.kernel.org/r/20091018232918.GA7385@Krystal} [Viewed December 29, 2009]" ,annotation={ Mathieu proposed defer_rcu() with fixed-size per-thread pool @@ -2518,7 +2518,7 @@ lot of {Linux} into your technology!!!" ,month="January" ,year="2011" ,note="Available: -\url{https://lkml.org/lkml/2011/1/18/322} +\url{https://lore.kernel.org/r/AANLkTimajU0x1v6y3rH2+jr-bZ=tNLs1S_agXdGGAa3S@mail.gmail.com} [Viewed March 4, 2011]" ,annotation={ "The RCU-based name lookup is at the other end of the spectrum - the @@ -2639,7 +2639,7 @@ lot of {Linux} into your technology!!!" RCU-protected hash tables, barriers vs. read-side traversal order. . If the updater is making changes in the opposite direction from - the read-side traveral order, the updater need only execute a + the read-side traversal order, the updater need only execute a memory-barrier instruction, but if in the same direction, the updater needs to wait for a grace period between the individual updates. diff --git a/Documentation/RCU/UP.rst b/Documentation/RCU/UP.rst index e26dda27430c..4060d7a2f62a 100644 --- a/Documentation/RCU/UP.rst +++ b/Documentation/RCU/UP.rst @@ -38,7 +38,7 @@ by having call_rcu() directly invoke its arguments only if it was called from process context. However, this can fail in a similar manner. Suppose that an RCU-based algorithm again scans a linked list containing -elements A, B, and C in process contexts, but that it invokes a function +elements A, B, and C in process context, but that it invokes a function on each element as it is scanned. Suppose further that this function deletes element B from the list, then passes it to call_rcu() for deferred freeing. This may be a bit unconventional, but it is perfectly legal @@ -59,7 +59,8 @@ Example 3: Death by Deadlock Suppose that call_rcu() is invoked while holding a lock, and that the callback function must acquire this same lock. In this case, if call_rcu() were to directly invoke the callback, the result would -be self-deadlock. +be self-deadlock *even if* this invocation occurred from a later +call_rcu() invocation a full grace period later. In some cases, it would possible to restructure to code so that the call_rcu() is delayed until after the lock is released. However, @@ -85,6 +86,14 @@ Quick Quiz #2: :ref:`Answers to Quick Quiz <answer_quick_quiz_up>` +It is important to note that userspace RCU implementations *do* +permit call_rcu() to directly invoke callbacks, but only if a full +grace period has elapsed since those callbacks were queued. This is +the case because some userspace environments are extremely constrained. +Nevertheless, people writing userspace RCU implementations are strongly +encouraged to avoid invoking callbacks from call_rcu(), thus obtaining +the deadlock-avoidance benefits called out above. + Summary ------- @@ -98,7 +107,7 @@ UP systems, including PREEMPT SMP builds running on UP systems. Quick Quiz #3: Why can't synchronize_rcu() return immediately on UP systems running - preemptable RCU? + preemptible RCU? .. _answer_quick_quiz_up: @@ -134,7 +143,7 @@ Answer to Quick Quiz #2: Answer to Quick Quiz #3: Why can't synchronize_rcu() return immediately on UP systems - running preemptable RCU? + running preemptible RCU? Because some other task might have been preempted in the middle of an RCU read-side critical section. If synchronize_rcu() diff --git a/Documentation/RCU/arrayRCU.rst b/Documentation/RCU/arrayRCU.rst deleted file mode 100644 index 4051ea3871ef..000000000000 --- a/Documentation/RCU/arrayRCU.rst +++ /dev/null @@ -1,165 +0,0 @@ -.. _array_rcu_doc: - -Using RCU to Protect Read-Mostly Arrays -======================================= - -Although RCU is more commonly used to protect linked lists, it can -also be used to protect arrays. Three situations are as follows: - -1. :ref:`Hash Tables <hash_tables>` - -2. :ref:`Static Arrays <static_arrays>` - -3. :ref:`Resizable Arrays <resizable_arrays>` - -Each of these three situations involves an RCU-protected pointer to an -array that is separately indexed. It might be tempting to consider use -of RCU to instead protect the index into an array, however, this use -case is **not** supported. The problem with RCU-protected indexes into -arrays is that compilers can play way too many optimization games with -integers, which means that the rules governing handling of these indexes -are far more trouble than they are worth. If RCU-protected indexes into -arrays prove to be particularly valuable (which they have not thus far), -explicit cooperation from the compiler will be required to permit them -to be safely used. - -That aside, each of the three RCU-protected pointer situations are -described in the following sections. - -.. _hash_tables: - -Situation 1: Hash Tables ------------------------- - -Hash tables are often implemented as an array, where each array entry -has a linked-list hash chain. Each hash chain can be protected by RCU -as described in the listRCU.txt document. This approach also applies -to other array-of-list situations, such as radix trees. - -.. _static_arrays: - -Situation 2: Static Arrays --------------------------- - -Static arrays, where the data (rather than a pointer to the data) is -located in each array element, and where the array is never resized, -have not been used with RCU. Rik van Riel recommends using seqlock in -this situation, which would also have minimal read-side overhead as long -as updates are rare. - -Quick Quiz: - Why is it so important that updates be rare when using seqlock? - -:ref:`Answer to Quick Quiz <answer_quick_quiz_seqlock>` - -.. _resizable_arrays: - -Situation 3: Resizable Arrays ------------------------------- - -Use of RCU for resizable arrays is demonstrated by the grow_ary() -function formerly used by the System V IPC code. The array is used -to map from semaphore, message-queue, and shared-memory IDs to the data -structure that represents the corresponding IPC construct. The grow_ary() -function does not acquire any locks; instead its caller must hold the -ids->sem semaphore. - -The grow_ary() function, shown below, does some limit checks, allocates a -new ipc_id_ary, copies the old to the new portion of the new, initializes -the remainder of the new, updates the ids->entries pointer to point to -the new array, and invokes ipc_rcu_putref() to free up the old array. -Note that rcu_assign_pointer() is used to update the ids->entries pointer, -which includes any memory barriers required on whatever architecture -you are running on:: - - static int grow_ary(struct ipc_ids* ids, int newsize) - { - struct ipc_id_ary* new; - struct ipc_id_ary* old; - int i; - int size = ids->entries->size; - - if(newsize > IPCMNI) - newsize = IPCMNI; - if(newsize <= size) - return newsize; - - new = ipc_rcu_alloc(sizeof(struct kern_ipc_perm *)*newsize + - sizeof(struct ipc_id_ary)); - if(new == NULL) - return size; - new->size = newsize; - memcpy(new->p, ids->entries->p, - sizeof(struct kern_ipc_perm *)*size + - sizeof(struct ipc_id_ary)); - for(i=size;i<newsize;i++) { - new->p[i] = NULL; - } - old = ids->entries; - - /* - * Use rcu_assign_pointer() to make sure the memcpyed - * contents of the new array are visible before the new - * array becomes visible. - */ - rcu_assign_pointer(ids->entries, new); - - ipc_rcu_putref(old); - return newsize; - } - -The ipc_rcu_putref() function decrements the array's reference count -and then, if the reference count has dropped to zero, uses call_rcu() -to free the array after a grace period has elapsed. - -The array is traversed by the ipc_lock() function. This function -indexes into the array under the protection of rcu_read_lock(), -using rcu_dereference() to pick up the pointer to the array so -that it may later safely be dereferenced -- memory barriers are -required on the Alpha CPU. Since the size of the array is stored -with the array itself, there can be no array-size mismatches, so -a simple check suffices. The pointer to the structure corresponding -to the desired IPC object is placed in "out", with NULL indicating -a non-existent entry. After acquiring "out->lock", the "out->deleted" -flag indicates whether the IPC object is in the process of being -deleted, and, if not, the pointer is returned:: - - struct kern_ipc_perm* ipc_lock(struct ipc_ids* ids, int id) - { - struct kern_ipc_perm* out; - int lid = id % SEQ_MULTIPLIER; - struct ipc_id_ary* entries; - - rcu_read_lock(); - entries = rcu_dereference(ids->entries); - if(lid >= entries->size) { - rcu_read_unlock(); - return NULL; - } - out = entries->p[lid]; - if(out == NULL) { - rcu_read_unlock(); - return NULL; - } - spin_lock(&out->lock); - - /* ipc_rmid() may have already freed the ID while ipc_lock - * was spinning: here verify that the structure is still valid - */ - if (out->deleted) { - spin_unlock(&out->lock); - rcu_read_unlock(); - return NULL; - } - return out; - } - -.. _answer_quick_quiz_seqlock: - -Answer to Quick Quiz: - Why is it so important that updates be rare when using seqlock? - - The reason that it is important that updates be rare when - using seqlock is that frequent updates can livelock readers. - One way to avoid this problem is to assign a seqlock for - each array entry rather than to the entire array. diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.rst index e98ff261a438..2d42998a89a6 100644 --- a/Documentation/RCU/checklist.txt +++ b/Documentation/RCU/checklist.rst @@ -1,4 +1,8 @@ +.. SPDX-License-Identifier: GPL-2.0 + +================================ Review Checklist for RCU Patches +================================ This document contains a checklist for producing and reviewing patches @@ -28,12 +32,12 @@ over a rather long period of time, but improvements are always welcome! for lockless updates. This does result in the mildly counter-intuitive situation where rcu_read_lock() and rcu_read_unlock() are used to protect updates, however, this - approach provides the same potential simplifications that garbage - collectors do. + approach can provide the same simplifications to certain types + of lockless algorithms that garbage collectors do. 1. Does the update code have proper mutual exclusion? - RCU does allow -readers- to run (almost) naked, but -writers- must + RCU does allow *readers* to run (almost) naked, but *writers* must still use some sort of mutual exclusion, such as: a. locking, @@ -45,12 +49,12 @@ over a rather long period of time, but improvements are always welcome! them -- even x86 allows later loads to be reordered to precede earlier stores), and be prepared to explain why this added complexity is worthwhile. If you choose #c, be prepared to - explain how this single task does not become a major bottleneck on - big multiprocessor machines (for example, if the task is updating - information relating to itself that other tasks can read, there - by definition can be no bottleneck). Note that the definition - of "large" has changed significantly: Eight CPUs was "large" - in the year 2000, but a hundred CPUs was unremarkable in 2017. + explain how this single task does not become a major bottleneck + on large systems (for example, if the task is updating information + relating to itself that other tasks can read, there by definition + can be no bottleneck). Note that the definition of "large" has + changed significantly: Eight CPUs was "large" in the year 2000, + but a hundred CPUs was unremarkable in 2017. 2. Do the RCU read-side critical sections make proper use of rcu_read_lock() and friends? These primitives are needed @@ -62,14 +66,19 @@ over a rather long period of time, but improvements are always welcome! As a rough rule of thumb, any dereference of an RCU-protected pointer must be covered by rcu_read_lock(), rcu_read_lock_bh(), rcu_read_lock_sched(), or by the appropriate update-side lock. - Disabling of preemption can serve as rcu_read_lock_sched(), but - is less readable and prevents lockdep from detecting locking issues. + Explicit disabling of preemption (preempt_disable(), for example) + can serve as rcu_read_lock_sched(), but is less readable and + prevents lockdep from detecting locking issues. + + Please note that you *cannot* rely on code known to be built + only in non-preemptible kernels. Such code can and will break, + especially in kernels built with CONFIG_PREEMPT_COUNT=y. Letting RCU-protected pointers "leak" out of an RCU read-side - critical section is every bid as bad as letting them leak out + critical section is every bit as bad as letting them leak out from under a lock. Unless, of course, you have arranged some other means of protection, such as a lock or a reference count - -before- letting them out of the RCU read-side critical section. + *before* letting them out of the RCU read-side critical section. 3. Does the update code tolerate concurrent accesses? @@ -88,33 +97,38 @@ over a rather long period of time, but improvements are always welcome! b. Proceed as in (a) above, but also maintain per-element locks (that are acquired by both readers and writers) - that guard per-element state. Of course, fields that - the readers refrain from accessing can be guarded by - some other lock acquired only by updaters, if desired. + that guard per-element state. Fields that the readers + refrain from accessing can be guarded by some other lock + acquired only by updaters, if desired. - This works quite well, also. + This also works quite well. c. Make updates appear atomic to readers. For example, pointer updates to properly aligned fields will appear atomic, as will individual atomic primitives. - Sequences of operations performed under a lock will -not- + Sequences of operations performed under a lock will *not* appear to be atomic to RCU readers, nor will sequences - of multiple atomic primitives. + of multiple atomic primitives. One alternative is to + move multiple individual fields to a separate structure, + thus solving the multiple-field problem by imposing an + additional level of indirection. This can work, but is starting to get a bit tricky. - d. Carefully order the updates and the reads so that - readers see valid data at all phases of the update. - This is often more difficult than it sounds, especially - given modern CPUs' tendency to reorder memory references. - One must usually liberally sprinkle memory barriers - (smp_wmb(), smp_rmb(), smp_mb()) through the code, - making it difficult to understand and to test. - - It is usually better to group the changing data into - a separate structure, so that the change may be made - to appear atomic by updating a pointer to reference - a new structure containing updated values. + d. Carefully order the updates and the reads so that readers + see valid data at all phases of the update. This is often + more difficult than it sounds, especially given modern + CPUs' tendency to reorder memory references. One must + usually liberally sprinkle memory-ordering operations + through the code, making it difficult to understand and + to test. Where it works, it is better to use things + like smp_store_release() and smp_load_acquire(), but in + some cases the smp_mb() full memory barrier is required. + + As noted earlier, it is usually better to group the + changing data into a separate structure, so that the + change may be made to appear atomic by updating a pointer + to reference a new structure containing updated values. 4. Weakly ordered CPUs pose special challenges. Almost all CPUs are weakly ordered -- even x86 CPUs allow later loads to be @@ -125,9 +139,7 @@ over a rather long period of time, but improvements are always welcome! accesses. The rcu_dereference() primitive ensures that the CPU picks up the pointer before it picks up the data that the pointer points to. This really is necessary - on Alpha CPUs. If you don't believe me, see: - - http://www.openvms.compaq.com/wizard/wiz_2637.html + on Alpha CPUs. The rcu_dereference() primitive is also an excellent documentation aid, letting the person reading the @@ -138,8 +150,7 @@ over a rather long period of time, but improvements are always welcome! prevents destructive compiler optimizations. However, with a bit of devious creativity, it is possible to mishandle the return value from rcu_dereference(). - Please see rcu_dereference.txt in this directory for - more information. + Please see rcu_dereference.rst for more information. The rcu_dereference() primitive is used by the various "_rcu()" list-traversal primitives, such @@ -149,7 +160,7 @@ over a rather long period of time, but improvements are always welcome! primitives. This is particularly useful in code that is common to readers and updaters. However, lockdep will complain if you access rcu_dereference() outside - of an RCU read-side critical section. See lockdep.txt + of an RCU read-side critical section. See lockdep.rst to learn what to do about this. Of course, neither rcu_dereference() nor the "_rcu()" @@ -182,23 +193,29 @@ over a rather long period of time, but improvements are always welcome! when publicizing a pointer to a structure that can be traversed by an RCU read-side critical section. -5. If call_rcu() or call_srcu() is used, the callback function will - be called from softirq context. In particular, it cannot block. +5. If any of call_rcu(), call_srcu(), call_rcu_tasks(), + call_rcu_tasks_rude(), or call_rcu_tasks_trace() is used, + the callback function may be invoked from softirq context, + and in any case with bottom halves disabled. In particular, + this callback function cannot block. If you need the callback + to block, run that code in a workqueue handler scheduled from + the callback. The queue_rcu_work() function does this for you + in the case of call_rcu(). 6. Since synchronize_rcu() can block, it cannot be called from any sort of irq context. The same rule applies - for synchronize_srcu(), synchronize_rcu_expedited(), and - synchronize_srcu_expedited(). + for synchronize_srcu(), synchronize_rcu_expedited(), + synchronize_srcu_expedited(), synchronize_rcu_tasks(), + synchronize_rcu_tasks_rude(), and synchronize_rcu_tasks_trace(). The expedited forms of these primitives have the same semantics - as the non-expedited forms, but expediting is both expensive and - (with the exception of synchronize_srcu_expedited()) unfriendly - to real-time workloads. Use of the expedited primitives should - be restricted to rare configuration-change operations that would - not normally be undertaken while a real-time workload is running. - However, real-time workloads can use rcupdate.rcu_normal kernel - boot parameter to completely disable expedited grace periods, - though this might have performance implications. + as the non-expedited forms, but expediting is more CPU intensive. + Use of the expedited primitives should be restricted to rare + configuration-change operations that would not normally be + undertaken while a real-time workload is running. Note that + IPI-sensitive real-time workloads can use the rcupdate.rcu_normal + kernel boot parameter to completely disable expedited grace + periods, though this might have performance implications. In particular, if you find yourself invoking one of the expedited primitives repeatedly in a loop, please do everyone a favor: @@ -206,40 +223,63 @@ over a rather long period of time, but improvements are always welcome! a single non-expedited primitive to cover the entire batch. This will very likely be faster than the loop containing the expedited primitive, and will be much much easier on the rest - of the system, especially to real-time workloads running on - the rest of the system. - -7. As of v4.20, a given kernel implements only one RCU flavor, - which is RCU-sched for PREEMPT=n and RCU-preempt for PREEMPT=y. - If the updater uses call_rcu() or synchronize_rcu(), - then the corresponding readers my use rcu_read_lock() and - rcu_read_unlock(), rcu_read_lock_bh() and rcu_read_unlock_bh(), - or any pair of primitives that disables and re-enables preemption, - for example, rcu_read_lock_sched() and rcu_read_unlock_sched(). - If the updater uses synchronize_srcu() or call_srcu(), - then the corresponding readers must use srcu_read_lock() and - srcu_read_unlock(), and with the same srcu_struct. The rules for - the expedited primitives are the same as for their non-expedited - counterparts. Mixing things up will result in confusion and - broken kernels, and has even resulted in an exploitable security - issue. - - One exception to this rule: rcu_read_lock() and rcu_read_unlock() - may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh() - in cases where local bottom halves are already known to be - disabled, for example, in irq or softirq context. Commenting - such cases is a must, of course! And the jury is still out on - whether the increased speed is worth it. - -8. Although synchronize_rcu() is slower than is call_rcu(), it - usually results in simpler code. So, unless update performance is - critically important, the updaters cannot block, or the latency of - synchronize_rcu() is visible from userspace, synchronize_rcu() - should be used in preference to call_rcu(). Furthermore, - kfree_rcu() usually results in even simpler code than does - synchronize_rcu() without synchronize_rcu()'s multi-millisecond - latency. So please take advantage of kfree_rcu()'s "fire and - forget" memory-freeing capabilities where it applies. + of the system, especially to real-time workloads running on the + rest of the system. Alternatively, instead use asynchronous + primitives such as call_rcu(). + +7. As of v4.20, a given kernel implements only one RCU flavor, which + is RCU-sched for PREEMPTION=n and RCU-preempt for PREEMPTION=y. + If the updater uses call_rcu() or synchronize_rcu(), then + the corresponding readers may use: (1) rcu_read_lock() and + rcu_read_unlock(), (2) any pair of primitives that disables + and re-enables softirq, for example, rcu_read_lock_bh() and + rcu_read_unlock_bh(), or (3) any pair of primitives that disables + and re-enables preemption, for example, rcu_read_lock_sched() and + rcu_read_unlock_sched(). If the updater uses synchronize_srcu() + or call_srcu(), then the corresponding readers must use + srcu_read_lock() and