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doc: Tasks RCU must protect instructions before trampoline

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Protecting the code in a trampoline can also require protecting a
number of instructions prior to actually entering the trampoline.
For example, these earlier instructions might be computing the address
of the trampoline.  This commit therefore updates RCU's requirements to
record this for posterity.

Link: https://lore.kernel.org/lkml/20200511154824.09a18c46@gandalf.local.home/
Reported-by: Lai Jiangshan <jiangshanlai@gmail.com>
Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Paul E. McKenney 2020-05-11 19:52:34 -07:00
parent b81898e3d2
commit d93d97cbe0
1 changed files with 6 additions and 1 deletions
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7
Documentation/RCU/Design/Requirements/Requirements.rst
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@ -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