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-rw-r--r--Documentation/RCU/Design/Requirements/Requirements.rst9
1 files changed, 7 insertions, 2 deletions
diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst
index 75b8ca007a11..8f41ad0aa753 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.rst
+++ b/Documentation/RCU/Design/Requirements/Requirements.rst
@@ -463,7 +463,7 @@ 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
+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
@@ -2583,7 +2583,12 @@ 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.
+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