srcu_read_unlock(), and with the same + srcu_struct. The rules for the expedited RCU grace-period-wait + primitives are the same as for their non-expedited counterparts. + + Similarly, it is necessary to correctly use the RCU Tasks flavors: + + a. If the updater uses synchronize_rcu_tasks() or + call_rcu_tasks(), then the readers must refrain from + executing voluntary context switches, that is, from + blocking. + + b. If the updater uses call_rcu_tasks_trace() + or synchronize_rcu_tasks_trace(), then the + corresponding readers must use rcu_read_lock_trace() + and rcu_read_unlock_trace(). + + c. If an updater uses call_rcu_tasks_rude() or + synchronize_rcu_tasks_rude(), then the corresponding + readers must use anything that disables preemption, + for example, preempt_disable() and preempt_enable(). + + Mixing things up will result in confusion and broken kernels, and + has even resulted in an exploitable security issue. Therefore, + when using non-obvious pairs of primitives, commenting is + of course a must. One example of non-obvious pairing is + the XDP feature in networking, which calls BPF programs from + network-driver NAPI (softirq) context. BPF relies heavily on RCU + protection for its data structures, but because the BPF program + invocation happens entirely within a single local_bh_disable() + section in a NAPI poll cycle, this usage is safe. The reason + that this usage is safe is that readers can use anything that + disables BH when updaters use call_rcu() or synchronize_rcu(). + +8. Although synchronize_rcu() is slower than is call_rcu(), + it usually results in simpler code. So, unless update + performance is critically important, the updaters cannot block, + or the latency of synchronize_rcu() is visible from userspace, + synchronize_rcu() should be used in preference to call_rcu(). + Furthermore, kfree_rcu() and kvfree_rcu() usually result + in even simpler code than does synchronize_rcu() without + synchronize_rcu()'s multi-millisecond latency. So please take + advantage of kfree_rcu()'s and kvfree_rcu()'s "fire and forget" + memory-freeing capabilities where it applies. An especially important property of the synchronize_rcu() primitive is that it automatically self-limits: if grace periods @@ -249,8 +289,8 @@ over a rather long period of time, but improvements are always welcome! cases where grace periods are delayed, as failing to do so can result in excessive realtime latencies or even OOM conditions. - Ways of gaining this self-limiting property when using call_rcu() - include: + Ways of gaining this self-limiting property when using call_rcu(), + kfree_rcu(), or kvfree_rcu() include: a. Keeping a count of the number of data-structure elements used by the RCU-protected data structure, including @@ -282,17 +322,21 @@ over a rather long period of time, but improvements are always welcome! here is that superuser already has lots of ways to crash the machine. - d. Periodically invoke synchronize_rcu(), permitting a limited + d. Periodically invoke rcu_barrier(), permitting a limited number of updates per grace period. - The same cautions apply to call_srcu() and kfree_rcu(). + The same cautions apply to call_srcu(), call_rcu_tasks(), + call_rcu_tasks_rude(), and call_rcu_tasks_trace(). This is + why there is an srcu_barrier(), rcu_barrier_tasks(), + rcu_barrier_tasks_rude(), and rcu_barrier_tasks_rude(), + respectively. - Note that although these primitives do take action to avoid memory - exhaustion when any given CPU has too many callbacks, a determined - user could still exhaust memory. This is especially the case - if a system with a large number of CPUs has been configured to - offload all of its RCU callbacks onto a single CPU, or if the - system has relatively little free memory. + Note that although these primitives do take action to avoid + memory exhaustion when any given CPU has too many callbacks, + a determined user or administrator can still exhaust memory. + This is especially the case if a system with a large number of + CPUs has been configured to offload all of its RCU callbacks onto + a single CPU, or if the system has relatively little free memory. 9. All RCU list-traversal primitives, which include rcu_dereference(), list_for_each_entry_rcu(), and @@ -308,20 +352,27 @@ over a rather long period of time, but improvements are always welcome! primitives when the update-side lock is held is that doing so can be quite helpful in reducing code bloat when common code is shared between readers and updaters. Additional primitives - are provided for this case, as discussed in lockdep.txt. + are provided for this case, as discussed in lockdep.rst. + + One exception to this rule is when data is only ever added to + the linked data structure, and is never removed during any + time that readers might be accessing that structure. In such + cases, READ_ONCE() may be used in place of rcu_dereference() + and the read-side markers (rcu_read_lock() and rcu_read_unlock(), + for example) may be omitted. 10. Conversely, if you are in an RCU read-side critical section, - and you don't hold the appropriate update-side lock, you -must- + and you don't hold the appropriate update-side lock, you *must* use the "_rcu()" variants of the list macros. Failing to do so will break Alpha, cause aggressive compilers to generate bad code, - and confuse people trying to read your code. + and confuse people trying to understand your code. 11. Any lock acquired by an RCU callback must be acquired elsewhere - with softirq disabled, e.g., via spin_lock_irqsave(), - spin_lock_bh(), etc. Failing to disable softirq on a given - acquisition of that lock will result in deadlock as soon as - the RCU softirq handler happens to run your RCU callback while - interrupting that acquisition's critical section. + with softirq disabled, e.g., via spin_lock_bh(). Failing to + disable softirq on a given acquisition of that lock will result + in deadlock as soon as the RCU softirq handler happens to run + your RCU callback while interrupting that acquisition's critical + section. 12. RCU callbacks can be and are executed in parallel. In many cases, the callback code simply wrappers around kfree(), so that this @@ -337,12 +388,22 @@ over a rather long period of time, but improvements are always welcome! callback pending, then that RCU callback will execute on some surviving CPU. (If this was not the case, a self-spawning RCU callback would prevent the victim CPU from ever going offline.) - Furthermore, CPUs designated by rcu_nocbs= might well -always- + Furthermore, CPUs designated by rcu_nocbs= might well *always* have their RCU callbacks executed on some other CPUs, in fact, for some real-time workloads, this is the whole point of using the rcu_nocbs= kernel boot parameter. -13. Unlike other forms of RCU, it -is- permissible to block in an + In addition, do not assume that callbacks queued in a given order + will be invoked in that order, even if they all are queued on the + same CPU. Furthermore, do not assume that same-CPU callbacks will + be invoked serially. For example, in recent kernels, CPUs can be + switched between offloaded and de-offloaded callback invocation, + and while a given CPU is undergoing such a switch, its callbacks + might be concurrently invoked by that CPU's softirq handler and + that CPU's rcuo kthread. At such times, that CPU's callbacks + might be executed both concurrently and out of order. + +13. Unlike most flavors of RCU, it *is* permissible to block in an SRCU read-side critical section (demarked by srcu_read_lock() and srcu_read_unlock()), hence the "SRCU": "sleepable RCU". Please note that if you don't need to sleep in read-side critical @@ -382,6 +443,12 @@ over a rather long period of time, but improvements are always welcome! never sends IPIs to other CPUs, so it is easier on real-time workloads than is synchronize_rcu_expedited(). + It is also permissible to sleep in RCU Tasks Trace read-side + critical, which are delimited by rcu_read_lock_trace() and + rcu_read_unlock_trace(). However, this is a specialized flavor + of RCU, and you should not use it without first checking with + its current users. In most cases, you should instead use SRCU. + Note that rcu_assign_pointer() relates to SRCU just as it does to other forms of RCU, but instead of rcu_dereference() you should use srcu_dereference() in order to avoid lockdep splats. @@ -389,16 +456,16 @@ over a rather long period of time, but improvements are always welcome! 14. The whole point of call_rcu(), synchronize_rcu(), and friends is to wait until all pre-existing readers have finished before carrying out some otherwise-destructive operation. It is - therefore critically important to -first- remove any path + therefore critically important to *first* remove any path that readers can follow that could be affected by the - destructive operation, and -only- -then- invoke call_rcu(), + destructive operation, and *only then* invoke call_rcu(), synchronize_rcu(), or friends. Because these primitives only wait for pre-existing readers, it is the caller's responsibility to guarantee that any subsequent readers will execute safely. -15. The various RCU read-side primitives do -not- necessarily contain +15. The various RCU read-side primitives do *not* necessarily contain memory barriers. You should therefore plan for the CPU and the compiler to freely reorder code into and out of RCU read-side critical sections. It is the responsibility of the @@ -411,48 +478,63 @@ over a rather long period of time, but improvements are always welcome! __rcu sparse checks to validate your RCU code. These can help find problems as follows: - CONFIG_PROVE_LOCKING: check that accesses to RCU-protected data - structures are carried out under the proper RCU - read-side critical section, while holding the right - combination of locks, or whatever other conditions - are appropriate. + CONFIG_PROVE_LOCKING: + check that accesses to RCU-protected data structures + are carried out under the proper RCU read-side critical + section, while holding the right combination of locks, + or whatever other conditions are appropriate. - CONFIG_DEBUG_OBJECTS_RCU_HEAD: check that you don't pass the - same object to call_rcu() (or friends) before an RCU - grace period has elapsed since the last time that you - passed that same object to call_rcu() (or friends). + CONFIG_DEBUG_OBJECTS_RCU_HEAD: + check that you don't pass the same object to call_rcu() + (or friends) before an RCU grace period has elapsed + since the last time that you passed that same object to + call_rcu() (or friends). - __rcu sparse checks: tag the pointer to the RCU-protected data - structure with __rcu, and sparse will warn you if you - access that pointer without the services of one of the - variants of rcu_dereference(). + __rcu sparse checks: + tag the pointer to the RCU-protected data structure + with __rcu, and sparse will warn you if you access that + pointer without the services of one of the variants + of rcu_dereference(). These debugging aids can help you find problems that are otherwise extremely difficult to spot. -17. If you register a callback using call_rcu() or call_srcu(), and - pass in a function defined within a loadable module, then it in - necessary to wait for all pending callbacks to be invoked after - the last invocation and before unloading that module. Note that - it is absolutely -not- sufficient to wait for a grace period! - The current (say) synchronize_rcu() implementation is -not- - guaranteed to wait for callbacks registered on other CPUs. - Or even on the current CPU if that CPU recently went offline - and came back online. +17. If you pass a callback function defined within a module to one of + call_rcu(), call_srcu(), call_rcu_tasks(), call_rcu_tasks_rude(), + or call_rcu_tasks_trace(), then it is necessary to wait for all + pending callbacks to be invoked before unloading that module. + Note that it is absolutely *not* sufficient to wait for a grace + period! For example, synchronize_rcu() implementation is *not* + guaranteed to wait for callbacks registered on other CPUs via + call_rcu(). Or even on the current CPU if that CPU recently + went offline and came back online. You instead need to use one of the barrier functions: - o call_rcu() -> rcu_barrier() - o call_srcu() -> srcu_barrier() - - However, these barrier functions are absolutely -not- guaranteed - to wait for a grace period. In fact, if there are no call_rcu() - callbacks waiting anywhere in the system, rcu_barrier() is within - its rights to return immediately. - - So if you need to wait for both an RCU grace period and for - all pre-existing call_rcu() callbacks, you will need to execute - both rcu_barrier() and synchronize_rcu(), if necessary, using - something like workqueues to to execute them concurrently. - - See rcubarrier.txt for more information. + - call_rcu() -> rcu_barrier() + - call_srcu() -> srcu_barrier() + - call_rcu_tasks() -> rcu_barrier_tasks() + - call_rcu_tasks_rude() -> rcu_barrier_tasks_rude() + - call_rcu_tasks_trace() -> rcu_barrier_tasks_trace() + + However, these barrier functions are absolutely *not* guaranteed + to wait for a grace period. For example, if there are no + call_rcu() callbacks queued anywhere in the system, rcu_barrier() + can and will return immediately. + + So if you need to wait for both a grace period and for all + pre-existing callbacks, you will need to invoke both functions, + with the pair depending on the flavor of RCU: + + - Either synchronize_rcu() or synchronize_rcu_expedited(), + together with rcu_barrier() + - Either synchronize_srcu() or synchronize_srcu_expedited(), + together with and srcu_barrier() + - synchronize_rcu_tasks() and rcu_barrier_tasks() + - synchronize_tasks_rude() and rcu_barrier_tasks_rude() + - synchronize_tasks_trace() and rcu_barrier_tasks_trace() + + If necessary, you can use something like workqueues to execute + the requisite pair of functions concurrently. + + See rcubarrier.rst for more information. diff --git a/Documentation/RCU/index.rst b/Documentation/RCU/index.rst index 81a0a1e5f767..84a79903f6a8 100644 --- a/Documentation/RCU/index.rst +++ b/Documentation/RCU/index.rst @@ -1,3 +1,5 @@ +.. SPDX-License-Identifier: GPL-2.0 + .. _rcu_concepts: ============ @@ -7,11 +9,17 @@ RCU concepts .. toctree:: :maxdepth: 3 - arrayRCU + checklist + lockdep + lockdep-splat rcubarrier rcu_dereference whatisRCU rcu + rculist_nulls + rcuref + torture + stallwarn listRCU NMI-RCU UP diff --git a/Documentation/RCU/listRCU.rst b/Documentation/RCU/listRCU.rst index 2a643e293fb4..ed5c9d8c9afe 100644 --- a/Documentation/RCU/listRCU.rst +++ b/Documentation/RCU/listRCU.rst @@ -3,11 +3,19 @@ Using RCU to Protect Read-Mostly Linked Lists ============================================= -One of the best applications of RCU is to protect read-mostly linked lists -(``struct list_head`` in list.h). One big advantage of this approach -is that all of the required memory barriers are included for you in -the list macros. This document describes several applications of RCU, -with the best fits first. +One of the most common uses of RCU is protecting read-mostly linked lists +(``struct list_head`` in list.h). One big advantage of this approach is +that all of the required memory ordering is provided by the list macros. +This document describes several list-based RCU use cases. + +When iterating a list while holding the rcu_read_lock(), writers may +modify the list. The reader is guaranteed to see all of the elements +which were added to the list before they acquired the rcu_read_lock() +and are still on the list when they drop the rcu_read_unlock(). +Elements which are added to, or removed from the list may or may not +be seen. If the writer calls list_replace_rcu(), the reader may see +either the old element or the new element; they will not see both, +nor will they see neither. Example 1: Read-mostly list: Deferred Destruction @@ -35,7 +43,8 @@ The code traversing the list of all processes typically looks like:: } rcu_read_unlock(); -The simplified code for removing a process from a task list is:: +The simplified and heavily inlined code for removing a process from a +task list is:: void release_task(struct task_struct *p) { @@ -45,39 +54,48 @@ The simplified code for removing a process from a task list is:: call_rcu(&p->rcu, delayed_put_task_struct); } -When a process exits, ``release_task()`` calls ``list_del_rcu(&p->tasks)`` under -``tasklist_lock`` writer lock protection, to remove the task from the list of -all tasks. The ``tasklist_lock`` prevents concurrent list additions/removals -from corrupting the list. Readers using ``for_each_process()`` are not protected -with the ``tasklist_lock``. To prevent readers from noticing changes in the list -pointers, the ``task_struct`` object is freed only after one or more grace -periods elapse (with the help of call_rcu()). This deferring of destruction -ensures that any readers traversing the list will see valid ``p->tasks.next`` -pointers and deletion/freeing can happen in parallel with traversal of the list. -This pattern is also called an **existence lock**, since RCU pins the object in -memory until all existing readers finish. +When a process exits, ``release_task()`` calls ``list_del_rcu(&p->tasks)`` +via __exit_signal() and __unhash_process() under ``tasklist_lock`` +writer lock protection. The list_del_rcu() invocation removes +the task from the list of all tasks. The ``tasklist_lock`` +prevents concurrent list additions/removals from corrupting the +list. Readers using ``for_each_process()`` are not protected with the +``tasklist_lock``. To prevent readers from noticing changes in the list +pointers, the ``task_struct`` object is freed only after one or more +grace periods elapse, with the help of call_rcu(), which is invoked via +put_task_struct_rcu_user(). This deferring of destruction ensures that +any readers traversing the list will see valid ``p->tasks.next`` pointers +and deletion/freeing can happen in parallel with traversal of the list. +This pattern is also called an **existence lock**, since RCU refrains +from invoking the delayed_put_task_struct() callback function until +all existing readers finish, which guarantees that the ``task_struct`` +object in question will remain in existence until after the completion +of all RCU readers that might possibly have a reference to that object. Example 2: Read-Side Action Taken Outside of Lock: No In-Place Updates ---------------------------------------------------------------------- -The best applications are cases where, if reader-writer locking were -used, the read-side lock would be dropped before taking any action -based on the results of the search. The most celebrated example is -the routing table. Because the routing table is tracking the state of -equipment outside of the computer, it will at times contain stale data. -Therefore, once the route has been computed, there is no need to hold -the routing table static during transmission of the packet. After all, -you can hold the routing table static all you want, but that won't keep -the external Internet from changing, and it is the state of the external -Internet that really matters. In addition, routing entries are typically -added or deleted, rather than being modified in place. - -A straightforward example of this use of RCU may be found in the -system-call auditing support. For example, a reader-writer locked +Some reader-writer locking use cases compute a value while holding +the read-side lock, but continue to use that value after that lock is +released. These use cases are often good candidates for conversion +to RCU. One prominent example involves network packet routing. +Because the packet-routing data tracks the state of equipment outside +of the computer, it will at times contain stale data. Therefore, once +the route has been computed, there is no need to hold the routing table +static during transmission of the packet. After all, you can hold the +routing table static all you want, but that won't keep the external +Internet from changing, and it is the state of the external Internet +that really matters. In addition, routing entries are typically added +or deleted, rather than being modified in place. This is a rare example +of the finite speed of light and the non-zero size of atoms actually +helping make synchronization be lighter weight. + +A straightforward example of this type of RCU use case may be found in +the system-call auditing support. For example, a reader-writer locked implementation of ``audit_filter_task()`` might be as follows:: - static enum audit_state audit_filter_task(struct task_struct *tsk) + static enum audit_state audit_filter_task(struct task_struct *tsk, char **key) { struct audit_entry *e; enum audit_state state; @@ -86,6 +104,8 @@ implementation of ``audit_filter_task()`` might be as follows:: /* Note: audit_filter_mutex held by caller. */ list_for_each_entry(e, &audit_tsklist, list) { if (audit_filter_rules(tsk, &e->rule, NULL, &state)) { + if (state == AUDIT_STATE_RECORD) + *key = kstrdup(e->rule.filterkey, GFP_ATOMIC); read_unlock(&auditsc_lock); return state; } @@ -101,7 +121,7 @@ you are turning auditing off, it is OK to audit a few extra system calls. This means that RCU can be easily applied to the read side, as follows:: - static enum audit_state audit_filter_task(struct task_struct *tsk) + static enum audit_state audit_filter_task(struct task_struct *tsk, char **key) { struct audit_entry *e; enum audit_state state; @@ -110,6 +130,8 @@ This means that RCU can be easily applied to the read side, as follows:: /* Note: audit_filter_mutex held by caller. */ list_for_each_entry_rcu(e, &audit_tsklist, list) { if (audit_filter_rules(tsk, &e->rule, NULL, &state)) { + if (state == AUDIT_STATE_RECORD) + *key = kstrdup(e->rule.filterkey, GFP_ATOMIC); rcu_read_unlock(); return state; } @@ -118,13 +140,15 @@ This means that RCU can be easily applied to the read side, as follows:: return AUDIT_BUILD_CONTEXT; } -The ``read_lock()`` and ``read_unlock()`` calls have become rcu_read_lock() -and rcu_read_unlock(), respectively, and