[tip:locking/core] locking/Documentation: Clarify failed cmpxchg( ) memory ordering semantics

[tip-bot for Will Deacon <tipbot@zytor.com>]

Commit-ID:  ed2de9f74ecbbf3063d29b2334e7b455d7f35189
Gitweb:     http://git.kernel.org/tip/ed2de9f74ecbbf3063d29b2334e7b455d7f35189
Author:     Will Deacon <will.deacon@arm.com>
AuthorDate: Thu, 16 Jul 2015 16:10:06 +0100
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 3 Aug 2015 10:57:09 +0200

locking/Documentation: Clarify failed cmpxchg() memory ordering semantics

A failed cmpxchg does not provide any memory ordering guarantees, a
property that is used to optimise the cmpxchg implementations on Alpha,
PowerPC and arm64.

This patch updates atomic_ops.txt and memory-barriers.txt to reflect
this.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Douglas Hatch <doug.hatch@hp.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Scott J Norton <scott.norton@hp.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman Long <waiman.long@hp.com>
Link: http://lkml.kernel.org/r/20150716151006.GH26390@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 Documentation/atomic_ops.txt      | 4 +++-
 Documentation/memory-barriers.txt | 6 +++---
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/Documentation/atomic_ops.txt b/Documentation/atomic_ops.txt
index dab6da3..b19fc34 100644
--- a/Documentation/atomic_ops.txt
+++ b/Documentation/atomic_ops.txt
@@ -266,7 +266,9 @@ with the given old and new values. Like all atomic_xxx operations,
 atomic_cmpxchg will only satisfy its atomicity semantics as long as all
 other accesses of *v are performed through atomic_xxx operations.
 
-atomic_cmpxchg must provide explicit memory barriers around the operation.
+atomic_cmpxchg must provide explicit memory barriers around the operation,
+although if the comparison fails then no memory ordering guarantees are
+required.
 
 The semantics for atomic_cmpxchg are the same as those defined for 'cas'
 below.
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 13feb69..18fc860 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -2383,9 +2383,7 @@ about the state (old or new) implies an SMP-conditional general memory barrier
 explicit lock operations, described later).  These include:
 
 	xchg();
-	cmpxchg();
 	atomic_xchg();			atomic_long_xchg();
-	atomic_cmpxchg();		atomic_long_cmpxchg();
 	atomic_inc_return();		atomic_long_inc_return();
 	atomic_dec_return();		atomic_long_dec_return();
 	atomic_add_return();		atomic_long_add_return();
@@ -2398,7 +2396,9 @@ explicit lock operations, described later).  These include:
 	test_and_clear_bit();
 	test_and_change_bit();
 
-	/* when succeeds (returns 1) */
+	/* when succeeds */
+	cmpxchg();
+	atomic_cmpxchg();		atomic_long_cmpxchg();
 	atomic_add_unless();		atomic_long_add_unless();
 
 These are used for such things as implementing ACQUIRE-class and RELEASE-class