diff options
Diffstat (limited to 'features/rt/futex-Fix-bug-on-when-a-requeued-RT-task-times-out.patch')
| -rw-r--r-- | features/rt/futex-Fix-bug-on-when-a-requeued-RT-task-times-out.patch | 117 |
1 files changed, 117 insertions, 0 deletions
diff --git a/features/rt/futex-Fix-bug-on-when-a-requeued-RT-task-times-out.patch b/features/rt/futex-Fix-bug-on-when-a-requeued-RT-task-times-out.patch new file mode 100644 index 00000000..1e9a9e63 --- /dev/null +++ b/features/rt/futex-Fix-bug-on-when-a-requeued-RT-task-times-out.patch @@ -0,0 +1,117 @@ +From 1301c0809848cdcd4a0fae002d13707fa3e3db5d Mon Sep 17 00:00:00 2001 +From: Steven Rostedt <rostedt@goodmis.org> +Date: Tue, 14 Jul 2015 14:26:34 +0200 +Subject: [PATCH 066/191] futex: Fix bug on when a requeued RT task times out + +Requeue with timeout causes a bug with PREEMPT_RT. + +The bug comes from a timed out condition. + + TASK 1 TASK 2 + ------ ------ + futex_wait_requeue_pi() + futex_wait_queue_me() + <timed out> + + double_lock_hb(); + + raw_spin_lock(pi_lock); + if (current->pi_blocked_on) { + } else { + current->pi_blocked_on = PI_WAKE_INPROGRESS; + run_spin_unlock(pi_lock); + spin_lock(hb->lock); <-- blocked! + + plist_for_each_entry_safe(this) { + rt_mutex_start_proxy_lock(); + task_blocks_on_rt_mutex(); + BUG_ON(task->pi_blocked_on)!!!! + +The BUG_ON() actually has a check for PI_WAKE_INPROGRESS, but the +problem is that, after TASK 1 sets PI_WAKE_INPROGRESS, it then tries to +grab the hb->lock, which it fails to do so. As the hb->lock is a mutex, +it will block and set the "pi_blocked_on" to the hb->lock. + +When TASK 2 goes to requeue it, the check for PI_WAKE_INPROGESS fails +because the task1's pi_blocked_on is no longer set to that, but instead, +set to the hb->lock. + +The fix: + +When calling rt_mutex_start_proxy_lock() a check is made to see +if the proxy tasks pi_blocked_on is set. If so, exit out early. +Otherwise set it to a new flag PI_REQUEUE_INPROGRESS, which notifies +the proxy task that it is being requeued, and will handle things +appropriately. + +Signed-off-by: Steven Rostedt <rostedt@goodmis.org> +Signed-off-by: Thomas Gleixner <tglx@linutronix.de> +--- + kernel/locking/rtmutex.c | 31 ++++++++++++++++++++++++++++++- + kernel/locking/rtmutex_common.h | 1 + + 2 files changed, 31 insertions(+), 1 deletion(-) + +diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c +index 7fff3b88b96b..4d2a57e8dcb9 100644 +--- a/kernel/locking/rtmutex.c ++++ b/kernel/locking/rtmutex.c +@@ -138,7 +138,8 @@ static void fixup_rt_mutex_waiters(struct rt_mutex *lock) + + static int rt_mutex_real_waiter(struct rt_mutex_waiter *waiter) + { +- return waiter && waiter != PI_WAKEUP_INPROGRESS; ++ return waiter && waiter != PI_WAKEUP_INPROGRESS && ++ waiter != PI_REQUEUE_INPROGRESS; + } + + /* +@@ -1707,6 +1708,34 @@ int __rt_mutex_start_proxy_lock(struct rt_mutex *lock, + if (try_to_take_rt_mutex(lock, task, NULL)) + return 1; + ++#ifdef CONFIG_PREEMPT_RT ++ /* ++ * In PREEMPT_RT there's an added race. ++ * If the task, that we are about to requeue, times out, ++ * it can set the PI_WAKEUP_INPROGRESS. This tells the requeue ++ * to skip this task. But right after the task sets ++ * its pi_blocked_on to PI_WAKEUP_INPROGRESS it can then ++ * block on the spin_lock(&hb->lock), which in RT is an rtmutex. ++ * This will replace the PI_WAKEUP_INPROGRESS with the actual ++ * lock that it blocks on. We *must not* place this task ++ * on this proxy lock in that case. ++ * ++ * To prevent this race, we first take the task's pi_lock ++ * and check if it has updated its pi_blocked_on. If it has, ++ * we assume that it woke up and we return -EAGAIN. ++ * Otherwise, we set the task's pi_blocked_on to ++ * PI_REQUEUE_INPROGRESS, so that if the task is waking up ++ * it will know that we are in the process of requeuing it. ++ */ ++ raw_spin_lock(&task->pi_lock); ++ if (task->pi_blocked_on) { ++ raw_spin_unlock(&task->pi_lock); ++ return -EAGAIN; ++ } ++ task->pi_blocked_on = PI_REQUEUE_INPROGRESS; ++ raw_spin_unlock(&task->pi_lock); ++#endif ++ + /* We enforce deadlock detection for futexes */ + ret = task_blocks_on_rt_mutex(lock, waiter, task, + RT_MUTEX_FULL_CHAINWALK); +diff --git a/kernel/locking/rtmutex_common.h b/kernel/locking/rtmutex_common.h +index 096b16cfb096..37cd6b3bf6f4 100644 +--- a/kernel/locking/rtmutex_common.h ++++ b/kernel/locking/rtmutex_common.h +@@ -126,6 +126,7 @@ enum rtmutex_chainwalk { + * PI-futex support (proxy locking functions, etc.): + */ + #define PI_WAKEUP_INPROGRESS ((struct rt_mutex_waiter *) 1) ++#define PI_REQUEUE_INPROGRESS ((struct rt_mutex_waiter *) 2) + + extern struct task_struct *rt_mutex_next_owner(struct rt_mutex *lock); + extern void rt_mutex_init_proxy_locked(struct rt_mutex *lock, +-- +2.19.1 + |