the list_for_each_entry() has -become list_for_each_entry_rcu(). The **_rcu()** list-traversal primitives -insert the read-side memory barriers that are required on DEC Alpha CPUs. +The read_lock() and read_unlock() calls have become rcu_read_lock() +and rcu_read_unlock(), respectively, and the list_for_each_entry() +has become list_for_each_entry_rcu(). The **_rcu()** list-traversal +primitives add READ_ONCE() and diagnostic checks for incorrect use +outside of an RCU read-side critical section. The changes to the update side are also straightforward. A reader-writer lock -might be used as follows for deletion and insertion:: +might be used as follows for deletion and insertion in these simplified +versions of audit_del_rule() and audit_add_rule():: static inline int audit_del_rule(struct audit_rule *rule, struct list_head *list) @@ -188,16 +212,16 @@ Following are the RCU equivalents for these two functions:: return 0; } -Normally, the ``write_lock()`` and ``write_unlock()`` would be replaced by a +Normally, the write_lock() and write_unlock() would be replaced by a spin_lock() and a spin_unlock(). But in this case, all callers hold ``audit_filter_mutex``, so no additional locking is required. The -``auditsc_lock`` can therefore be eliminated, since use of RCU eliminates the +auditsc_lock can therefore be eliminated, since use of RCU eliminates the need for writers to exclude readers. The list_del(), list_add(), and list_add_tail() primitives have been replaced by list_del_rcu(), list_add_rcu(), and list_add_tail_rcu(). -The **_rcu()** list-manipulation primitives add memory barriers that are needed on -weakly ordered CPUs (most of them!). The list_del_rcu() primitive omits the +The **_rcu()** list-manipulation primitives add memory barriers that are +needed on weakly ordered CPUs. The list_del_rcu() primitive omits the pointer poisoning debug-assist code that would otherwise cause concurrent readers to fail spectacularly. @@ -238,7 +262,9 @@ need to be filled in):: The RCU version creates a copy, updates the copy, then replaces the old entry with the newly updated entry. This sequence of actions, allowing concurrent reads while making a copy to perform an update, is what gives -RCU (*read-copy update*) its name. The RCU code is as follows:: +RCU (*read-copy update*) its name. + +The RCU version of audit_upd_rule() is as follows:: static inline int audit_upd_rule(struct audit_rule *rule, struct list_head *list, @@ -267,6 +293,9 @@ RCU (*read-copy update*) its name. The RCU code is as follows:: Again, this assumes that the caller holds ``audit_filter_mutex``. Normally, the writer lock would become a spinlock in this sort of code. +The update_lsm_rule() does something very similar, for those who would +prefer to look at real Linux-kernel code. + Another use of this pattern can be found in the openswitch driver's *connection tracking table* code in ``ct_limit_set()``. The table holds connection tracking entries and has a limit on the maximum entries. There is one such table @@ -281,9 +310,10 @@ Example 4: Eliminating Stale Data --------------------------------- The auditing example above tolerates stale data, as do most algorithms -that are tracking external state. Because there is a delay from the -time the external state changes before Linux becomes aware of the change, -additional RCU-induced staleness is generally not a problem. +that are tracking external state. After all, given there is a delay +from the time the external state changes before Linux becomes aware +of the change, and so as noted earlier, a small quantity of additional +RCU-induced staleness is generally not a problem. However, there are many examples where stale data cannot be tolerated. One example in the Linux kernel is the System V IPC (see the shm_lock() @@ -302,7 +332,7 @@ Quick Quiz: If the system-call audit module were to ever need to reject stale data, one way to accomplish this would be to add a ``deleted`` flag and a ``lock`` spinlock to the -audit_entry structure, and modify ``audit_filter_task()`` as follows:: +``audit_entry`` structure, and modify audit_filter_task() as follows:: static enum audit_state audit_filter_task(struct task_struct *tsk) { @@ -319,6 +349,8 @@ audit_entry structure, and modify ``audit_filter_task()`` as follows:: return AUDIT_BUILD_CONTEXT; } rcu_read_unlock(); + if (state == AUDIT_STATE_RECORD) + *key = kstrdup(e->rule.filterkey, GFP_ATOMIC); return state; } } @@ -326,12 +358,6 @@ audit_entry structure, and modify ``audit_filter_task()`` as follows:: return AUDIT_BUILD_CONTEXT; } -Note that this example assumes that entries are only added and deleted. -Additional mechanism is required to deal correctly with the update-in-place -performed by ``audit_upd_rule()``. For one thing, ``audit_upd_rule()`` would -need additional memory barriers to ensure that the list_add_rcu() was really -executed before the list_del_rcu(). - The ``audit_del_rule()`` function would need to set the ``deleted`` flag under the spinlock as follows:: @@ -357,24 +383,32 @@ spinlock as follows:: This too assumes that the caller holds ``audit_filter_mutex``. +Note that this example assumes that entries are only added and deleted. +Additional mechanism is required to deal correctly with the update-in-place +performed by audit_upd_rule(). For one thing, audit_upd_rule() would +need to hold the locks of both the old ``audit_entry`` and its replacement +while executing the list_replace_rcu(). + Example 5: Skipping Stale Objects --------------------------------- -For some usecases, reader performance can be improved by skipping stale objects -during read-side list traversal if the object in concern is pending destruction -after one or more grace periods. One such example can be found in the timerfd -subsystem. When a ``CLOCK_REALTIME`` clock is reprogrammed - for example due to -setting of the system time, then all programmed timerfds that depend on this -clock get triggered and processes waiting on them to expire are woken up in -advance of their scheduled expiry. To facilitate this, all such timers are added -to an RCU-managed ``cancel_list`` when they are setup in +For some use cases, reader performance can be improved by skipping +stale objects during read-side list traversal, where stale objects +are those that will be removed and destroyed after one or more grace +periods. One such example can be found in the timerfd subsystem. When a +``CLOCK_REALTIME`` clock is reprogrammed (for example due to setting +of the system time) then all programmed ``timerfds`` that depend on +this clock get triggered and processes waiting on them are awakened in +advance of their scheduled expiry. To facilitate this, all such timers +are added to an RCU-managed ``cancel_list`` when they are setup in ``timerfd_setup_cancel()``:: static void timerfd_setup_cancel(struct timerfd_ctx *ctx, int flags) { spin_lock(&ctx->cancel_lock); - if ((ctx->clockid == CLOCK_REALTIME && + if ((ctx->clockid == CLOCK_REALTIME || + ctx->clockid == CLOCK_REALTIME_ALARM) && (flags & TFD_TIMER_ABSTIME) && (flags & TFD_TIMER_CANCEL_ON_SET)) { if (!ctx->might_cancel) { ctx->might_cancel = true; @@ -382,13 +416,16 @@ to an RCU-managed ``cancel_list`` when they are setup in list_add_rcu(&ctx->clist, &cancel_list); spin_unlock(&cancel_lock); } + } else { + __timerfd_remove_cancel(ctx); } spin_unlock(&ctx->cancel_lock); } -When a timerfd is freed (fd is closed), then the ``might_cancel`` flag of the -timerfd object is cleared, the object removed from the ``cancel_list`` and -destroyed:: +When a timerfd is freed (fd is closed), then the ``might_cancel`` +flag of the timerfd object is cleared, the object removed from the +``cancel_list`` and destroyed, as shown in this simplified and inlined +version of timerfd_release():: int timerfd_release(struct inode *inode, struct file *file) { @@ -403,7 +440,10 @@ destroyed:: } spin_unlock(&ctx->cancel_lock); - hrtimer_cancel(&ctx->t.tmr); + if (isalarm(ctx)) + alarm_cancel(&ctx->t.alarm); + else + hrtimer_cancel(&ctx->t.tmr); kfree_rcu(ctx, rcu); return 0; } @@ -416,6 +456,7 @@ objects:: void timerfd_clock_was_set(void) { + ktime_t moffs = ktime_mono_to_real(0); struct timerfd_ctx *ctx; unsigned long flags; @@ -424,7 +465,7 @@ objects:: if (!ctx->might_cancel) continue; spin_lock_irqsave(&ctx->wqh.lock, flags); - if (ctx->moffs != ktime_mono_to_real(0)) { + if (ctx->moffs != moffs) { ctx->moffs = KTIME_MAX; ctx->ticks++; wake_up_locked_poll(&ctx->wqh, EPOLLIN); @@ -434,10 +475,10 @@ objects:: rcu_read_unlock(); } -The key point here is, because RCU-traversal of the ``cancel_list`` happens -while objects are being added and removed to the list, sometimes the traversal -can step on an object that has been removed from the list. In this example, it -is seen that it is better to skip such objects using a flag. +The key point is that because RCU-protected traversal of the +``cancel_list`` happens concurrently with object addition and removal, +sometimes the traversal can access an object that has been removed from +the list. In this example, a flag is used to skip such objects. Summary diff --git a/Documentation/RCU/lockdep-splat.txt b/Documentation/RCU/lockdep-splat.rst index b8096316fd11..bcbc4b3c88d7 100644 --- a/Documentation/RCU/lockdep-splat.txt +++ b/Documentation/RCU/lockdep-splat.rst @@ -1,10 +1,16 @@ +.. SPDX-License-Identifier: GPL-2.0 + +================= +Lockdep-RCU Splat +================= + Lockdep-RCU was added to the Linux kernel in early 2010 (http://lwn.net/Articles/371986/). This facility checks for some common misuses of the RCU API, most notably using one of the rcu_dereference() family to access an RCU-protected pointer without the proper protection. When such misuse is detected, an lockdep-RCU splat is emitted. -The usual cause of a lockdep-RCU slat is someone accessing an +The usual cause of a lockdep-RCU splat is someone accessing an RCU-protected data structure without either (1) being in the right kind of RCU read-side critical section or (2) holding the right update-side lock. This problem can therefore be serious: it might result in random memory @@ -12,55 +18,54 @@ overwriting or worse. There can of course be false positives, this being the real world and all that. So let's look at an example RCU lockdep splat from 3.0-rc5, one that -has long since been fixed: - -============================= -WARNING: suspicious RCU usage ------------------------------ -block/cfq-iosched.c:2776 suspicious rcu_dereference_protected() usage! - -other info that might help us debug this: - - -rcu_scheduler_active = 1, debug_locks = 0 -3 locks held by scsi_scan_6/1552: - #0: (&shost->scan_mutex){+.+.}, at: [<ffffffff8145efca>] -scsi_scan_host_selected+0x5a/0x150 - #1: (&eq->sysfs_lock){+.+.}, at: [<ffffffff812a5032>] -elevator_exit+0x22/0x60 - #2: (&(&q->__queue_lock)->rlock){-.-.}, at: [<ffffffff812b6233>] -cfq_exit_queue+0x43/0x190 - -stack backtrace: -Pid: 1552, comm: scsi_scan_6 Not tainted 3.0.0-rc5 #17 -Call Trace: - [<ffffffff810abb9b>] lockdep_rcu_dereference+0xbb/0xc0 - [<ffffffff812b6139>] __cfq_exit_single_io_context+0xe9/0x120 - [<ffffffff812b626c>] cfq_exit_queue+0x7c/0x190 - [<ffffffff812a5046>] elevator_exit+0x36/0x60 - [<ffffffff812a802a>] blk_cleanup_queue+0x4a/0x60 - [<ffffffff8145cc09>] scsi_free_queue+0x9/0x10 - [<ffffffff81460944>] __scsi_remove_device+0x84/0xd0 - [<ffffffff8145dca3>] scsi_probe_and_add_lun+0x353/0xb10 - [<ffffffff817da069>] ? error_exit+0x29/0xb0 - [<ffffffff817d98ed>] ? _raw_spin_unlock_irqrestore+0x3d/0x80 - [<ffffffff8145e722>] __scsi_scan_target+0x112/0x680 - [<ffffffff812c690d>] ? trace_hardirqs_off_thunk+0x3a/0x3c - [<ffffffff817da069>] ? error_exit+0x29/0xb0 - [<ffffffff812bcc60>] ? kobject_del+0x40/0x40 - [<ffffffff8145ed16>] scsi_scan_channel+0x86/0xb0 - [<ffffffff8145f0b0>] scsi_scan_host_selected+0x140/0x150 - [<ffffffff8145f149>] do_scsi_scan_host+0x89/0x90 - [<ffffffff8145f170>] do_scan_async+0x20/0x160 - [<ffffffff8145f150>] ? do_scsi_scan_host+0x90/0x90 - [<ffffffff810975b6>] kthread+0xa6/0xb0 - [<ffffffff817db154>] kernel_thread_helper+0x4/0x10 - [<ffffffff81066430>] ? finish_task_switch+0x80/0x110 - [<ffffffff817d9c04>] ? retint_restore_args+0xe/0xe - [<ffffffff81097510>] ? __kthread_init_worker+0x70/0x70 - [<ffffffff817db150>] ? gs_change+0xb/0xb - -Line 2776 of block/cfq-iosched.c in v3.0-rc5 is as follows: +has long since been fixed:: + + ============================= + WARNING: suspicious RCU usage + ----------------------------- + block/cfq-iosched.c:2776 suspicious rcu_dereference_protected() usage! + +other info that might help us debug this:: + + rcu_scheduler_active = 1, debug_locks = 0 + 3 locks held by scsi_scan_6/1552: + #0: (&shost->scan_mutex){+.+.}, at: [<ffffffff8145efca>] + scsi_scan_host_selected+0x5a/0x150 + #1: (&eq->sysfs_lock){+.+.}, at: [<ffffffff812a5032>] + elevator_exit+0x22/0x60 + #2: (&(&q->__queue_lock)->rlock){-.-.}, at: [<ffffffff812b6233>] + cfq_exit_queue+0x43/0x190 + + stack backtrace: + Pid: 1552, comm: scsi_scan_6 Not tainted 3.0.0-rc5 #17 + Call Trace: + [<ffffffff810abb9b>] lockdep_rcu_dereference+0xbb/0xc0 + [<ffffffff812b6139>] __cfq_exit_single_io_context+0xe9/0x120 + [<ffffffff812b626c>] cfq_exit_queue+0x7c/0x190 + [<ffffffff812a5046>] elevator_exit+0x36/0x60 + [<ffffffff812a802a>] blk_cleanup_queue+0x4a/0x60 + [<ffffffff8145cc09>] scsi_free_queue+0x9/0x10 + [<ffffffff81460944>] __scsi_remove_device+0x84/0xd0 + [<ffffffff8145dca3>] scsi_probe_and_add_lun+0x353/0xb10 + [<ffffffff817da069>] ? error_exit+0x29/0xb0 + [<ffffffff817d98ed>] ? _raw_spin_unlock_irqrestore+0x3d/0x80 + [<ffffffff8145e722>] __scsi_scan_target+0x112/0x680 + [<ffffffff812c690d>] ? trace_hardirqs_off_thunk+0x3a/0x3c + [<ffffffff817da069>] ? error_exit+0x29/0xb0 + [<ffffffff812bcc60>] ? kobject_del+0x40/0x40 + [<ffffffff8145ed16>] scsi_scan_channel+0x86/0xb0 + [<ffffffff8145f0b0>] scsi_scan_host_selected+0x140/0x150 + [<ffffffff8145f149>] do_scsi_scan_host+0x89/0x90 + [<ffffffff8145f170>] do_scan_async+0x20/0x160 + [<ffffffff8145f150>] ? do_scsi_scan_host+0x90/0x90 + [<ffffffff810975b6>] kthread+0xa6/0xb0 + [<ffffffff817db154>] kernel_thread_helper+0x4/0x10 + [<ffffffff81066430>] ? finish_task_switch+0x80/0x110 + [<ffffffff817d9c04>] ? retint_restore_args+0xe/0xe + [<ffffffff81097510>] ? __kthread_init_worker+0x70/0x70 + [<ffffffff817db150>] ? gs_change+0xb/0xb + +Line 2776 of block/cfq-iosched.c in v3.0-rc5 is as follows:: if (rcu_dereference(ioc->ioc_data) == cic) { @@ -70,7 +75,7 @@ case. Instead, we hold three locks, one of which might be RCU related. And maybe that lock really does protect this reference. If so, the fix is to inform RCU, perhaps by changing __cfq_exit_single_io_context() to take the struct request_queue "q" from cfq_exit_queue() as an argument, -which would permit us to invoke rcu_dereference_protected as follows: +which would permit us to invoke rcu_dereference_protected as follows:: if (rcu_dereference_protected(ioc->ioc_data, lockdep_is_held(&q->queue_lock)) == cic) { @@ -85,7 +90,7 @@ On the other hand, perhaps we really do need an RCU read-side critical section. In this case, the critical section must span the use of the return value from rcu_dereference(), or at least until there is some reference count incremented or some such. One way to handle this is to -add rcu_read_lock() and rcu_read_unlock() as follows: +add rcu_read_lock() and rcu_read_unlock() as follows:: rcu_read_lock(); if (rcu_dereference(ioc->ioc_data) == cic) { @@ -102,7 +107,7 @@ above lockdep-RCU splat. But in this particular case, we don't actually dereference the pointer returned from rcu_dereference(). Instead, that pointer is just compared to the cic pointer, which means that the rcu_dereference() can be replaced -by rcu_access_pointer() as follows: +by rcu_access_pointer() as follows:: if (rcu_access_pointer(ioc->ioc_data) == cic) { diff --git a/Documentation/RCU/lockdep.txt b/Documentation/RCU/lockdep.rst index 89db949eeca0..69e73a39bd11 100644 --- a/Documentation/RCU/lockdep.txt +++ b/Documentation/RCU/lockdep.rst @@ -1,4 +1,8 @@ +.. SPDX-License-Identifier: GPL-2.0 + +======================== RCU and lockdep checking +======================== All flavors of RCU have lockdep checking available, so that lockdep is aware of when each task enters and leaves any flavor of RCU read-side @@ -8,12 +12,14 @@ tracking to include RCU state, which can sometimes help when debugging deadlocks and the like. In addition, RCU provides the following primitives that check lockdep's -state: +state:: rcu_read_lock_held() for normal RCU. rcu_read_lock_bh_held() for RCU-bh. rcu_read_lock_sched_held() for RCU-sched. + rcu_read_lock_any_held() for any of normal RCU, RCU-bh, and RCU-sched. srcu_read_lock_held() for SRCU. + rcu_read_lock_trace_held() for RCU Tasks Trace. These functions are conservative, and will therefore return 1 if they aren't certain (for example, if CONFIG_DEBUG_LOCK_ALLOC is not set). @@ -45,10 +51,12 @@ checking of rcu_dereference() primitives: is invoked by both RCU-sched readers and updaters. srcu_dereference_check(p, c): Use explicit check expression "c" along with - srcu_read_lock_held()(). This is useful in code that + srcu_read_lock_held(). This is useful in code that is invoked by both SRCU readers and updaters. rcu_dereference_raw(p): Don't check. (Use sparingly, if at all.) + rcu_dereference_raw_check(p): + Don't do lockdep at all. (Use sparingly, if at all.) rcu_dereference_protected(p, c): Use explicit check expression "c", and omit all barriers and compiler constraints. This is useful when the data @@ -57,13 +65,12 @@ checking of rcu_dereference() primitives: rcu_access_pointer(p): Return the value of the pointer and omit all barriers, but retain the compiler constraints that prevent duplicating - or coalescsing. This is useful when when testing the + or coalescing. This is useful when testing the value of the pointer itself, for example, against NULL. The rcu_dereference_check() check expression can be any boolean -expression, but would normally include a lockdep expression. However, -any boolean expression can be used. For a moderately ornate example, -consider the following: +expression, but would normally include a lockdep expression. For a +moderately ornate example, consider the following:: file = rcu_dereference_check(fdt->fd[fd], lockdep_is_held(&files->file_lock) || @@ -82,17 +89,17 @@ RCU read-side critical sections, in case (2) the ->file_lock prevents any change from taking place, and finally, in case (3) the current task is the only task accessing the file_struct, again preventing any change from taking place. If the above statement was invoked only from updater -code, it could instead be written as follows: +code, it could instead be written as follows:: file = rcu_dereference_protected(fdt->fd[fd], lockdep_is_held(&files->file_lock) || atomic_read(&files->count) == 1); This would verify cases #2 and #3 above, and furthermore lockdep would -complain if this was used in an RCU read-side critical section unless one -of these two cases held. Because rcu_dereference_protected() omits all -barriers and compiler constraints, it generates better code than do the -other flavors of rcu_dereference(). On the other hand, it is illegal +complain even if this was used in an RCU read-side critical section unless +one of these two cases held. Because rcu_dereference_protected() omits +all barriers and compiler constraints, it generates better code than do +the other flavors of rcu_dereference(). On the other hand, it is illegal to use rcu_dereference_protected() if either the RCU-protected pointer or the RCU-protected data that it points to can change concurrently. @@ -105,7 +112,7 @@ false and they are called from outside any RCU read-side critical section. For example, the workqueue for_each_pwq() macro is intended to be used either within an RCU read-side critical section or with wq->mutex held. -It is thus implemented as follows: +It is thus implemented as follows:: #define for_each_pwq(pwq, wq) list_for_each_entry_rcu((pwq), &(wq)->pwqs, pwqs_node, diff --git a/Documentation/RCU/rcu.rst b/Documentation/RCU/rcu.rst index 0e03c6ef3147..bf6617b330a7 100644 --- a/Documentation/RCU/rcu.rst +++ b/Documentation/RCU/rcu.rst @@ -10,9 +10,8 @@ A "grace period" must elapse between the two parts, and this grace period must be long enough that any readers accessing the item being deleted have since dropped their references. For example, an RCU-protected deletion from a linked list would first remove the item from the list, wait for -a grace period to elapse, then free the element. See the -:ref:`Documentation/RCU/listRCU.rst <list_rcu_doc>` for more information on -using RCU with linked lists. +a grace period to elapse, then free the element. See listRCU.rst for more +information on using RCU with linked lists. Frequently Asked Questions -------------------------- @@ -50,7 +49,7 @@ Frequently Asked Questions - If I am running on a uniprocessor kernel, which can only do one thing at a time, why should I wait for a grace period? - See :ref:`Documentation/RCU/UP.rst <up_doc>` for more information. + See UP.rst for more information. - How can I see where RCU is currently used in the Linux kernel? @@ -64,13 +63,13 @@ Frequently Asked Questions - What guidelines should I follow when writing code that uses RCU? - See the checklist.txt file in this directory. + See checklist.rst. - Why the name "RCU"? "RCU" stands for "read-copy update". - :ref:`Documentation/RCU/listRCU.rst <list_rcu_doc>` has more information on where - this name came from, search for "read-copy update" to find it. + listRCU.rst has more information on where this name came from, search + for "read-copy update" to find it. - I hear that RCU is patented? What is with that? @@ -78,15 +77,17 @@ Frequently Asked Questions search for the string "Patent" in Documentation/RCU/RTFP.txt to find them. Of these, one was allowed to lapse by the assignee, and the others have been contributed to the Linux kernel under GPL. + Many (but not all) have long since expired. There are now also LGPL implementations of user-level RCU available (https://liburcu.org/). - I hear that RCU