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

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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@efficios.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 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

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[PATCH memory-barriers.txt 1/7] documentation: Clarify relationship of barrier() to control dependencies "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-04-12 18:00 +0200
  [tip:locking/core] locking/Documentation: Clarify relationship of  barrier() to control dependencies "tip-bot for Paul E. McKenney" <tipbot@zytor.com> - 2016-04-13 09:30 +0200
  Re: [PATCH memory-barriers.txt 1/7] documentation: Clarify  relationship of barrier() to control dependencies Steven Rostedt <rostedt@goodmis.org> - 2016-04-14 06:00 +0200
    Re: [PATCH memory-barriers.txt 1/7] documentation: Clarify  relationship of barrier() to control dependencies "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2016-04-14 17:30 +0200

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