[PATCH tip/core/rcu 02/14] documentation: Fix control dependency and identical stores

From: Paul E. McKenney
Date: Wed Feb 24 2016 - 00:11:55 EST

The summary of the "CONTROL DEPENDENCIES" section incorrectly states that
barrier() may be used to prevent compiler reordering when more than one
leg of the control-dependent "if" statement start with identical stores.
This is incorrect at high optimization levels. This commit therefore
updates the summary to match the detailed description.

Reported by: Jianyu Zhan <nasa4836@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Documentation/memory-barriers.txt | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 904ee42d078e..e26058d3e253 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -800,9 +800,13 @@ In summary:
use smp_rmb(), smp_wmb(), or, in the case of prior stores and
later loads, smp_mb().

- (*) If both legs of the "if" statement begin with identical stores
- to the same variable, a barrier() statement is required at the
- beginning of each leg of the "if" statement.
+ (*) If both legs of the "if" statement begin with identical stores to
+ the same variable, then those stores must be ordered, either by
+ preceding both of them with smp_mb() or by using smp_store_release()
+ to carry out the stores. Please note that it is -not- sufficient
+ to use barrier() at beginning of each leg of the "if" statement,
+ as optimizing compilers do not necessarily respect barrier()
+ in this case.

(*) Control dependencies require at least one run-time conditional
between the prior load and the subsequent store, and this