needs work in order to support realtime kernels? - Realtime-friendly RCU can be enabled via the CONFIG_PREEMPT_RCU + Realtime-friendly RCU are enabled via the CONFIG_PREEMPTION kernel configuration parameter. - Where can I find more information on RCU? See the Documentation/RCU/RTFP.txt file. - Or point your browser at (http://www.rdrop.com/users/paulmck/RCU/). + Or point your browser at (https://docs.google.com/document/d/1X0lThx8OK0ZgLMqVoXiR4ZrGURHrXK6NyLRbeXe3Xac/edit) + or (https://docs.google.com/document/d/1GCdQC8SDbb54W1shjEXqGZ0Rq8a6kIeYutdSIajfpLA/edit?usp=sharing). diff --git a/Documentation/RCU/rcu_dereference.rst b/Documentation/RCU/rcu_dereference.rst index c9667eb0d444..659d5913784d 100644 --- a/Documentation/RCU/rcu_dereference.rst +++ b/Documentation/RCU/rcu_dereference.rst @@ -3,13 +3,26 @@ PROPER CARE AND FEEDING OF RETURN VALUES FROM rcu_dereference() =============================================================== -Most of the time, you can use values from rcu_dereference() or one of -the similar primitives without worries. Dereferencing (prefix "*"), -field selection ("->"), assignment ("="), address-of ("&"), addition and -subtraction of constants, and casts all work quite naturally and safely. - -It is nevertheless possible to get into trouble with other operations. -Follow these rules to keep your RCU code working properly: +Proper care and feeding of address and data dependencies is critically +important to correct use of things like RCU. To this end, the pointers +returned from the rcu_dereference() family of primitives carry address and +data dependencies. These dependencies extend from the rcu_dereference() +macro's load of the pointer to the later use of that pointer to compute +either the address of a later memory access (representing an address +dependency) or the value written by a later memory access (representing +a data dependency). + +Most of the time, these dependencies are preserved, permitting you to +freely use values from rcu_dereference(). For example, dereferencing +(prefix "*"), field selection ("->"), assignment ("="), address-of +("&"), casts, and addition or subtraction of constants all work quite +naturally and safely. However, because current compilers do not take +either address or data dependencies into account it is still possible +to get into trouble. + +Follow these rules to preserve the address and data dependencies emanating +from your calls to rcu_dereference() and friends, thus keeping your RCU +readers working properly: - You must use one of the rcu_dereference() family of primitives to load an RCU-protected pointer, otherwise CONFIG_PROVE_RCU @@ -19,8 +32,9 @@ Follow these rules to keep your RCU code working properly: can reload the value, and won't your code have fun with two different values for a single pointer! Without rcu_dereference(), DEC Alpha can load a pointer, dereference that pointer, and - return data preceding initialization that preceded the store of - the pointer. + return data preceding initialization that preceded the store + of the pointer. (As noted later, in recent kernels READ_ONCE() + also prevents DEC Alpha from playing these tricks.) In addition, the volatile cast in rcu_dereference() prevents the compiler from deducing the resulting pointer value. Please see @@ -28,7 +42,13 @@ Follow these rules to keep your RCU code working properly: for an example where the compiler can in fact deduce the exact value of the pointer, and thus cause misordering. -- You are only permitted to use rcu_dereference on pointer values. +- In the special case where data is added but is never removed + while readers are accessing the structure, READ_ONCE() may be used + instead of rcu_dereference(). In this case, use of READ_ONCE() + takes on the role of the lockless_dereference() primitive that + was removed in v4.15. + +- You are only permitted to use rcu_dereference() on pointer values. The compiler simply knows too much about integral values to trust it to carry dependencies through integer operations. There are a very few exceptions, namely that you can temporarily @@ -37,7 +57,7 @@ Follow these rules to keep your RCU code working properly: - Set bits and clear bits down in the must-be-zero low-order bits of that pointer. This clearly means that the pointer must have alignment constraints, for example, this does - -not- work in general for char* pointers. + *not* work in general for char* pointers. - XOR bits to translate pointers, as is done in some classic buddy-allocator algorithms. @@ -122,10 +142,16 @@ Follow these rules to keep your RCU code working properly: This sort of comparison occurs frequently when scanning RCU-protected circular linked lists. - Note that if checks for being within an RCU read-side - critical section are not required and the pointer is never - dereferenced, rcu_access_pointer() should be used in place - of rcu_dereference(). + Note that if the pointer comparison is done outside + of an RCU read-side critical section, and the pointer + is never dereferenced, rcu_access_pointer() should be + used in place of rcu_dereference(). In most cases, + it is best to avoid accidental dereferences by testing + the rcu_access_pointer() return value directly, without + assigning it to a variable. + + Within an RCU read-side critical section, there is little + reason to use rcu_access_pointer(). - The comparison is against a pointer that references memory that was initialized "a long time ago." The reason @@ -168,7 +194,7 @@ Follow these rules to keep your RCU code working properly: Please see the "CONTROL DEPENDENCIES" section of Documentation/memory-barriers.txt for more details. - - The pointers are not equal -and- the compiler does + - The pointers are not equal *and* the compiler does not have enough information to deduce the value of the pointer. Note that the volatile cast in rcu_dereference() will normally prevent the compiler from knowing too much. @@ -228,6 +254,7 @@ precautions. To see this, consider the following code fragment:: struct foo *q; int r1, r2; + rcu_read_lock(); p = rcu_dereference(gp2); if (p == NULL) return; @@ -236,7 +263,10 @@ precautions. To see this, consider the following code fragment:: if (p == q) { /* The compiler decides that q->c is same as p->c. */ r2 = p->c; /* Could get 44 on weakly order system. */ + } else { + r2 = p->c - r1; /* Unconditional access to p->c. */ } + rcu_read_unlock(); do_something_with(r1, r2); } @@ -285,6 +315,7 @@ Then one approach is to use locking, for example, as follows:: struct foo *q; int r1, r2; + rcu_read_lock(); p = rcu_dereference(gp2); if (p == NULL) return; @@ -294,7 +325,12 @@ Then one approach is to use locking, for example, as follows:: if (p == q) { /* The compiler decides that q->c is same as p->c. */ r2 = p->c; /* Locking guarantees r2 == 144. */ + } else { + spin_lock(&q->lock); + r2 = q->c - r1; + spin_unlock(&q->lock); } + rcu_read_unlock(); spin_unlock(&p->lock); do_something_with(r1, r2); } @@ -352,9 +388,9 @@ the exact value of "p" even in the not-equals case. This allows the compiler to make the return values independent of the load from "gp", in turn destroying the ordering between this load and the loads of the return values. This can result in "p->b" returning pre-initialization -garbage values. +garbage values on weakly ordered systems. -In short, rcu_dereference() is -not- optional when you are going to +In short, rcu_dereference() is *not* optional when you are going to dereference the resulting pointer. @@ -418,7 +454,7 @@ member of the rcu_dereference() to use in various situations: SPARSE CHECKING OF RCU-PROTECTED POINTERS ----------------------------------------- -The sparse static-analysis tool checks for direct access to RCU-protected +The sparse static-analysis tool checks for non-RCU access to RCU-protected pointers, which can result in "interesting" bugs due to compiler optimizations involving invented loads and perhaps also load tearing. For example, suppose someone mistakenly does something like this:: diff --git a/Documentation/RCU/rcubarrier.rst b/Documentation/RCU/rcubarrier.rst index f64f4413a47c..6da7f66da2a8 100644 --- a/Documentation/RCU/rcubarrier.rst +++ b/Documentation/RCU/rcubarrier.rst @@ -5,37 +5,12 @@ RCU and Unloadable Modules [Originally published in LWN Jan. 14, 2007: http://lwn.net/Articles/217484/] -RCU (read-copy update) is a synchronization mechanism that can be thought -of as a replacement for read-writer locking (among other things), but with -very low-overhead readers that are immune to deadlock, priority inversion, -and unbounded latency. RCU read-side critical sections are delimited -by rcu_read_lock() and rcu_read_unlock(), which, in non-CONFIG_PREEMPT -kernels, generate no code whatsoever. - -This means that RCU writers are unaware of the presence of concurrent -readers, so that RCU updates to shared data must be undertaken quite -carefully, leaving an old version of the data structure in place until all -pre-existing readers have finished. These old versions are needed because -such readers might hold a reference to them. RCU updates can therefore be -rather expensive, and RCU is thus best suited for read-mostly situations. - -How can an RCU writer possibly determine when all readers are finished, -given that readers might well leave absolutely no trace of their -presence? There is a synchronize_rcu() primitive that blocks until all -pre-existing readers have completed. An updater wishing to delete an -element p from a linked list might do the following, while holding an -appropriate lock, of course:: - - list_del_rcu(p); - synchronize_rcu(); - kfree(p); - -But the above code cannot be used in IRQ context -- the call_rcu() -primitive must be used instead. This primitive takes a pointer to an -rcu_head struct placed within the RCU-protected data structure and -another pointer to a function that may be invoked later to free that -structure. Code to delete an element p from the linked list from IRQ -context might then be as follows:: +RCU updaters sometimes use call_rcu() to initiate an asynchronous wait for +a grace period to elapse. This primitive takes a pointer to an rcu_head +struct placed within the RCU-protected data structure and another pointer +to a function that may be invoked later to free that structure. Code to +delete an element p from the linked list from IRQ context might then be +as follows:: list_del_rcu(p); call_rcu(&p->rcu, p_callback); @@ -54,7 +29,7 @@ IRQ context. The function p_callback() might be defined as follows:: Unloading Modules That Use call_rcu() ------------------------------------- -But what if p_callback is defined in an unloadable module? +But what if the p_callback() function is defined in an unloadable module? If we unload the module while some RCU callbacks are pending, the CPUs executing these callbacks are going to be severely @@ -67,20 +42,21 @@ grace period to elapse, it does not wait for the callbacks to complete. One might be tempted to try several back-to-back synchronize_rcu() calls, but this is still not guaranteed to work. If there is a very -heavy RCU-callback load, then some of the callbacks might be deferred -in order to allow other processing to proceed. Such deferral is required -in realtime kernels in order to avoid excessive scheduling latencies. +heavy RCU-callback load, then some of the callbacks might be deferred in +order to allow other processing to proceed. For but one example, such +deferral is required in realtime kernels in order to avoid excessive +scheduling latencies. rcu_barrier() ------------- -We instead need the rcu_barrier() primitive. Rather than waiting for -a grace period to elapse, rcu_barrier() waits for all outstanding RCU -callbacks to complete. Please note that rcu_barrier() does **not** imply -synchronize_rcu(), in particular, if there are no RCU callbacks queued -anywhere, rcu_barrier() is within its rights to return immediately, -without waiting for a grace period to elapse. +This situation can be handled by the rcu_barrier() primitive. Rather +than waiting for a grace period to elapse, rcu_barrier() waits for all +outstanding RCU callbacks to complete. Please note that rcu_barrier() +does **not** imply synchronize_rcu(), in particular, if there are no RCU +callbacks queued anywhere, rcu_barrier() is within its rights to return +immediately, without waiting for anything, let alone a grace period. Pseudo-code using rcu_barrier() is as follows: @@ -89,83 +65,86 @@ Pseudo-code using rcu_barrier() is as follows: 3. Allow the module to be unloaded. There is also an srcu_barrier() function for SRCU, and you of course -must match the flavor of rcu_barrier() with that of call_rcu(). If your -module uses multiple flavors of call_rcu(), then it must also use multiple -flavors of rcu_barrier() when unloading that module. For example, if -it uses call_rcu(), call_srcu() on srcu_struct_1, and call_srcu() on -srcu_struct_2, then the following three lines of code will be required -when unloading:: - - 1 rcu_barrier(); - 2 srcu_barrier(&srcu_struct_1); - 3 srcu_barrier(&srcu_struct_2); - -The rcutorture module makes use of rcu_barrier() in its exit function -as follows:: - - 1 static void - 2 rcu_torture_cleanup(void) - 3 { - 4 int i; - 5 - 6 fullstop = 1; - 7 if (shuffler_task != NULL) { - 8 VERBOSE_PRINTK_STRING("Stopping rcu_torture_shuffle task"); - 9 kthread_stop(shuffler_task); - 10 } - 11 shuffler_task = NULL; +must match the flavor of srcu_barrier() with that of call_srcu(). +If your module uses multiple srcu_struct structures, then it must also +use multiple invocations of srcu_barrier() when unloading that module. +For example, if it uses call_rcu(), call_srcu() on srcu_struct_1, and +call_srcu() on srcu_struct_2, then the following three lines of code +will be required when unloading:: + + 1 rcu_barrier(); + 2 srcu_barrier(&srcu_struct_1); + 3 srcu_barrier(&srcu_struct_2); + +If latency is of the essence, workqueues could be used to run these +three functions concurrently. + +An ancient version of the rcutorture module makes use of rcu_barrier() +in its exit function as follows:: + + 1 static void + 2 rcu_torture_cleanup(void) + 3 { + 4 int i; + 5 + 6 fullstop = 1; + 7 if (shuffler_task != NULL) { + 8 VERBOSE_PRINTK_STRING("Stopping rcu_torture_shuffle task"); + 9 kthread_stop(shuffler_task); + 10 } + 11 shuffler_task = NULL; 12 - 13 if (writer_task != NULL) { - 14 VERBOSE_PRINTK_STRING("Stopping rcu_torture_writer task"); - 15 kthread_stop(writer_task); - 16 } - 17 writer_task = NULL; + 13 if (writer_task != NULL) { + 14 VERBOSE_PRINTK_STRING("Stopping rcu_torture_writer task"); + 15 kthread_stop(writer_task); + 16 } + 17 writer_task = NULL; 18 - 19 if (reader_tasks != NULL) { - 20 for (i = 0; i < nrealreaders; i++) { - 21 if (reader_tasks[i] != NULL) { - 22 VERBOSE_PRINTK_STRING( - 23 "Stopping rcu_torture_reader task"); - 24 kthread_stop(reader_tasks[i]); - 25 } - 26 reader_tasks[i] = NULL; - 27 } - 28 kfree(reader_tasks); - 29 reader_tasks = NULL; - 30 } - 31 rcu_torture_current = NULL; + 19 if (reader_tasks != NULL) { + 20 for (i = 0; i < nrealreaders; i++) { + 21 if (reader_tasks[i] != NULL) { + 22 VERBOSE_PRINTK_STRING( + 23 "Stopping rcu_torture_reader task"); + 24 kthread_stop(reader_tasks[i]); + 25 } + 26 reader_tasks[i] = NULL; + 27 } + 28 kfree(reader_tasks); + 29 reader_tasks = NULL; + 30 } + 31 rcu_torture_current = NULL; 32 - 33 if (fakewriter_tasks != NULL) { - 34 for (i = 0; i < nfakewriters; i++) { - 35 if (fakewriter_tasks[i] != NULL) { - 36 VERBOSE_PRINTK_STRING( - 37 "Stopping rcu_torture_fakewriter task"); - 38 kthread_stop(fakewriter_tasks[i]); - 39 } - 40 fakewriter_tasks[i] = NULL; - 41 } - 42 kfree(fakewriter_tasks); - 43 fakewriter_tasks = NULL; - 44 } + 33 if (fakewriter_tasks != NULL) { + 34 for (i = 0; i < nfakewriters; i++) { + 35 if (fakewriter_tasks[i] != NULL) { + 36 VERBOSE_PRINTK_STRING( + 37 "Stopping rcu_torture_fakewriter task"); + 38 kthread_stop(fakewriter_tasks[i]); + 39 } + 40 fakewriter_tasks[i] = NULL; + 41 } + 42 kfree(fakewriter_tasks); + 43 fakewriter_tasks = NULL; + 44 } 45 - 46 if (stats_task != NULL) { - 47 VERBOSE_PRINTK_STRING("Stopping rcu_torture_stats task"); - 48 kthread_stop(stats_task); - 49 } - 50 stats_task = NULL; + 46 if (stats_task != NULL) { + 47 VERBOSE_PRINTK_STRING("Stopping rcu_torture_stats task"); + 48 kthread_stop(stats_task); + 49 } + 50 stats_task = NULL; 51 - 52 /* Wait for all RCU callbacks to fire. */ - 53 rcu_barrier(); + 52 /* Wait for all RCU callbacks to fire. */ + 53 rcu_barrier(); 54 - 55 rcu_torture_stats_print(); /* -After- the stats thread is stopped! */ + 55 rcu_torture_stats_print(); /* -After- the stats thread is stopped! */ 56 - 57 if (cur_ops->cleanup != NULL) - 58 cur_ops->cleanup(); - 59 if (atomic_read(&n_rcu_torture_error)) - 60 rcu_torture_print_module_parms("End of test: FAILURE"); - 61 else - 62 rcu_torture_print_module_parms("End of test: SUCCESS"); - 63 } + 57 if (cur_ops->cleanup != NULL) + 58 cur_ops->cleanup(); + 59 if (atomic_read(&n_rcu_torture_error)) + 60 rcu_torture_print_module_parms("End of test: FAILURE"); + 61 else + 62 rcu_torture_print_module_parms("End of test: SUCCESS"); + 63 } Line 6 sets a global variable that prevents any RCU callbacks from re-posting themselves. This will not be necessary in most cases, since @@ -190,16 +169,17 @@ Quick Quiz #1: :ref:`Answer to Quick Quiz #1 <answer_rcubarrier_quiz_1>` Your module might have additional complications. For example, if your -module invokes call_rcu() from timers, you will need to first cancel all -the timers, and only then invoke rcu_barrier() to wait for any remaining +module invokes call_rcu() from timers, you will need to first refrain +from posting new timers, cancel (or wait for) all the already-posted +timers, and only then invoke rcu_barrier() to wait for any remaining RCU callbacks to complete. -Of course, if you module uses call_rcu(), you will need to invoke +Of course, if your module uses call_rcu(), you will need to invoke rcu_barrier() before unloading. Similarly, if your module uses call_srcu(), you will need to invoke srcu_barrier() before unloading, and on the same srcu_struct structure. If your module uses call_rcu() -**and** call_srcu(), then you will need to invoke rcu_barrier() **and** -srcu_barrier(). +**and** call_srcu(), then (as noted above) you will need to invoke +rcu_barrier() **and** srcu_barrier(). Implementing rcu_barrier() @@ -211,27 +191,40 @@ queues. His implementation queues an RCU callback on each of the per-CPU callback queues, and then waits until they have all started executing, at which point, all earlier RCU callbacks are guaranteed to have completed. -The original code for rcu_barrier() was as follows:: - - 1 void rcu_barrier(void) - 2 { - 3 BUG_ON(in_interrupt()); - 4 /* Take cpucontrol mutex to protect against CPU hotplug */ - 5 mutex_lock(&rcu_barrier_mutex); - 6 init_completion(&rcu_barrier_completion); - 7 atomic_set(&rcu_barrier_cpu_count, 0); - 8 on_each_cpu(rcu_barrier_func, NULL, 0, 1); - 9 wait_for_completion(&rcu_barrier_completion); - 10 mutex_unlock(&rcu_barrier_mutex); - 11 } - -Line 3 verifies that the caller is in process context, and lines 5 and 10 +The original code for rcu_barrier() was roughly as follows:: + + 1 void rcu_barrier(void) + 2 { + 3 BUG_ON(in_interrupt()); + 4 /* Take cpucontrol mutex to protect against CPU hotplug */ + 5 mutex_lock(&rcu_barrier_mutex); + 6 init_completion(&rcu_barrier_completion); + 7 atomic_set(&rcu_barrier_cpu_count, 1); + 8 on_each_cpu(rcu_barrier_func, NULL, 0, 1); + 9 if (atomic_dec_and_test(&rcu_barrier_cpu_count)) + 10 complete(&rcu_barrier_completion); + 11 wait_for_completion(&rcu_barrier_completion); + 12 mutex_unlock(&rcu_barrier_mutex); + 13 } + +Line 3 verifies that the caller is in process context, and lines 5 and 12 use rcu_barrier_mutex to ensure that only one rcu_barrier() is using the global completion and counters at a time, which are initialized on lines 6 and 7. Line 8 causes each CPU to invoke rcu_barrier_func(), which is shown below. Note that the final "1" in on_each_cpu()'s argument list ensures that all the calls to rcu_barrier_func() will have completed -before on_each_cpu() returns. Line 9 then waits for the completion. +before on_each_cpu() returns. Line 9 removes the initial count from +rcu_barrier_cpu_count, and if this count is now zero, line 10 finalizes +the completion, which prevents line 11 from blocking. Either way, +line 11 then waits (if needed) for the completion. + +.. _rcubarrier_quiz_2: + +Quick Quiz #2: + Why doesn't line 8 initialize rcu_barrier_cpu_count to zero, + thereby avoiding the need for lines 9 and 10? + +:ref:`Answer to Quick Quiz #2 <answer_rcubarrier_quiz_2>` This code was rewritten in 2008 and several times thereafter, but this still gives the general idea. @@ -239,21 +232,21 @@ still gives the general idea. The rcu_barrier_func() runs on each CPU, where it invokes call_rcu() to post an RCU callback, as follows:: - 1 static void rcu_barrier_func(void *notused) - 2 { - 3 int cpu = smp_processor_id(); - 4 struct rcu_data *rdp = &per_cpu(rcu_data, cpu); - 5 struct rcu_head *head; - 6 - 7 head = &rdp->barrier; - 8 atomic_inc(&rcu_barrier_cpu_count); - 9 call_rcu(head, rcu_barrier_callback); - 10 } + 1 static void rcu_barrier_func(void *notused) + 2 { + 3 int cpu = smp_processor_id(); + 4 struct rcu_data *rdp = &per_cpu(rcu_data, cpu); + 5 struct rcu_head *head; + 6 + 7 head = &rdp->barrier; + 8 atomic_inc(&rcu_barrier_cpu_count); + 9 call_rcu(head, rcu_barrier_callback); + 10 } Lines 3 and 4 locate RCU's internal per-CPU rcu_data structure, which contains the struct rcu_head that needed for the later call to call_rcu(). Line 7 picks up a pointer to this struct rcu_head, and line -8 increments a global counter. This counter will later be decremented +8 increments the global counter. This counter will later be decremented by the callback. Line 9 then registers the rcu_barrier_callback() on the current CPU's queue. @@ -261,33 +254,34 @@ The rcu_barrier_callback() function simply atomically decrements the rcu_barrier_cpu_count variable and finalizes the completion when it