[tip: core/rcu] doc: Tasks RCU must protect instructions before trampoline

[tip-bot2 for Paul E. McKenney]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     d93d97cbe0d4369153fb04954f1481a9f42aa5b6
Gitweb:        https://git.kernel.org/tip/d93d97cbe0d4369153fb04954f1481a9f42aa5b6
Author:        Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate:    Mon, 11 May 2020 19:52:34 -07:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Mon, 29 Jun 2020 11:58:11 -07:00

doc: Tasks RCU must protect instructions before trampoline

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@xxxxxxxxxxxxxxxxxx/
Reported-by: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
Reported-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 Documentation/RCU/Design/Requirements/Requirements.rst | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst
index 75b8ca0..a69b5c4 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.rst
+++ b/Documentation/RCU/Design/Requirements/Requirements.rst
@@ -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