[PATCH tip/core/rcu 2/3] memory-barriers: Retain barrier() in fold-to-zero example

From: Paul E. McKenney
Date: Thu Aug 28 2014 - 14:13:07 EST

From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>

The transformation in the fold-to-zero example incorrectly omits the
barrier() directive. This commit therefore adds it back in.

Reported-by: Pranith Kumar <pranith@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Documentation/memory-barriers.txt | 9 ++++++---
1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index d67c508eb660..600b45c6e2ad 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -679,12 +679,15 @@ equal to zero, in which case the compiler is within its rights to
transform the above code into the following:

+ barrier();

-This transformation loses the ordering between the load from variable 'a'
-and the store to variable 'b'. If you are relying on this ordering, you
-should do something like the following:
+This transformation fails to require that the CPU respect the ordering
+between the load from variable 'a' and the store to variable 'b'.
+Yes, the barrier() is still there, but it affects only the compiler,
+not the CPU. Therefore, if you are relying on this ordering, you should
+do something like the following:

BUILD_BUG_ON(MAX <= 1); /* Order load from a with store to b. */

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