reaches zero, as follows:: - 1 static void rcu_barrier_callback(struct rcu_head *notused) - 2 { - 3 if (atomic_dec_and_test(&rcu_barrier_cpu_count)) - 4 complete(&rcu_barrier_completion); - 5 } + 1 static void rcu_barrier_callback(struct rcu_head *notused) + 2 { + 3 if (atomic_dec_and_test(&rcu_barrier_cpu_count)) + 4 complete(&rcu_barrier_completion); + 5 } -.. _rcubarrier_quiz_2: +.. _rcubarrier_quiz_3: -Quick Quiz #2: +Quick Quiz #3: What happens if CPU 0's rcu_barrier_func() executes immediately (thus incrementing rcu_barrier_cpu_count to the value one), but the other CPU's rcu_barrier_func() invocations are delayed for a full grace period? Couldn't this result in rcu_barrier() returning prematurely? -:ref:`Answer to Quick Quiz #2 <answer_rcubarrier_quiz_2>` +:ref:`Answer to Quick Quiz #3 <answer_rcubarrier_quiz_3>` The current rcu_barrier() implementation is more complex, due to the need to avoid disturbing idle CPUs (especially on battery-powered systems) and the need to minimally disturb non-idle CPUs in real-time systems. -However, the code above illustrates the concepts. +In addition, a great many optimizations have been applied. However, +the code above illustrates the concepts. rcu_barrier() Summary --------------------- -The rcu_barrier() primitive has seen relatively little use, since most +The rcu_barrier() primitive is used relatively infrequently, since most code using RCU is in the core kernel rather than in modules. However, if you are using RCU from an unloadable module, you need to use rcu_barrier() so that your module may be safely unloaded. @@ -302,7 +296,8 @@ Quick Quiz #1: Is there any other situation where rcu_barrier() might be required? -Answer: Interestingly enough, rcu_barrier() was not originally +Answer: + Interestingly enough, rcu_barrier() was not originally implemented for module unloading. Nikita Danilov was using RCU in a filesystem, which resulted in a similar situation at filesystem-unmount time. Dipankar Sarma coded up rcu_barrier() @@ -318,36 +313,68 @@ Answer: Interestingly enough, rcu_barrier() was not originally .. _answer_rcubarrier_quiz_2: Quick Quiz #2: + Why doesn't line 8 initialize rcu_barrier_cpu_count to zero, + thereby avoiding the need for lines 9 and 10? + +Answer: + Suppose that the on_each_cpu() function shown on line 8 was + delayed, so that CPU 0's rcu_barrier_func() executed and + the corresponding grace period elapsed, all before CPU 1's + rcu_barrier_func() started executing. This would result in + rcu_barrier_cpu_count being decremented to zero, so that line + 11's wait_for_completion() would return immediately, failing to + wait for CPU 1's callbacks to be invoked. + + Note that this was not a problem when the rcu_barrier() code + was first added back in 2005. This is because on_each_cpu() + disables preemption, which acted as an RCU read-side critical + section, thus preventing CPU 0's grace period from completing + until on_each_cpu() had dealt with all of the CPUs. However, + with the advent of preemptible RCU, rcu_barrier() no longer + waited on nonpreemptible regions of code in preemptible kernels, + that being the job of the new rcu_barrier_sched() function. + + However, with the RCU flavor consolidation around v4.20, this + possibility was once again ruled out, because the consolidated + RCU once again waits on nonpreemptible regions of code. + + Nevertheless, that extra count might still be a good idea. + Relying on these sort of accidents of implementation can result + in later surprise bugs when the implementation changes. + +:ref:`Back to Quick Quiz #2 <rcubarrier_quiz_2>` + +.. _answer_rcubarrier_quiz_3: + +Quick Quiz #3: What happens if CPU 0's rcu_barrier_func() executes immediately (thus incrementing rcu_barrier_cpu_count to the value one), but the other CPU's rcu_barrier_func() invocations are delayed for a full grace period? Couldn't this result in rcu_barrier() returning prematurely? -Answer: This cannot happen. The reason is that on_each_cpu() has its last +Answer: + This cannot happen. The reason is that on_each_cpu() has its last argument, the wait flag, set to "1". This flag is passed through to smp_call_function() and further to smp_call_function_on_cpu(), causing this latter to spin until the cross-CPU invocation of rcu_barrier_func() has completed. This by itself would prevent - a grace period from completing on non-CONFIG_PREEMPT kernels, + a grace period from completing on non-CONFIG_PREEMPTION kernels, since each CPU must undergo a context switch (or other quiescent state) before the grace period can complete. However, this is - of no use in CONFIG_PREEMPT kernels. + of no use in CONFIG_PREEMPTION kernels. Therefore, on_each_cpu() disables preemption across its call to smp_call_function() and also across the local call to - rcu_barrier_func(). This prevents the local CPU from context - switching, again preventing grace periods from completing. This + rcu_barrier_func(). Because recent RCU implementations treat + preemption-disabled regions of code as RCU read-side critical + sections, this prevents grace periods from completing. This means that all CPUs have executed rcu_barrier_func() before the first rcu_barrier_callback() can possibly execute, in turn preventing rcu_barrier_cpu_count from prematurely reaching zero. - Currently, -rt implementations of RCU keep but a single global - queue for RCU callbacks, and thus do not suffer from this - problem. However, when the -rt RCU eventually does have per-CPU - callback queues, things will have to change. One simple change - is to add an rcu_read_lock() before line 8 of rcu_barrier() - and an rcu_read_unlock() after line 8 of this same function. If - you can think of a better change, please let me know! + But if on_each_cpu() ever decides to forgo disabling preemption, + as might well happen due to real-time latency considerations, + initializing rcu_barrier_cpu_count to one will save the day. -:ref:`Back to Quick Quiz #2 <rcubarrier_quiz_2>` +:ref:`Back to Quick Quiz #3 <rcubarrier_quiz_3>` diff --git a/Documentation/RCU/rculist_nulls.rst b/Documentation/RCU/rculist_nulls.rst new file mode 100644 index 000000000000..21e40fcc08de --- /dev/null +++ b/Documentation/RCU/rculist_nulls.rst @@ -0,0 +1,215 @@ +.. SPDX-License-Identifier: GPL-2.0 + +================================================= +Using RCU hlist_nulls to protect list and objects +================================================= + +This section describes how to use hlist_nulls to +protect read-mostly linked lists and +objects using SLAB_TYPESAFE_BY_RCU allocations. + +Please read the basics in listRCU.rst. + +Using 'nulls' +============= + +Using special makers (called 'nulls') is a convenient way +to solve following problem. + +Without 'nulls', a typical RCU linked list managing objects which are +allocated with SLAB_TYPESAFE_BY_RCU kmem_cache can use the following +algorithms. Following examples assume 'obj' is a pointer to such +objects, which is having below type. + +:: + + struct object { + struct hlist_node obj_node; + atomic_t refcnt; + unsigned int key; + }; + +1) Lookup algorithm +------------------- + +:: + + begin: + rcu_read_lock(); + obj = lockless_lookup(key); + if (obj) { + if (!try_get_ref(obj)) { // might fail for free objects + rcu_read_unlock(); + goto begin; + } + /* + * Because a writer could delete object, and a writer could + * reuse these object before the RCU grace period, we + * must check key after getting the reference on object + */ + if (obj->key != key) { // not the object we expected + put_ref(obj); + rcu_read_unlock(); + goto begin; + } + } + rcu_read_unlock(); + +Beware that lockless_lookup(key) cannot use traditional hlist_for_each_entry_rcu() +but a version with an additional memory barrier (smp_rmb()) + +:: + + lockless_lookup(key) + { + struct hlist_node *node, *next; + for (pos = rcu_dereference((head)->first); + pos && ({ next = pos->next; smp_rmb(); prefetch(next); 1; }) && + ({ obj = hlist_entry(pos, typeof(*obj), obj_node); 1; }); + pos = rcu_dereference(next)) + if (obj->key == key) + return obj; + return NULL; + } + +And note the traditional hlist_for_each_entry_rcu() misses this smp_rmb():: + + struct hlist_node *node; + for (pos = rcu_dereference((head)->first); + pos && ({ prefetch(pos->next); 1; }) && + ({ obj = hlist_entry(pos, typeof(*obj), obj_node); 1; }); + pos = rcu_dereference(pos->next)) + if (obj->key == key) + return obj; + return NULL; + +Quoting Corey Minyard:: + + "If the object is moved from one list to another list in-between the + time the hash is calculated and the next field is accessed, and the + object has moved to the end of a new list, the traversal will not + complete properly on the list it should have, since the object will + be on the end of the new list and there's not a way to tell it's on a + new list and restart the list traversal. I think that this can be + solved by pre-fetching the "next" field (with proper barriers) before + checking the key." + +2) Insertion algorithm +---------------------- + +We need to make sure a reader cannot read the new 'obj->obj_node.next' value +and previous value of 'obj->key'. Otherwise, an item could be deleted +from a chain, and inserted into another chain. If new chain was empty +before the move, 'next' pointer is NULL, and lockless reader can not +detect the fact that it missed following items in original chain. + +:: + + /* + * Please note that new inserts are done at the head of list, + * not in the middle or end. + */ + obj = kmem_cache_alloc(...); + lock_chain(); // typically a spin_lock() + obj->key = key; + atomic_set_release(&obj->refcnt, 1); // key before refcnt + hlist_add_head_rcu(&obj->obj_node, list); + unlock_chain(); // typically a spin_unlock() + + +3) Removal algorithm +-------------------- + +Nothing special here, we can use a standard RCU hlist deletion. +But thanks to SLAB_TYPESAFE_BY_RCU, beware a deleted object can be reused +very very fast (before the end of RCU grace period) + +:: + + if (put_last_reference_on(obj) { + lock_chain(); // typically a spin_lock() + hlist_del_init_rcu(&obj->obj_node); + unlock_chain(); // typically a spin_unlock() + kmem_cache_free(cachep, obj); + } + + + +-------------------------------------------------------------------------- + +Avoiding extra smp_rmb() +======================== + +With hlist_nulls we can avoid extra smp_rmb() in lockless_lookup(). + +For example, if we choose to store the slot number as the 'nulls' +end-of-list marker for each slot of the hash table, we can detect +a race (some writer did a delete and/or a move of an object +to another chain) checking the final 'nulls' value if +the lookup met the end of chain. If final 'nulls' value +is not the slot number, then we must restart the lookup at +the beginning. If the object was moved to the same chain, +then the reader doesn't care: It might occasionally +scan the list again without harm. + +Note that using hlist_nulls means the type of 'obj_node' field of +'struct object' becomes 'struct hlist_nulls_node'. + + +1) lookup algorithm +------------------- + +:: + + head = &table[slot]; + begin: + rcu_read_lock(); + hlist_nulls_for_each_entry_rcu(obj, node, head, obj_node) { + if (obj->key == key) { + if (!try_get_ref(obj)) { // might fail for free objects + rcu_read_unlock(); + goto begin; + } + if (obj->key != key) { // not the object we expected + put_ref(obj); + rcu_read_unlock(); + goto begin; + } + goto out; + } + } + + // If the nulls value we got at the end of this lookup is + // not the expected one, we must restart lookup. + // We probably met an item that was moved to another chain. + if (get_nulls_value(node) != slot) { + put_ref(obj); + rcu_read_unlock(); + goto begin; + } + obj = NULL; + + out: + rcu_read_unlock(); + +2) Insert algorithm +------------------- + +Same to the above one, but uses hlist_nulls_add_head_rcu() instead of +hlist_add_head_rcu(). + +:: + + /* + * Please note that new inserts are done at the head of list, + * not in the middle or end. + */ + obj = kmem_cache_alloc(cachep); + lock_chain(); // typically a spin_lock() + obj->key = key; + atomic_set_release(&obj->refcnt, 1); // key before refcnt + /* + * insert obj in RCU way (readers might be traversing chain) + */ + hlist_nulls_add_head_rcu(&obj->obj_node, list); + unlock_chain(); // typically a spin_unlock() diff --git a/Documentation/RCU/rculist_nulls.txt b/Documentation/RCU/rculist_nulls.txt deleted file mode 100644 index 23f115dc87cf..000000000000 --- a/Documentation/RCU/rculist_nulls.txt +++ /dev/null @@ -1,172 +0,0 @@ -Using hlist_nulls to protect read-mostly linked lists and -objects using SLAB_TYPESAFE_BY_RCU allocations. - -Please read the basics in Documentation/RCU/listRCU.rst - -Using special makers (called 'nulls') is a convenient way -to solve following problem : - -A typical RCU linked list managing objects which are -allocated with SLAB_TYPESAFE_BY_RCU kmem_cache can -use following algos : - -1) Lookup algo --------------- -rcu_read_lock() -begin: -obj = lockless_lookup(key); -if (obj) { - if (!try_get_ref(obj)) // might fail for free objects - goto begin; - /* - * Because a writer could delete object, and a writer could - * reuse these object before the RCU grace period, we - * must check key after getting the reference on object - */ - if (obj->key != key) { // not the object we expected - put_ref(obj); - goto begin; - } -} -rcu_read_unlock(); - -Beware that lockless_lookup(key) cannot use traditional hlist_for_each_entry_rcu() -but a version with an additional memory barrier (smp_rmb()) - -lockless_lookup(key) -{ - struct hlist_node *node, *next; - for (pos = rcu_dereference((head)->first); - pos && ({ next = pos->next; smp_rmb(); prefetch(next); 1; }) && - ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1; }); - pos = rcu_dereference(next)) - if (obj->key == key) - return obj; - return NULL; - -And note the traditional hlist_for_each_entry_rcu() misses this smp_rmb() : - - struct hlist_node *node; - for (pos = rcu_dereference((head)->first); - pos && ({ prefetch(pos->next); 1; }) && - ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1; }); - pos = rcu_dereference(pos->next)) - if (obj->key == key) - return obj; - return NULL; -} - -Quoting Corey Minyard : - -"If the object is moved from one list to another list in-between the - time the hash is calculated and the next field is accessed, and the - object has moved to the end of a new list, the traversal will not - complete properly on the list it should have, since the object will - be on the end of the new list and there's not a way to tell it's on a - new list and restart the list traversal. I think that this can be - solved by pre-fetching the "next" field (with proper barriers) before - checking the key." - -2) Insert algo : ----------------- - -We need to make sure a reader cannot read the new 'obj->obj_next' value -and previous value of 'obj->key'. Or else, an item could be deleted -from a chain, and inserted into another chain. If new chain was empty -before the move, 'next' pointer is NULL, and lockless reader can -not detect it missed following items in original chain. - -/* - * Please note that new inserts are done at the head of list, - * not in the middle or end. - */ -obj = kmem_cache_alloc(...); -lock_chain(); // typically a spin_lock() -obj->key = key; -/* - * we need to make sure obj->key is updated before obj->next - * or obj->refcnt - */ -smp_wmb(); -atomic_set(&obj->refcnt, 1); -hlist_add_head_rcu(&obj->obj_node, list); -unlock_chain(); // typically a spin_unlock() - - -3) Remove algo --------------- -Nothing special here, we can use a standard RCU hlist deletion. -But thanks to SLAB_TYPESAFE_BY_RCU, beware a deleted object can be reused -very very fast (before the end of RCU grace period) - -if (put_last_reference_on(obj) { - lock_chain(); // typically a spin_lock() - hlist_del_init_rcu(&obj->obj_node); - unlock_chain(); // typically a spin_unlock() - kmem_cache_free(cachep, obj); -} - - - --------------------------------------------------------------------------- -With hlist_nulls we can avoid extra smp_rmb() in lockless_lookup() -and extra smp_wmb() in insert function. - -For example, if we choose to store the slot number as the 'nulls' -end-of-list marker for each slot of the hash table, we can detect -a race (some writer did a delete and/or a move of an object -to another chain) checking the final 'nulls' value if -the lookup met the end of chain. If final 'nulls' value -is not the slot number, then we must restart the lookup at -the beginning. If the object was moved to the same chain, -then the reader doesn't care : It might eventually -scan the list again without harm. - - -1) lookup algo - - head = &table[slot]; - rcu_read_lock(); -begin: - hlist_nulls_for_each_entry_rcu(obj, node, head, member) { - if (obj->key == key) { - if (!try_get_ref(obj)) // might fail for free objects - goto begin; - if (obj->key != key) { // not the object we expected - put_ref(obj); - goto begin; - } - goto out; - } -/* - * if the nulls value we got at the end of this lookup is - * not the expected one, we must restart lookup. - * We probably met an item that was moved to another chain. - */ - if (get_nulls_value(node) != slot) - goto begin; - obj = NULL; - -out: - rcu_read_unlock(); - -2) Insert function : --------------------- - -/* - * Please note that new inserts are done at the head of list, - * not in the middle or end. - */ -obj = kmem_cache_alloc(cachep); -lock_chain(); // typically a spin_lock() -obj->key = key; -/* - * changes to obj->key must be visible before refcnt one - */ -smp_wmb(); -atomic_set(&obj->refcnt, 1); -/* - * insert obj in RCU way (readers might be traversing chain) - */ -hlist_nulls_add_head_rcu(&obj->obj_node, list); -unlock_chain(); // typically a spin_unlock() diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.rst index 5e6429d66c24..b33aeb14fde3 100644 --- a/Documentation/RCU/rcuref.txt +++ b/Documentation/RCU/rcuref.rst @@ -1,4 +1,8 @@ -Reference-count design for elements of lists/arrays protected by RCU. +.. SPDX-License-Identifier: GPL-2.0 + +==================================================================== +Reference-count design for elements of lists/arrays protected by RCU +==================================================================== Please note that the percpu-ref feature is likely your first @@ -12,32 +16,33 @@ please read on. Reference counting on elements of lists which are protected by traditional reader/writer spinlocks or semaphores are straightforward: -CODE LISTING A: -1. 2. -add() search_and_reference() -{ { - alloc_object read_lock(&list_lock); - ... search_for_element - atomic_set(&el->rc, 1); atomic_inc(&el->rc); - write_lock(&list_lock); ... - add_element read_unlock(&list_lock); - ... ... - write_unlock(&list_lock); } -} - -3. 4. -release_referenced() delete() -{ { - ... write_lock(&list_lock); - if(atomic_dec_and_test(&el->rc)) ... - kfree(el); - ... remove_element -} write_unlock(&list_lock); - ... - if (atomic_dec_and_test(&el->rc)) - kfree(el); - ... - } +CODE LISTING A:: + + 1. 2. + add() search_and_reference() + { { + alloc_object read_lock(&list_lock); + ... search_for_element + atomic_set(&el->rc, 1); atomic_inc(&el->rc); + write_lock(&list_lock); ... + add_element read_unlock(&list_lock); + ... ... + write_unlock(&list_lock); } + } + + 3. 4. + release_referenced() delete() + { { + ... write_lock(&list_lock); + if(atomic_dec_and_test(&el->rc)) ... + kfree(el); + ... remove_element + } write_unlock(&list_lock); + ... + if (atomic_dec_and_test(&el->rc)) + kfree(el); + ... + } If this list/array is made lock free using RCU as in changing the write_lock() in add() and delete() to spin_lock() and changing read_lock() @@ -46,34 +51,35 @@ search_and_reference() could potentially hold reference to an element which has already been deleted from the list/array. Use atomic_inc_not_zero() in this scenario as follows: -CODE LISTING B: -1. 2. -add() search_and_reference() -{ { - alloc_object rcu_read_lock(); - ... search_for_element - atomic_set(&el->rc, 1); if (!atomic_inc_not_zero(&el->rc)) { - spin_lock(&list_lock); rcu_read_unlock(); - return FAIL; - add_element } - ... ... - spin_unlock(&list_lock); rcu_read_unlock(); -} } -3. 4. -release_referenced() delete() -{ { - ... spin_lock(&list_lock); - if (atomic_dec_and_test(&el->rc)) ... - call_rcu(&el->head, el_free); remove_element - ... spin_unlock(&list_lock); -} ... - if (atomic_dec_and_test(&el->rc)) - call_rcu(&el->head, el_free); - ... - } +CODE LISTING B:: + + 1. 2. + add() search_and_reference() + { { + alloc_object rcu_read_lock(); + ... search_for_element + atomic_set(&el->rc, 1); if (!atomic_inc_not_zero(&el->rc)) { + spin_lock(&list_lock); rcu_read_unlock(); + return FAIL; + add_element } + ... ... + spin_unlock(&list_lock); rcu_read_unlock(); + } } + 3. 4. + release_referenced() delete() + { { + ... spin_lock(&list_lock); + if (atomic_dec_and_test(&el->rc)) ... + call_rcu(&el->head, el_free); remove_element + ... spin_unlock(&list_lock); + } ... + if (atomic_dec_and_test(&el->rc)) + call_rcu(&el->head, el_free); + ... + } Sometimes, a reference to the element needs to be obtained in the -update (write) stream. In such cases, atomic_inc_not_zero() might be +update (write) stream. In such cases, atomic_inc_not_zero() might be overkill, since we hold the update-side spinlock. One might instead use atomic_inc() in such cases. @@ -82,39 +88,40 @@ search_and_reference() code path. In such cases, the atomic_dec_and_test() may be moved from delete() to el_free() as follows: -CODE LISTING C: -1. 2. -add() search_and_reference() -{ { - alloc_object rcu_read_lock(); - ... search_for_element - atomic_set(&el->rc, 1); atomic_inc(&el->rc); - spin_lock(&list_lock); ... - - add_element rcu_read_unlock(); - ... } - spin_unlock(&list_lock); 4. -} delete() -3. { -release_referenced() spin_lock(&list_lock); -{ ... - ... remove_element - if (atomic_dec_and_test(&el->rc)) spin_unlock(&list_lock); - kfree(el); ... - ... call_rcu(&el->head, el_free); -} ... -5. } -void el_free(struct rcu_head *rhp) -{ - release_referenced(); -} +CODE LISTING C:: + + 1. 