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locking/Documentation: Clarify relationship of barrier() to control dependencies 

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>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: bobby.prani@gmail.com
Cc: dhowells@redhat.com
Cc: dipankar@in.ibm.com
Cc: dvhart@linux.intel.com
Cc: edumazet@google.com
Cc: fweisbec@gmail.com
Cc: jiangshanlai@gmail.com
Cc: josh@joshtriplett.org
Cc: oleg@redhat.com
Cc: rostedt@goodmis.org
Link: http://lkml.kernel.org/r/1460476375-27803-1-git-send-email-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Paul E. McKenney 2016-04-12 08:52:49 -07:00 committed by Ingo Molnar
parent 5c8a010c24
commit a5052657c1
1 changed files with 4 additions and 3 deletions
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7
Documentation/memory-barriers.txt
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@ -813,9 +813,10 @@ In summary:
the same variable, then those stores must be ordered, either by the same variable, then those stores must be ordered, either by
preceding both of them with smp_mb() or by using smp_store_release() 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 carry out the stores. Please note that it is -not- sufficient
to use barrier() at beginning of each leg of the "if" statement, to use barrier() at beginning of each leg of the "if" statement
as optimizing compilers do not necessarily respect barrier() because, as shown by the example above, optimizing compilers can
in this case. destroy the control dependency while respecting the letter of the
barrier() law.
(*) Control dependencies require at least one run-time conditional (*) Control dependencies require at least one run-time conditional
between the prior load and the subsequent store, and this between the prior load and the subsequent store, and this