[PATCH memory-barriers.txt 1/7] documentation: Clarify relationship of barrier() to control dependencies

[Paul E. McKenney]

The current documentation claims that the compiler ignores barrier(),
which is not the case.  Instead, the compiler carefully pays attention
to barrier(), but in a creative way that still manages to destroy
the control dependency.  This commit sets the story straight.

Reported-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
 Documentation/memory-barriers.txt | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 3729cbe60e41..ec1289042396 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -813,9 +813,10 @@ In summary:
       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.
+      to use barrier() at beginning of each leg of the "if" statement
+      because, as shown by the example above, optimizing compilers can
+      destroy the control dependency while respecting the letter of the
+      barrier() law.
 
   (*) Control dependencies require at least one run-time conditional
       between the prior load and the subsequent store, and this
-- 
2.5.2