2. + add() search_and_reference() + { { + alloc_object rcu_read_lock(); + ... search_for_element + atomic_set(&el->rc, 1); atomic_inc(&el->rc); + spin_lock(&list_lock); ... + + add_element rcu_read_unlock(); + ... } + spin_unlock(&list_lock); 4. + } delete() + 3. { + release_referenced() spin_lock(&list_lock); + { ... + ... remove_element + if (atomic_dec_and_test(&el->rc)) spin_unlock(&list_lock); + kfree(el); ... + ... call_rcu(&el->head, el_free); + } ... + 5. } + void el_free(struct rcu_head *rhp) + { + release_referenced(); + } The key point is that the initial reference added by add() is not removed until after a grace period has elapsed following removal. This means that search_and_reference() cannot find this element, which means that the value of el->rc cannot increase. Thus, once it reaches zero, there are no -readers that can or ever will be able to reference the element. The -element can therefore safely be freed. This in turn guarantees that if +readers that can or ever will be able to reference the element. The +element can therefore safely be freed. This in turn guarantees that if any reader finds the element, that reader may safely acquire a reference without checking the value of the reference counter. @@ -130,21 +137,21 @@ the eventual invocation of kfree(), which is usually not a problem on modern computer systems, even the small ones. In cases where delete() can sleep, synchronize_rcu() can be called from -delete(), so that el_free() can be subsumed into delete as follows: - -4. -delete() -{ - spin_lock(&list_lock); - ... - remove_element - spin_unlock(&list_lock); - ... - synchronize_rcu(); - if (atomic_dec_and_test(&el->rc)) - kfree(el); - ... -} +delete(), so that el_free() can be subsumed into delete as follows:: + + 4. + delete() + { + spin_lock(&list_lock); + ... + remove_element + spin_unlock(&list_lock); + ... + synchronize_rcu(); + if (atomic_dec_and_test(&el->rc)) + kfree(el); + ... + } As additional examples in the kernel, the pattern in listing C is used by reference counting of struct pid, while the pattern in listing B is used by diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.rst index a360a8796710..ca7b7cd806a1 100644 --- a/Documentation/RCU/stallwarn.txt +++ b/Documentation/RCU/stallwarn.rst @@ -1,4 +1,8 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============================== Using RCU's CPU Stall Detector +============================== This document first discusses what sorts of issues RCU's CPU stall detector can locate, and then discusses kernel parameters and Kconfig @@ -7,39 +11,40 @@ this document explains the stall detector's "splat" format. What Causes RCU CPU Stall Warnings? +=================================== So your kernel printed an RCU CPU stall warning. The next question is "What caused it?" The following problems can result in RCU CPU stall warnings: -o A CPU looping in an RCU read-side critical section. +- A CPU looping in an RCU read-side critical section. -o A CPU looping with interrupts disabled. +- A CPU looping with interrupts disabled. -o A CPU looping with preemption disabled. +- A CPU looping with preemption disabled. -o A CPU looping with bottom halves disabled. +- A CPU looping with bottom halves disabled. -o For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the kernel - without invoking schedule(). If the looping in the kernel is - really expected and desirable behavior, you might need to add - some calls to cond_resched(). +- For !CONFIG_PREEMPTION kernels, a CPU looping anywhere in the + kernel without potentially invoking schedule(). If the looping + in the kernel is really expected and desirable behavior, you + might need to add some calls to cond_resched(). -o Booting Linux using a console connection that is too slow to +- Booting Linux using a console connection that is too slow to keep up with the boot-time console-message rate. For example, - a 115Kbaud serial console can be -way- too slow to keep up + a 115Kbaud serial console can be *way* too slow to keep up with boot-time message rates, and will frequently result in RCU CPU stall warning messages. Especially if you have added debug printk()s. -o Anything that prevents RCU's grace-period kthreads from running. +- Anything that prevents RCU's grace-period kthreads from running. This can result in the "All QSes seen" console-log message. This message will include information on when the kthread last ran and how often it should be expected to run. It can also - result in the "rcu_.*kthread starved for" console-log message, + result in the ``rcu_.*kthread starved for`` console-log message, which will include additional debugging information. -o A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might +- A CPU-bound real-time task in a CONFIG_PREEMPTION kernel, which might happen to preempt a low-priority task in the middle of an RCU read-side critical section. This is especially damaging if that low-priority task is not permitted to run on any other CPU, @@ -48,7 +53,7 @@ o A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might While the system is in the process of running itself out of memory, you might see stall-warning messages. -o A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that +- A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that is running at a higher priority than the RCU softirq threads. This will prevent RCU callbacks from ever being invoked, and in a CONFIG_PREEMPT_RCU kernel will further prevent @@ -63,7 +68,7 @@ o A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that can increase your system's context-switch rate and thus degrade performance. -o A periodic interrupt whose handler takes longer than the time +- A periodic interrupt whose handler takes longer than the time interval between successive pairs of interrupts. This can prevent RCU's kthreads and softirq handlers from running. Note that certain high-overhead debugging options, for example @@ -71,29 +76,49 @@ o A periodic interrupt whose handler takes longer than the time considerably longer than normal, which can in turn result in RCU CPU stall warnings. -o Testing a workload on a fast system, tuning the stall-warning +- Testing a workload on a fast system, tuning the stall-warning timeout down to just barely avoid RCU CPU stall warnings, and then running the same workload with the same stall-warning timeout on a slow system. Note that thermal throttling and on-demand governors can cause a single system to be sometimes fast and sometimes slow! -o A hardware or software issue shuts off the scheduler-clock +- A hardware or software issue shuts off the scheduler-clock interrupt on a CPU that is not in dyntick-idle mode. This problem really has happened, and seems to be most likely to result in RCU CPU stall warnings for CONFIG_NO_HZ_COMMON=n kernels. -o A bug in the RCU implementation. - -o A hardware failure. This is quite unlikely, but has occurred - at least once in real life. A CPU failed in a running system, - becoming unresponsive, but not causing an immediate crash. - This resulted in a series of RCU CPU stall warnings, eventually - leading the realization that the CPU had failed. +- A hardware or software issue that prevents time-based wakeups + from occurring. These issues can range from misconfigured or + buggy timer hardware through bugs in the interrupt or exception + path (whether hardware, firmware, or software) through bugs + in Linux's timer subsystem through bugs in the scheduler, and, + yes, even including bugs in RCU itself. It can also result in + the ``rcu_.*timer wakeup didn't happen for`` console-log message, + which will include additional debugging information. -The RCU, RCU-sched, and RCU-tasks implementations have CPU stall warning. -Note that SRCU does -not- have CPU stall warnings. Please note that -RCU only detects CPU stalls when there is a grace period in progress. -No grace period, no CPU stall warnings. +- A low-level kernel issue that either fails to invoke one of the + variants of rcu_eqs_enter(true), rcu_eqs_exit(true), ct_idle_enter(), + ct_idle_exit(), ct_irq_enter(), or ct_irq_exit() on the one + hand, or that invokes one of them too many times on the other. + Historically, the most frequent issue has been an omission + of either irq_enter() or irq_exit(), which in turn invoke + ct_irq_enter() or ct_irq_exit(), respectively. Building your + kernel with CONFIG_RCU_EQS_DEBUG=y can help track down these types + of issues, which sometimes arise in architecture-specific code. + +- A bug in the RCU implementation. + +- A hardware failure. This is quite unlikely, but is not at all + uncommon in large datacenter. In one memorable case some decades + back, a CPU failed in a running system, becoming unresponsive, + but not causing an immediate crash. This resulted in a series + of RCU CPU stall warnings, eventually leading the realization + that the CPU had failed. + +The RCU, RCU-sched, RCU-tasks, and RCU-tasks-trace implementations have +CPU stall warning. Note that SRCU does *not* have CPU stall warnings. +Please note that RCU only detects CPU stalls when there is a grace period +in progress. No grace period, no CPU stall warnings. To diagnose the cause of the stall, inspect the stack traces. The offending function will usually be near the top of the stack. @@ -109,6 +134,7 @@ see include/trace/events/rcu.h. Fine-Tuning the RCU CPU Stall Detector +====================================== The rcuupdate.rcu_cpu_stall_suppress module parameter disables RCU's CPU stall detector, which detects conditions that unduly delay RCU grace @@ -118,6 +144,7 @@ The stall detector's idea of what constitutes "unduly delayed" is controlled by a set of kernel configuration variables and cpp macros: CONFIG_RCU_CPU_STALL_TIMEOUT +---------------------------- This kernel configuration parameter defines the period of time that RCU will wait from the beginning of a grace period until it @@ -129,14 +156,35 @@ CONFIG_RCU_CPU_STALL_TIMEOUT this parameter is checked only at the beginning of a cycle. So if you are 10 seconds into a 40-second stall, setting this sysfs parameter to (say) five will shorten the timeout for the - -next- stall, or the following warning for the current stall + *next* stall, or the following warning for the current stall (assuming the stall lasts long enough). It will not affect the timing of the next warning for the current stall. Stall-warning messages may be enabled and disabled completely via /sys/module/rcupdate/parameters/rcu_cpu_stall_suppress. +CONFIG_RCU_EXP_CPU_STALL_TIMEOUT +-------------------------------- + + Same as the CONFIG_RCU_CPU_STALL_TIMEOUT parameter but only for + the expedited grace period. This parameter defines the period + of time that RCU will wait from the beginning of an expedited + grace period until it issues an RCU CPU stall warning. This time + period is normally 20 milliseconds on Android devices. A zero + value causes the CONFIG_RCU_CPU_STALL_TIMEOUT value to be used, + after conversion to milliseconds. + + This configuration parameter may be changed at runtime via the + /sys/module/rcupdate/parameters/rcu_exp_cpu_stall_timeout, however + this parameter is checked only at the beginning of a cycle. If you + are in a current stall cycle, setting it to a new value will change + the timeout for the -next- stall. + + Stall-warning messages may be enabled and disabled completely via + /sys/module/rcupdate/parameters/rcu_cpu_stall_suppress. + RCU_STALL_DELAY_DELTA +--------------------- Although the lockdep facility is extremely useful, it does add some overhead. Therefore, under CONFIG_PROVE_RCU, the @@ -145,6 +193,7 @@ RCU_STALL_DELAY_DELTA macro, not a kernel configuration parameter.) RCU_STALL_RAT_DELAY +------------------- The CPU stall detector tries to make the offending CPU print its own warnings, as this often gives better-quality stack traces. @@ -155,22 +204,29 @@ RCU_STALL_RAT_DELAY parameter.) rcupdate.rcu_task_stall_timeout +------------------------------- - This boot/sysfs parameter controls the RCU-tasks stall warning - interval. A value of zero or less suppresses RCU-tasks stall - warnings. A positive value sets the stall-warning interval - in seconds. An RCU-tasks stall warning starts with the line: + This boot/sysfs parameter controls the RCU-tasks and + RCU-tasks-trace stall warning intervals. A value of zero or less + suppresses RCU-tasks stall warnings. A positive value sets the + stall-warning interval in seconds. An RCU-tasks stall warning + starts with the line: INFO: rcu_tasks detected stalls on tasks: And continues with the output of sched_show_task() for each task stalling the current RCU-tasks grace period. + An RCU-tasks-trace stall warning starts (and continues) similarly: + + INFO: rcu_tasks_trace detected stalls on tasks + Interpreting RCU's CPU Stall-Detector "Splats" +============================================== -For non-RCU-tasks flavors of RCU, when a CPU detects that it is stalling, -it will print a message similar to the following: +For non-RCU-tasks flavors of RCU, when a CPU detects that some other +CPU is stalling, it will print a message similar to the following:: INFO: rcu_sched detected stalls on CPUs/tasks: 2-...: (3 GPs behind) idle=06c/0/0 softirq=1453/1455 fqs=0 @@ -182,8 +238,10 @@ causing stalls, and that the stall was affecting RCU-sched. This message will normally be followed by stack dumps for each CPU. Please note that PREEMPT_RCU builds can be stalled by tasks as well as by CPUs, and that the tasks will be indicated by PID, for example, "P3421". It is even -possible for an rcu_state stall to be caused by both CPUs -and- tasks, +possible for an rcu_state stall to be caused by both CPUs *and* tasks, in which case the offending CPUs and tasks will all be called out in the list. +In some cases, CPUs will detect themselves stalling, which will result +in a self-detected stall. CPU 2's "(3 GPs behind)" indicates that this CPU has not interacted with the RCU core for the past three grace periods. In contrast, CPU 16's "(0 @@ -196,7 +254,8 @@ dynticks counter, which will have an even-numbered value if the CPU is in dyntick-idle mode and an odd-numbered value otherwise. The hex number between the two "/"s is the value of the nesting, which will be a small non-negative number if in the idle loop (as shown above) and a -very large positive number otherwise. +very large positive number otherwise. The number following the final +"/" is the NMI nesting, which will be a small non-negative number. The "softirq=" portion of the message tracks the number of RCU softirq handlers that the stalled CPU has executed. The number before the "/" @@ -204,7 +263,7 @@ is the number that had executed since boot at the time that this CPU last noted the beginning of a grace period, which might be the current (stalled) grace period, or it might be some earlier grace period (for example, if the CPU might have been in dyntick-idle mode for an extended -time period. The number after the "/" is the number that have executed +time period). The number after the "/" is the number that have executed since boot until the current time. If this latter number stays constant across repeated stall-warning messages, it is possible that RCU's softirq handlers are no longer able to execute on this CPU. This can happen if @@ -222,20 +281,9 @@ period (in this case 2603), the grace-period sequence number (7075), and an estimate of the total number of RCU callbacks queued across all CPUs (625 in this case). -In kernels with CONFIG_RCU_FAST_NO_HZ, more information is printed -for each CPU: - - 0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 dyntick_enabled: 1 - -The "last_accelerate:" prints the low-order 16 bits (in hex) of the -jiffies counter when this CPU last invoked rcu_try_advance_all_cbs() -from rcu_needs_cpu() or last invoked rcu_accelerate_cbs() from -rcu_prepare_for_idle(). "dyntick_enabled: 1" indicates that dyntick-idle -processing is enabled. - If the grace period ends just as the stall warning starts printing, there will be a spurious stall-warning message, which will include -the following: +the following:: INFO: Stall ended before state dump start @@ -248,7 +296,7 @@ which is overkill for this sort of problem. If all CPUs and tasks have passed through quiescent states, but the grace period has nevertheless failed to end, the stall-warning splat -will include something like the following: +will include something like the following:: All QSes seen, last rcu_preempt kthread activity 23807 (4297905177-4297881370), jiffies_till_next_fqs=3, root ->qsmask 0x0 @@ -261,9 +309,10 @@ which is way less than 23807. Finally, the root rcu_node structure's If the relevant grace-period kthread has been unable to run prior to the stall warning, as was the case in the "All QSes seen" line above, -the following additional line is printed: +the following additional line is printed:: - kthread starved for 23807 jiffies! g7075 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1 ->cpu=5 + rcu_sched kthread starved for 23807 jiffies! g7075 f0x0 RCU_GP_WAIT_FQS(3) ->state=0x1 ->cpu=5 + Unless rcu_sched kthread gets sufficient CPU time, OOM is now expected behavior. Starving the grace-period kthreads of CPU time can of course result in RCU CPU stall warnings even when all CPUs and tasks have passed @@ -274,20 +323,47 @@ kthread is waiting for a short timeout, the "state" precedes value of the task_struct ->state field, and the "cpu" indicates that the grace-period kthread last ran on CPU 5. +If the relevant grace-period kthread does not wake from FQS wait in a +reasonable time, then the following additional line is printed:: + + kthread timer wakeup didn't happen for 23804 jiffies! g7076 f0x0 RCU_GP_WAIT_FQS(5) ->state=0x402 + +The "23804" indicates that kthread's timer expired more than 23 thousand +jiffies ago. The rest of the line has meaning similar to the kthread +starvation case. + +Additionally, the following line is printed:: + + Possible timer handling issue on cpu=4 timer-softirq=11142 + +Here "cpu" indicates that the grace-period kthread last ran on CPU 4, +where it queued the fqs timer. The number following the "timer-softirq" +is the current ``TIMER_SOFTIRQ`` count on cpu 4. If this value does not +change on successive RCU CPU stall warnings, there is further reason to +suspect a timer problem. + +These messages are usually followed by stack dumps of the CPUs and tasks +involved in the stall. These stack traces can help you locate the cause +of the stall, keeping in mind that the CPU detecting the stall will have +an interrupt frame that is mainly devoted to detecting the stall. + Multiple Warnings From One Stall +================================ -If a stall lasts long enough, multiple stall-warning messages will be -printed for it. The second and subsequent messages are printed at +If a stall lasts long enough, multiple stall-warning messages will +be printed for it. The second and subsequent messages are printed at longer intervals, so that the time between (say) the first and second message will be about three times the interval between the beginning -of the stall and the first message. +of the stall and the first message. It can be helpful to compare the +stack dumps for the different messages for the same stalled grace period. Stall Warnings for Expedited Grace Periods +========================================== If an expedited grace period detects a stall, it will place a message -like the following in dmesg: +like the following in dmesg:: INFO: rcu_sched detected expedited stalls on CPUs/tasks: { 7-... } 21119 jiffies s: 73 root: 0x2/. @@ -314,3 +390,95 @@ for example, "P3421". It is entirely possible to see stall warnings from normal and from expedited grace periods at about the same time during the same run. + +RCU_CPU_STALL_CPUTIME +===================== + +In kernels built with CONFIG_RCU_CPU_STALL_CPUTIME=y or booted with +rcupdate.rcu_cpu_stall_cputime=1, the following additional information +is supplied with each RCU CPU stall warning:: + + rcu: hardirqs softirqs csw/system + rcu: number: 624 45 0 + rcu: cputime: 69 1 2425 ==> 2500(ms) + +These statistics are collected during the sampling period. The values +in row "number:" are the number of hard interrupts, number of soft +interrupts, and number of context switches on the stalled CPU. The +first three values in row "cputime:" indicate the CPU time in +milliseconds consumed by hard interrupts, soft interrupts, and tasks +on the stalled CPU. The last number is the measurement interval, again +in milliseconds. Because user-mode tasks normally do not cause RCU CPU +stalls, these tasks are typically kernel tasks, which is why only the +system CPU time are considered. + +The sampling period is shown as follows:: + + |<------------first timeout---------->|<-----second timeout----->| + |<--half timeout-->|<--half timeout-->| | + | |<--first period-->| | + | |<-----------second sampling period---------->| + | | | | + snapshot time point 1st-stall 2nd-stall + +The following describes four typical scenarios: + +1. A CPU looping with interrupts disabled. + + :: + + rcu: hardirqs softirqs csw/system + rcu: number: 0 0 0 + rcu: cputime: 0 0 0 ==> 2500(ms) + + Because interrupts have been disabled throughout the measurement + interval, there are no interrupts and no context switches. + Furthermore, because CPU time consumption was measured using interrupt + handlers, the system CPU consumption is misleadingly measured as zero. + This scenario will normally also have "(0 ticks this GP)" printed on + this CPU's summary line. + +2. A CPU looping with bottom halves disabled. + + This is similar to the previous example, but with non-zero number of + and CPU time consumed by hard interrupts, along with non-zero CPU + time consumed by in-kernel execution:: + + rcu: hardirqs softirqs csw/system + rcu: number: 624 0 0 + rcu: cputime: 49 0 2446 ==> 2500(ms) + + The fact that there are zero softirqs gives a hint that these were + disabled, perhaps via local_bh_disable(). It is of course possible + that there were no softirqs, perhaps because all events that would + result in softirq execution are confined to other CPUs. In this case, + the diagnosis should continue as shown in the next example. + +3. A CPU looping with preemption disabled. + + Here, only the number of context switches is zero:: + + rcu: hardirqs softirqs csw/system + rcu: number: 624 45 0 + rcu: cputime: 69 1 2425 ==> 2500(ms) + + This situation hints that the stalled CPU was looping with preemption + disabled. + +4. No looping, but massive hard and soft interrupts. + + :: + + rcu: hardirqs softirqs csw/system + rcu: number: xx xx 0 + rcu: cputime: xx xx 0 ==> 2500(ms) + + Here, the number and CPU time of hard interrupts are all non-zero, + but the number of context switches and the in-kernel CPU time consumed + are zero. The number and cputime of soft interrupts will usually be + non-zero, but could be zero, for example, if the CPU was spinning + within a single hard interrupt handler. + + If this type of RCU CPU stall warning can be reproduced, you can + narrow it down by looking at /proc/interrupts or by writing code to + trace each interrupt, for example, by referring to show_interrupts(). diff --git a/Documentation/RCU/torture.txt b/Documentation/RCU/torture.rst index af712a3c5b6a..49e7beea6ae1 100644 --- a/Documentation/RCU/torture.txt +++ b/Documentation/RCU/torture.rst @@ -1,7 +1,12 @@ +.. SPDX-License-Identifier: GPL-2.0 + +========================== RCU Torture Test Operation +========================== CONFIG_RCU_TORTURE_TEST +======================= The CONFIG_RCU_TORTURE_TEST config option is available for all RCU implementations. It creates an rcutorture kernel module that can @@ -13,9 +18,10 @@ when the module is loaded, and stops when the module is unloaded. Module parameters are prefixed by "rcutorture." in Documentation/admin-guide/kernel-parameters.txt. -OUTPUT +Output +====== -The statistics output is as follows: +The statistics output is as follows:: rcu-torture:--- Start of test: nreaders=16 nfakewriters=4 stat_interval=30 verbose=0 test_no_idle_hz=1 shuffle_interval=3 stutter=5 irqreader=1 fqs_duration=0 fqs_holdoff=0 fqs_stutter=3 test_boost=1/0 test_boost_interval=7 test_boost_duration=4 rcu-torture: rtc: (null) ver: 155441 tfle: 0 rta: 155441 rtaf: 8884 rtf: 155440 rtmbe: 0 rtbe: 0 rtbke: 0 rtbre: 0 rtbf: 0 rtb: 0 nt: 3055767 @@ -36,53 +42,53 @@ automatic determination as to whether RCU operated correctly. The entries are as follows: -o "rtc": The hexadecimal address of the structure currently visible +* "rtc": The hexadecimal address of the structure currently visible to readers. -o "ver": The number of times since boot that the RCU writer task +* "ver": The number of times since boot that the RCU writer task has changed the structure visible to readers. -o "tfle": If non-zero, indicates that the "torture freelist" +* "tfle": If non-zero, indicates that the "torture freelist" containing structures to be placed into the "rtc" area is empty. This condition is important, since it can fool you into thinking that RCU is working when it is not. :-/ -o "rta": Number of structures allocated from the torture freelist. +* "rta": Number of structures allocated from the torture freelist. -o "rtaf": Number of allocations from the torture freelist that have +* "rtaf": Number of allocations from the torture freelist that have failed due to the list being empty. It is not unusual for this to be non-zero, but it is bad for it to be a large fraction of the value indicated by "rta". -o "rtf": Number of frees into the torture freelist. +* "rtf": Number of frees into the torture freelist. -o "rtmbe": A non-zero value indicates that rcutorture believes that +* "rtmbe": A non-zero value indicates that rcutorture believes that rcu_assign_pointer() and rcu_dereference() are not working correctly. This value should be zero. -o "rtbe": A non-zero value indicates that one of the rcu_barrier() +* "rtbe": A non-zero value indicates that one of the rcu_barrier() family of functions is not working correctly. -o "rtbke": rcutorture was unable to create the real-time kthreads +* "rtbke": rcutorture was unable to create the real-time kthreads used to force RCU priority inversion. This value should be zero. -o "rtbre": Although rcutorture successfully created the kthreads +* "rtbre": Although rcutorture successfully created the kthreads used to force RCU priority inversion, it was unable to set them to the real-time priority level of 1. This value should be zero. -o "rtbf": The number of times that RCU priority boosting failed +* "rtbf": The number of times that RCU priority boosting failed to resolve RCU priority inversion. -o "rtb": The number of times that rcutorture attempted to force +* "rtb": The number of times that rcutorture attempted to force an RCU priority inversion condition. If you are testing RCU priority boosting via the "test_boost" module parameter, this value should be non-zero. -o "nt": The number of times rcutorture ran RCU read-side code from +* "nt": The number of times rcutorture ran RCU read-side code from within a timer handler. This value should be non-zero only if you specified the "irqreader" module parameter. -o "Reader Pipe": Histogram of "ages" of structures seen by readers. +* "Reader Pipe": Histogram of "ages" of structures seen by readers. If any entries past the first two are non-zero, RCU is broken. And rcutorture prints the error flag string "!!!" to make sure you notice. The age of a newly allocated structure is zero, @@ -94,14 +100,14 @@ o "Reader Pipe": Histogram of "ages" of structures seen by readers. RCU. If you want to see what it looks like when broken, break it yourself. ;-) -o "Reader Batch": Another histogram of "ages" of structures seen +* "Reader Batch": Another histogram of "ages" of structures seen by readers, but in terms of counter flips (or batches) rather than in terms of grace periods. The legal number of non-zero entries is again two. The reason for this separate view is that it is sometimes easier to get the third entry to show up in the "Reader Batch" list than in the "Reader Pipe" list. -o "Free-Block Circulation": Shows the number of torture structures +* "Free-Block Circulation": Shows the number of torture structures that have reached a given point in the pipeline. The first element should closely correspond to the number of structures allocated, the second to the number that have been removed from reader view, @@ -112,7 +118,7 @@ o "Free-Block Circulation": Shows the number of torture structures Different implementations of RCU can provide implementation-specific additional information. For example, Tree SRCU provides the following -additional line: +additional line:: srcud-torture: Tree SRCU per-CPU(idx=0): 0(35,-21) 1(-4,24) 2(1,1) 3(-26,20) 4(28,-47) 5(-9,4) 6(-10,14) 7(-14,11) T(1,6) @@ -123,15 +129,15 @@ using a dynamically allocated srcu_struct (hence "srcud-" rather than "old" and "current" values to the underlying array, and is useful for debugging. The final "T" entry contains the totals of the counters. - -USAGE ON SPECIFIC KERNEL BUILDS +Usage on Specific Kernel Builds +=============================== It is sometimes desirable to torture RCU on a specific kernel build, for example, when preparing to put that kernel build into production. In that case, the kernel should be built with CONFIG_RCU_TORTURE_TEST=m so that the test can be started using modprobe and terminated using rmmod. -For example, the following script may be used to torture RCU: +For example, the following script may be used to torture RCU:: #!/bin/sh @@ -148,7 +154,8 @@ two are self-explanatory, while the last indicates that while there were no RCU failures, CPU-hotplug problems were detected. -USAGE ON MAINLINE KERNELS +Usage on Mainline Kernels +========================= When using rcutorture to test changes to RCU itself, it is often necessary to build a number of kernels in order to test that change @@ -178,18 +185,18 @@ argument. Not all changes require that all scenarios be run. For example, a change to Tree SRCU might run only the SRCU-N and SRCU-P scenarios using the --configs argument to kvm.sh as follows: "--configs 'SRCU-N SRCU-P'". -Large systems can run multiple copies of of the full set of scenarios, +Large systems can run multiple copies of the full set of scenarios, for example, a system with 448 hardware threads can run five instances -of the full set concurrently. To make this happen: +of the full set concurrently. To make this happen:: kvm.sh --cpus 448 --configs '5*CFLIST' Alternatively, such a system can run 56 concurrent instances of a single -eight-CPU scenario: +eight-CPU scenario:: kvm.sh --cpus 448 --configs '56*TREE04' -Or 28 concurrent instances of each of two eight-CPU scenarios: +Or 28 concurrent instances of each of two eight-CPU scenarios:: kvm.sh --cpus 448 --configs '28*TREE03 28*TREE04' @@ -199,23 +206,34 @@ values for memory may require disabling the callback-flooding tests using the --bootargs parameter discussed below. Sometimes additional debugging is useful, and in such cases the --kconfig -parameter to kvm.sh may be used, for example, "--kconfig 'CONFIG_KASAN=y'". +parameter to kvm.sh may be used, for example, ``--kconfig 'CONFIG_RCU_EQS_DEBUG=y'``. +In addition, there are the --gdb, --kasan, and --kcsan parameters. +Note that --gdb limits you to one scenario per kvm.sh run and requires +that you have another window open from which to run ``gdb`` as instructed +by the script. Kernel boot arguments can also be supplied, for example, to control rcutorture's module parameters. For example, to test a change to RCU's CPU stall-warning code, use "--bootargs 'rcutorture.stall_cpu=30'". This will of course result in the scripting reporting a failure, namely -the resuling RCU CPU stall warning. As noted above, reducing memory may -require disabling rcutorture's callback-flooding tests: +the resulting RCU CPU stall warning. As noted above, reducing memory may +require disabling rcutorture's callback-flooding tests:: kvm.sh --cpus 448 --configs '56*TREE04' --memory 128M \ --bootargs 'rcutorture.fwd_progress=0' Sometimes all that is needed is a full set of kernel builds. This is -what the --buildonly argument does. +what the --buildonly parameter does. -Finally, the --trust-make argument allows each kernel build to reuse what -it can from the previous kernel build. +The --duration parameter can override the default run time of 30 minutes. +For example, ``--duration 2d`` would run for two days, ``--duration 3h`` +would run for three hours, ``--duration 5m`` would run for five minutes, +and ``--duration 45s`` would run for 45 seconds. This last can be useful +for tracking down rare boot-time failures. + +Finally, the --trust-make parameter allows each kernel build to reuse what +it can from the previous kernel build. Please note that without the +--trust-make parameter, your tags files may be demolished. There are additional more arcane arguments that are documented in the source code of the kvm.sh script. @@ -225,7 +243,7 @@ is listed at the end of the kvm.sh output, which you really should redirect to a file. The build products and console output of each run is kept in tools/testing/selftests/rcutorture/res in timestamped directories. A given directory can be supplied to kvm-find-errors.sh in order to have -it cycle you through summaries of errors and full error logs. For example: +it cycle you through summaries of errors and full error logs. For example:: tools/testing/selftests/rcutorture/bin/kvm-find-errors.sh \ tools/testing/selftests/rcutorture/res/2020.01.20-15.54.23 @@ -245,38 +263,112 @@ that was tested and any uncommitted changes in diff format. The most frequently used files in each per-scenario-run directory are: -.config: This file contains the Kconfig options. +.config: + This file contains the Kconfig options. -Make.out: This contains build output for a specific scenario. +Make.out: + This contains build output for a specific scenario. -console.log: This contains the console output for a specific scenario. +console.log: + This contains the console output for a specific scenario. This file may be examined once the kernel has booted, but it might not exist if the build failed. -vmlinux: This contains the kernel, which can be useful with tools like +vmlinux: + This contains the kernel, which can be useful with tools like objdump and gdb. A number of additional files are available, but are less frequently used. Many are intended for debugging of rcutorture itself or of its scripting. As of v5.4, a successful run with the default set of scenarios produces -the following summary at the end of the run on a 12-CPU system: - -SRCU-N ------- 804233 GPs (148.932/s) [srcu: g10008272 f0x0 ] -SRCU-P ------- 202320 GPs (37.4667/s) [srcud: g1809476 f0x0 ] -SRCU-t ------- 1122086 GPs (207.794/s) [srcu: g0 f0x0 ] -SRCU-u ------- 1111285 GPs (205.794/s) [srcud: g1 f0x0 ] -TASKS01 ------- 19666 GPs (3.64185/s) [tasks: g0 f0x0 ] -TASKS02 ------- 20541 GPs (3.80389/s) [tasks: g0 f0x0 ] -TASKS03 ------- 19416 GPs (3.59556/s) [tasks: g0 f0x0 ] -TINY01 ------- 836134 GPs (154.84/s) [rcu: g0 f0x0 ] n_max_cbs: 34198 -TINY02 ------- 850371 GPs (157.476/s) [rcu: g0 f0x0 ] n_max_cbs: 2631 -TREE01 ------- 162625 GPs (30.1157/s) [rcu: g1124169 f0x0 ] -TREE02 ------- 333003 GPs (61.6672/s) [rcu: g2647753 f0x0 ] n_max_cbs: 35844 -TREE03 ------- 306623 GPs (56.782/s) [rcu: g2975325 f0x0 ] n_max_cbs: 1496497 -CPU count limited from 16 to 12 -TREE04 ------- 246149 GPs (45.5831/s) [rcu: g1695737 f0x0 ] n_max_cbs: 434961 -TREE05 ------- 314603 GPs (58.2598/s) [rcu: g2257741 f0x2 ] n_max_cbs: 193997 -TREE07 ------- 167347 GPs (30.9902/s) [rcu: g1079021 f0x0 ] n_max_cbs: 478732 -CPU count limited from 16 to 12 -TREE09 ------- 752238 GPs (139.303/s) [rcu: g13075057 f0x0 ] n_max_cbs: 99011 +the following summary at the end of the run on a 12-CPU system:: + + SRCU-N ------- 804233 GPs (148.932/s) [srcu: g10008272 f0x0 ] + SRCU-P ------- 202320 GPs (37.4667/s) [srcud: g1809476 f0x0 ] + SRCU-t ------- 1122086 GPs (207.794/s) [srcu: g0 f0x0 ] + SRCU-u ------- 1111285 GPs (205.794/s) [srcud: g1 f0x0 ] + TASKS01 ------- 19666 GPs (3.64185/s) [tasks: g0 f0x0 ] + TASKS02 ------- 20541 GPs (3.80389/s) [tasks: g0 f0x0 ] + TASKS03 ------- 19416 GPs (3.59556/s) [tasks: g0 f0x0 ] + TINY01 ------- 836134 GPs (154.84/s) [rcu: g0 f0x0 ] n_max_cbs: 34198 + TINY02 ------- 850371 GPs (157.476/s) [rcu: g0 f0x0 ] n_max_cbs: 2631 + TREE01 ------- 162625 GPs (30.1157/s) [rcu: g1124169 f0x0 ] + TREE02 ------- 333003 GPs (61.6672/s) [rcu: g2647753 f0x0 ] n_max_cbs: 35844 + TREE03 ------- 306623 GPs (56.782/s) [rcu: g2975325 f0x0 ] n_max_cbs: 1496497 + CPU count limited from 16 to 12 + TREE04 ------- 246149 GPs (45.5831/s) [rcu: g1695737 f0x0 ] n_max_cbs: 434961 + TREE05 ------- 314603 GPs (58.2598/s) [rcu: g2257741 f0x2 ] n_max_cbs: 193997 + TREE07 ------- 167347 GPs (30.9902/s) [rcu: g1079021 f0x0 ] n_max_cbs: 478732 + CPU count limited from 16 to 12 + TREE09 ------- 752238 GPs (139.303/s) [rcu: g13075057 f0x0 ] n_max_cbs: 99011 + + +Repeated Runs +============= + +Suppose that you are chasing down a rare boot-time failure. Although you +could use kvm.sh, doing so will rebuild the kernel on each run. If you +need (say) 1,000 runs to have confidence that you have fixed the bug, +these pointless rebuilds can become extremely annoying. + +This is why kvm-again.sh exists. + +Suppose that a previous kvm.sh run left its output in this directory:: + + tools/testing/selftests/rcutorture/res/2022.11.03-11.26.28 + +Then this run can be re-run without rebuilding as follow: + + kvm-again.sh tools/testing/selftests/rcutorture/res/2022.11.03-11.26.28 + +A few of the original run's kvm.sh parameters may be overridden, perhaps +most notably --duration and --bootargs. For example:: + + kvm-again.sh tools/testing/selftests/rcutorture/res/2022.11.03-11.26.28 \ + --duration 45s + +would re-run the previous test, but for only 45 seconds, thus facilitating +tracking down the aforementioned rare boot-time failure. + + +Distributed Runs +================ + +Although kvm.sh is quite useful, its testing is confined to a single +system. It is not all that hard to use your favorite framework to cause +(say) 5 instances of kvm.sh to run on your 5 systems, but this will very +likely unnecessarily rebuild kernels. In addition, manually distributing +the desired rcutorture scenarios across the available systems can be +painstaking and error-prone. + +And this is why the kvm-remote.sh script exists. + +If you the following command works:: + + ssh system0 date + +and if it also works for system1, system2, system3, system4, and system5, +and all of these systems have 64 CPUs, you can type:: + + kvm-remote.sh "system0 system1 system2 system3 system4 system5" \ + --cpus 64 --duration 8h --configs "5*CFLIST" + +This will build each default scenario's kernel on the local system, then +spread each of five instances of each scenario over the systems listed, +running each scenario for eight hours. At the end of the runs, the +results will be gathered, recorded, and printed. Most of the parameters +that kvm.sh will accept can be passed to kvm-remote.sh, but the list of +systems must come first. + +The kvm.sh ``--dryrun scenarios`` argument is useful for working out +how many scenarios may be run in one batch across a group of systems. + +You can also re-run a previous remote run in a manner similar to kvm.sh: + + kvm-remote.sh "system0 system1 system2 system3 system4 system5" \ + tools/testing/selftests/rcutorture/res/2022.11.03-11.26.28-remote \ + --duration 24h + +In this case, most of the kvm-again.sh parameters may be supplied following +the pathname of the old run-results directory. diff --git a/Documentation/RCU/whatisRCU.rst b/Documentation/RCU/whatisRCU.rst index c7f147b8034f..60ce02475142 100644 --- a/Documentation/RCU/whatisRCU.rst +++ b/Documentation/RCU/whatisRCU.rst @@ -6,26 +6,33 @@ What is RCU? -- "Read, Copy, Update" Please note that the "What is RCU?" LWN series is an excellent place to start learning about RCU: -| 1. What is RCU, Fundamentally? http://lwn.net/Articles/262464/ -| 2. What is RCU? Part 2: Usage http://lwn.net/Articles/263130/ -| 3. RCU part 3: the RCU API http://lwn.net/Articles/264090/ -| 4. The RCU API, 2010 Edition http://lwn.net/Articles/418853/ -| 2010 Big API Table http://lwn.net/Articles/419086/ -| 5. The RCU API, 2014 Edition http://lwn.net/Articles/609904/ -| 2014 Big API Table http://lwn.net/Articles/609973/ +| 1. What is RCU, Fundamentally? https://lwn.net/Articles/262464/ +| 2. What is RCU? Part 2: Usage https://lwn.net/Articles/263130/ +| 3. RCU part 3: the RCU API https://lwn.net/Articles/264090/ +| 4. The RCU API, 2010 Edition https://lwn.net/Articles/418853/ +| 2010 Big API Table https://lwn.net/Articles/419086/ +| 5. The RCU API, 2014 Edition https://lwn.net/Articles/609904/ +| 2014 Big API Table https://lwn.net/Articles/609973/ +| 6. The RCU API, 2019 Edition https://lwn.net/Articles/777036/ +| 2019 Big API Table https://lwn.net/Articles/777165/ + +For those preferring video: + +| 1. Unraveling RCU Mysteries: Fundamentals https://www.linuxfoundation.org/webinars/unraveling-rcu-usage-mysteries +| 2. Unraveling RCU Mysteries: Additional Use Cases https://www.linuxfoundation.org/webinars/unraveling-rcu-usage-mysteries-additional-use-cases What is RCU? RCU is a synchronization mechanism that was added to the Linux kernel during the 2.5 development effort that is optimized for read-mostly -situations. Although RCU is actually quite simple once you understand it, -getting there can sometimes be a challenge. Part of the problem is that -most of the past descriptions of RCU have been written with the mistaken -assumption that there is "one true way" to describe RCU. Instead, -the experience has been that different people must take different paths -to arrive at an understanding of RCU. This document provides several -different paths, as follows: +situations. Although RCU is actually quite simple, making effective use +of it requires you to think differently about your code. Another part +of the problem is the mistaken assumption that there is "one true way" to +describe and to use RCU. Instead, the experience has been that different +people must take different paths to arrive at an understanding of RCU, +depending on their experiences and use cases. This document provides +several different paths, as follows: :ref:`1. RCU OVERVIEW <1_whatisRCU>` @@ -39,9 +46,11 @@ different paths, as follows: :ref:`6. ANALOGY WITH READER-WRITER LOCKING <6_whatisRCU>` -:ref:`7. FULL LIST OF RCU APIs <7_whatisRCU>` +:ref:`7. ANALOGY WITH REFERENCE COUNTING <7_whatisRCU>` + +:ref:`8. FULL LIST OF RCU APIs <8_whatisRCU>` -:ref:`8. ANSWERS TO QUICK QUIZZES <8_whatisRCU>` +:ref:`9. ANSWERS TO QUICK QUIZZES <9_whatisRCU>` People who prefer starting with a conceptual overview should focus on Section 1, though most readers will profit by reading this section at @@ -50,8 +59,8 @@ experiment with should focus on Section 2. People who prefer to start with example uses should focus on Sections 3 and 4. People who need to understand the RCU implementation should focus on Section 5, then dive into the kernel source code. People who reason best by analogy should -focus on Section 6. Section 7 serves as an index to the docbook API -documentation, and Section 8 is the traditional answer key. +focus on Section 6 and 7. Section 8 serves as an index to the docbook +API documentation, and Section 9 is the traditional answer key. So, start with the section that makes the most sense to you and your preferred method of learning. If you need to know everything about @@ -153,34 +162,36 @@ rcu_read_lock() ^^^^^^^^^^^^^^^ void rcu_read_lock(void); - Used by a reader to inform the reclaimer that the reader is - entering an RCU read-side critical section. It is illegal - to block while in an RCU read-side critical section, though - kernels built with CONFIG_PREEMPT_RCU can preempt RCU - read-side critical sections. Any RCU-protected data structure - accessed during an RCU read-side critical section is guaranteed to - remain unreclaimed for the full duration of that critical section. - Reference counts may be used in conjunction with RCU to maintain - longer-term references to data structures. + This temporal primitive is used by a reader to inform the + reclaimer that the reader is entering an RCU read-side critical + section. It is illegal to block while in an RCU read-side + critical section, though kernels built with CONFIG_PREEMPT_RCU + can preempt RCU read-side critical sections. Any RCU-protected + data structure accessed during an RCU read-side critical section + is guaranteed to remain unreclaimed for the full duration of that + critical section. Reference counts may be used in conjunction + with RCU to maintain longer-term references to data structures. rcu_read_unlock() ^^^^^^^^^^^^^^^^^ void rcu_read_unlock(void); - Used by a reader to inform the reclaimer that the reader is - exiting an RCU read-side critical section. Note that RCU - read-side critical sections may be nested and/or overlapping. + This temporal primitives is used by a reader to inform the + reclaimer that the reader is exiting an RCU read-side critical + section. Note that RCU read-side critical sections may be nested + and/or overlapping. synchronize_rcu() ^^^^^^^^^^^^^^^^^ void synchronize_rcu(void); - Marks the end of updater code and the beginning of reclaimer - code. It does this by blocking until all pre-existing RCU - read-side critical sections on all CPUs have completed. - Note that synchronize_rcu() will **not** necessarily wait for - any subsequent RCU read-side critical sections to complete. - For example, consider the following sequence of events:: + This temporal primitive marks the end of updater code and the + beginning of reclaimer code. It does this by blocking until + all pre-existing RCU read-side critical sections on all CPUs + have completed. Note that synchronize_rcu() will **not** + necessarily wait for any subsequent RCU read-side critical + sections to complete. For example, consider the following + sequence of events:: CPU 0 CPU 1 CPU 2 ----------------- ------------------------- --------------- @@ -207,13 +218,13 @@ synchronize_rcu() to be useful in all but the most read-intensive situations, synchronize_rcu()'s overhead must also be quite small. - The call_rcu() API is a callback form of synchronize_rcu(), - and is described in more detail in a later section. Instead of - blocking, it registers a function and argument which are invoked - after all ongoing RCU read-side critical sections have completed. - This callback variant is particularly useful in situations where - it is illegal to block or where update-side performance is - critically important. + The call_rcu() API is an asynchronous callback form of + synchronize_rcu(), and is described in more detail in a later + section. Instead of blocking, it registers a function and + argument which are invoked after all ongoing RCU read-side + critical sections have completed. This callback variant is + particularly useful in situations where it is illegal to block + or where update-side performance is critically important. However, the call_rcu() API should not be used lightly, as use of the synchronize_rcu() API generally results in simpler code. @@ -222,7 +233,7 @@ synchronize_rcu() be delayed. This property results in system resilience in face of denial-of-service attacks. Code using call_rcu() should limit update rate in order to gain this same sort of resilience. See - checklist.txt for some approaches to limiting the update rate. + checklist.rst for some approaches to limiting the update rate. rcu_assign_pointer() ^^^^^^^^^^^^^^^^^^^^ @@ -232,11 +243,13 @@ rcu_assign_pointer() would be cool to be able to declare a function in this manner. (Compiler experts will no doubt disagree.) - The updater uses this function to assign a new value to an + The updater uses this spatial macro to assign a new value to an RCU-protected pointer, in order to safely communicate the change - in value from the updater to the reader. This macro does not - evaluate to an rvalue, but it does execute any memory-barrier - instructions required for a given CPU architecture. + in value from the updater to the reader. This is a spatial (as + opposed to temporal) macro. It does not evaluate to an rvalue, + but it does execute any memory-barrier instructions required + for a given CPU architecture. Its ordering properties are that + of a store-release operation. Perhaps just as important, it serves to document (1) which pointers are protected by RCU and (2) the point at which a @@ -251,14 +264,15 @@ rcu_dereference() Like rcu_assign_pointer(), rcu_dereference() must be implemented as a macro. - The reader uses rcu_dereference() to fetch an RCU-protected - pointer, which returns a value that may then be safely - dereferenced. Note that rcu_dereference() does not actually - dereference the pointer, instead, it protects the pointer for - later dereferencing. It also executes any needed memory-barrier - instructions for a given CPU architecture. Currently, only Alpha - needs memory barriers within rcu_dereference() -- on other CPUs, - it compiles to nothing, not even a compiler directive. + The reader uses the spatial rcu_dereference() macro to fetch + an RCU-protected pointer, which returns a value that may + then be safely dereferenced. Note that rcu_dereference() + does not actually dereference the pointer, instead, it + protects the pointer for later dereferencing. It also + executes any needed memory-barrier instructions for a given + CPU architecture. Currently, only Alpha needs memory barriers + within rcu_dereference() -- on other CPUs, it compiles to a + volatile load. Common coding practice uses rcu_dereference() to copy an RCU-protected pointer to a local variable, then dereferences @@ -316,7 +330,7 @@ rcu_dereference() must prohibit. The rcu_dereference_protected() variant takes a lockdep expression to indicate which locks must be acquired by the caller. If the indicated protection is not provided, - a lockdep splat is emitted. See Documentation/RCU/Design/Requirements/Requirements.rst + a lockdep splat is emitted. See Design/Requirements/Requirements.rst and the API's code comments for more details and example usage. .. [2] If the list_for_each_entry_rcu() instance might be used by @@ -351,16 +365,19 @@ reader, updater, and reclaimer. synchronize_rcu() & call_rcu() -The RCU infrastructure observes the time sequence of rcu_read_lock(), +The RCU infrastructure observes the temporal sequence of rcu_read_lock(), rcu_read_unlock(), synchronize_rcu(), and call_rcu() invocations in order to determine when (1) synchronize_rcu() invocations may return to their callers and (2) call_rcu() callbacks may be invoked. Efficient implementations of the RCU infrastructure make heavy use of batching in order to amortize their overhead over many uses of the corresponding APIs. +The rcu_assign_pointer() and rcu_dereference() invocations communicate +spatial changes via stores to and loads from the RCU-protected pointer in +question. There are at least three flavors of RCU usage in the Linux kernel. The diagram above shows the most common one. On the updater side, the rcu_assign_pointer(), -sychronize_rcu() and call_rcu() primitives used are the same for all three +synchronize_rcu() and call_rcu() primitives used are the same for all three flavors. However for protection (on the reader side), the primitives used vary depending on the flavor: @@ -388,7 +405,9 @@ b. RCU applied to networking data structures that may be subjected c. RCU applied to scheduler and interrupt/NMI-handler tasks. Again, most uses will be of (a). The (b) and (c) cases are important -for specialized uses, but are relatively uncommon. +for specialized uses, but are relatively uncommon. The SRCU, RCU-Tasks, +RCU-Tasks-Rude, and RCU-Tasks-Trace have similar relationships among +their assorted primitives. .. _3_whatisRCU: @@ -397,8 +416,7 @@ for specialized uses, but are relatively uncommon. This section shows a simple use of the core RCU API to protect a global pointer to a dynamically allocated structure. More-typical -uses of RCU may be found in :ref:`listRCU.rst <list_rcu_doc>`, -:ref:`arrayRCU.rst <array_rcu_doc>`, and :ref:`NMI-RCU.rst <NMI_rcu_doc>`. +uses of RCU may be found in listRCU.rst, arrayRCU.rst, and NMI-RCU.rst. :: struct foo { @@ -465,7 +483,7 @@ So, to sum up: - Within an RCU read-side critical section, use rcu_dereference() to dereference RCU-protected pointers. -- Use some solid scheme (such as locks or semaphores) to +- Use some solid design (such as locks or semaphores) to keep concurrent updates from interfering with each other. - Use rcu_assign_pointer() to update an RCU-protected pointer. @@ -480,10 +498,9 @@ So, to sum up: RCU read-side critical sections that might be referencing that data item. -See checklist.txt for additional rules to follow when using RCU. -And again, more-typical uses of RCU may be found in :ref:`listRCU.rst -<list_rcu_doc>`, :ref:`arrayRCU.rst <array_rcu_doc>`, and :ref:`NMI-RCU.rst -<NMI_rcu_doc>`. +See checklist.rst for additional rules to follow when using RCU. +And again, more-typical uses of RCU may be found in listRCU.rst, +arrayRCU.rst, and NMI-RCU.rst. .. _4_whatisRCU: @@ -497,8 +514,7 @@ long -- there might be other high-priority work to be done. In such cases, one uses call_rcu() rather than synchronize_rcu(). The call_rcu() API is as follows:: - void call_rcu(struct rcu_head * head, - void (*func)(struct rcu_head *head)); + void call_rcu(struct rcu_head *head, rcu_callback_t func); This function invokes func(head) after a grace period has elapsed. This invocation might happen from either softirq or process context, @@ -578,7 +594,15 @@ to avoid having to write your own callback:: kfree_rcu(old_fp, rcu); -Again, see checklist.txt for additional rules governing the use of RCU. +If the occasional sleep is permitted, the single-argument form may +be used, omitting the rcu_head structure from struct foo. + + kfree_rcu_mightsleep(old_fp); + +This variant almost never blocks, but might do so by invoking +synchronize_rcu() in response to memory-allocation failure. + +Again, see checklist.rst for additional rules governing the use of RCU. .. _5_whatisRCU: @@ -595,7 +619,7 @@ lacking both functionality and performance. However, they are useful in getting a feel for how RCU works. See kernel/rcu/update.c for a production-quality implementation, and see: - http://www.rdrop.com/users/paulmck/RCU + https://docs.google.com/document/d/1X0lThx8OK0ZgLMqVoXiR4ZrGURHrXK6NyLRbeXe3Xac/edit for papers describing the Linux kernel RCU implementation. The OLS'01 and OLS'02 papers are a good introduction, and the dissertation provides @@ -662,7 +686,7 @@ been able to write-acquire the lock otherwise. The smp_mb__after_spinlock() promotes synchronize_rcu() to a full memory barrier in compliance with the "Memory-Barrier Guarantees" listed in: - Documentation/RCU/Design/Requirements/Requirements.rst + Design/Requirements/Requirements.rst It is possible to nest rcu_read_lock(), since reader-writer locks may be recursively acquired. Note also that rcu_read_lock() is immune @@ -678,13 +702,13 @@ Quick Quiz #1: occur when using this algorithm in a real-world Linux kernel? How could this deadlock be avoided? -:ref:`Answers to Quick Quiz <8_whatisRCU>` +:ref:`Answers to Quick Quiz <9_whatisRCU>` 5B. "TOY" EXAMPLE #2: CLASSIC RCU ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ This section presents a "toy" RCU implementation that is based on "classic RCU". It is also short on performance (but only for updates) and -on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT +on features such as hotplug CPU and the ability to run in CONFIG_PREEMPTION kernels. The definitions of rcu_dereference() and rcu_assign_pointer() are the same as those shown in the preceding section, so they are omitted. :: @@ -733,16 +757,16 @@ Quick Quiz #2: Give an example where Classic RCU's read-side overhead is **negative**. -:ref:`Answers to Quick Quiz <8_whatisRCU>` +:ref:`Answers to Quick Quiz <9_whatisRCU>` .. _quiz_3: Quick Quiz #3: If it is illegal to block in an RCU read-side critical section, what the heck do you do in - PREEMPT_RT, where normal spinlocks can block??? + CONFIG_PREEMPT_RT, where normal spinlocks can block??? -:ref:`Answers to Quick Quiz <8_whatisRCU>` +:ref:`Answers to Quick Quiz <9_whatisRCU>` .. _6_whatisRCU: @@ -873,7 +897,86 @@ be used in place of synchronize_rcu(). .. _7_whatisRCU: -7. FULL LIST OF RCU APIs +7. ANALOGY WITH REFERENCE COUNTING +----------------------------------- + +The reader-writer analogy (illustrated by the previous section) is not +always the best way to think about using RCU. Another helpful analogy +considers RCU an effective reference count on everything which is +protected by RCU. + +A reference count typically does not prevent the referenced object's +values from changing, but does prevent changes to type -- particularly the +gross change of type that happens when that object's memory is freed and +re-allocated for some other purpose. Once a type-safe reference to the +object is obtained, some other mechanism is needed to ensure consistent +access to the data in the object. This could involve taking a spinlock, +but with RCU the typical approach is to perform reads with SMP-aware +operations such as smp_load_acquire(), to perform updates with atomic +read-modify-write operations, and to provide the necessary ordering. +RCU provides a number of support functions that embed the required +operations and ordering, such as the list_for_each_entry_rcu() macro +used in the previous section. + +A more focused view of the reference counting behavior is that, +between rcu_read_lock() and rcu_read_unlock(), any reference taken with +rcu_dereference() on a pointer marked as ``__rcu`` can be treated as +though a reference-count on that object has been temporarily increased. +This prevents the object from changing type. Exactly what this means +will depend on normal expectations of objects of that type, but it +typically includes that spinlocks can still be safely locked, normal +reference counters can be safely manipulated, and ``__rcu`` pointers +can be safely dereferenced. + +Some operations that one might expect to see on an object for +which an RCU reference is held include: + + - Copying out data that is guaranteed to be stable by the object's type. + - Using kref_get_unless_zero() or similar to get a longer-term + reference. This may fail of course. + - Acquiring a spinlock in the object, and checking if the object still + is the expected object and if so, manipulating it freely. + +The understanding that RCU provides a reference that only prevents a +change of type is particularly visible with objects allocated from a +slab cache marked ``SLAB_TYPESAFE_BY_RCU``. RCU operations may yield a +reference to an object from such a cache that has been concurrently freed +and the memory reallocated to a completely different object, though of +the same type. In this case RCU doesn't even protect the identity of the +object from changing, only its type. So the object found may not be the +one expected, but it will be one where it is safe to take a reference +(and then potentially acquiring a spinlock), allowing subsequent code +to check whether the identity matches expectations. It is tempting +to simply acquire the spinlock without first taking the reference, but +unfortunately any spinlock in a ``SLAB_TYPESAFE_BY_RCU`` object must be +initialized after each and every call to kmem_cache_alloc(), which renders +reference-free spinlock acquisition completely unsafe. Therefore, when +using ``SLAB_TYPESAFE_BY_RCU``, make proper use of a reference counter. +(Those willing to use a kmem_cache constructor may also use locking, +including cache-friendly sequence locking.) + +With traditional reference counting -- such as that implemented by the +kref library in Linux -- there is typically code that runs when the last +reference to an object is dropped. With kref, this is the function +passed to kref_put(). When RCU is being used, such finalization code +must not be run until all ``__rcu`` pointers referencing the object have +been updated, and then a grace period has passed. Every remaining +globally visible pointer to the object must be considered to be a +potential counted reference, and the finalization code is typically run +using call_rcu() only after all those pointers have been changed. + +To see how to choose between these two analogies -- of RCU as a +reader-writer lock and RCU as a reference counting system -- it is useful +to reflect on the scale of the thing being protected. The reader-writer +lock analogy looks at larger multi-part objects such as a linked list +and shows how RCU can facilitate concurrency while elements are added +to, and removed from, the list. The reference-count analogy looks at +the individual objects and looks at how they can be accessed safely +within whatever whole they are a part of. + +.. _8_whatisRCU: + +8. FULL LIST OF RCU APIs ------------------------- The RCU APIs are documented in docbook-format header comments in the @@ -969,6 +1072,30 @@ sched:: rcu_read_lock_sched_held +RCU-Tasks:: + + Critical sections Grace period Barrier + + N/A call_rcu_tasks rcu_barrier_tasks + synchronize_rcu_tasks + + +RCU-Tasks-Rude:: + + Critical sections Grace period Barrier + + N/A call_rcu_tasks_rude rcu_barrier_tasks_rude + synchronize_rcu_tasks_rude + + +RCU-Tasks-Trace:: + + Critical sections Grace period Barrier + + rcu_read_lock_trace call_rcu_tasks_trace rcu_barrier_tasks_trace + rcu_read_unlock_trace synchronize_rcu_tasks_trace + + SRCU:: Critical sections Grace period Barrier @@ -986,13 +1113,18 @@ SRCU: Initialization/cleanup:: init_srcu_struct cleanup_srcu_struct -All: lockdep-checked RCU-protected pointer access:: +All: lockdep-checked RCU utility APIs:: - rcu_access_pointer - rcu_dereference_raw RCU_LOCKDEP_WARN rcu_sleep_check - RCU_NONIDLE + +All: Unchecked RCU-protected pointer access:: + + rcu_dereference_raw + +All: Unchecked RCU-protected pointer access with dereferencing prohibited:: + + rcu_access_pointer See the comment headers in the source code (or the docbook generated from them) for more information. @@ -1003,42 +1135,50 @@ list can be helpful: a. Will readers need to block? If so, you need SRCU. -b. What about the -rt patchset? If readers would need to block - in an non-rt kernel, you need SRCU. If readers would block - in a -rt kernel, but not in a non-rt kernel, SRCU is not - necessary. (The -rt patchset turns spinlocks into sleeplocks, - hence this distinction.) +b. Will readers need to block and are you doing tracing, for + example, ftrace or BPF? If so, you need RCU-tasks, + RCU-tasks-rude, and/or RCU-tasks-trace. -c. Do you need to treat NMI handlers, hardirq handlers, +c. What about the -rt patchset? If readers would need to block in + an non-rt kernel, you need SRCU. If readers would block when + acquiring spinlocks in a -rt kernel, but not in a non-rt kernel, + SRCU is not necessary. (The -rt patchset turns spinlocks into + sleeplocks, hence this distinction.) + +d. Do you need to treat NMI handlers, hardirq handlers, and code segments with preemption disabled (whether via preempt_disable(), local_irq_save(), local_bh_disable(), or some other mechanism) as if they were explicit RCU readers? - If so, RCU-sched is the only choice that will work for you. - -d. Do you need RCU grace periods to complete even in the face - of softirq monopolization of one or more of the CPUs? For - example, is your code subject to network-based denial-of-service - attacks? If so, you should disable softirq across your readers, - for example, by using rcu_read_lock_bh(). - -e. Is your workload too update-intensive for normal use of + If so, RCU-sched readers are the only choice that will work + for you, but since about v4.20 you use can use the vanilla RCU + update primitives. + +e. Do you need RCU grace periods to complete even in the face of + softirq monopolization of one or more of the CPUs? For example, + is your code subject to network-based denial-of-service attacks? + If so, you should disable softirq across your readers, for + example, by using rcu_read_lock_bh(). Since about v4.20 you + use can use the vanilla RCU update primitives. + +f. Is your workload too update-intensive for normal use of RCU, but inappropriate for other synchronization mechanisms? If so, consider SLAB_TYPESAFE_BY_RCU (which was originally named SLAB_DESTROY_BY_RCU). But please be careful! -f. Do you need read-side critical sections that are respected - even though they are in the middle of the idle loop, during - user-mode execution, or on an offlined CPU? If so, SRCU is the - only choice that will work for you. +g. Do you need read-side critical sections that are respected even + on CPUs that are deep in the idle loop, during entry to or exit + from user-mode execution, or on an offlined CPU? If so, SRCU + and RCU Tasks Trace are the only choices that will work for you, + with SRCU being strongly preferred in almost all cases. -g. Otherwise, use RCU. +h. Otherwise, use RCU. Of course, this all assumes that you have determined that RCU is in fact the right tool for your job. -.. _8_whatisRCU: +.. _9_whatisRCU: -8. ANSWERS TO QUICK QUIZZES +9. ANSWERS TO QUICK QUIZZES ---------------------------- Quick Quiz #1: @@ -1094,7 +1234,7 @@ Quick Quiz #2: overhead is **negative**. Answer: - Imagine a single-CPU system with a non-CONFIG_PREEMPT + Imagine a single-CPU system with a non-CONFIG_PREEMPTION kernel where a routing table is used by process-context code, but can be updated by irq-context code (for example, by an "ICMP REDIRECT" packet). The usual way of handling @@ -1121,10 +1261,10 @@ Answer: Quick Quiz #3: If it is illegal to block in an RCU read-side critical section, what the heck do you do in - PREEMPT_RT, where normal spinlocks can block??? + CONFIG_PREEMPT_RT, where normal spinlocks can block??? Answer: - Just as PREEMPT_RT permits preemption of spinlock + Just as CONFIG_PREEMPT_RT permits preemption of spinlock critical sections, it permits preemption of RCU read-side critical sections. It also permits spinlocks blocking while in RCU read-side critical |
