linux-kernel - [PATCH] doc: atomic_add_unless() doesn't imply mb() on failure

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Date:	Thu, 30 Nov 2006 03:35:06 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Andrew Morton <akpm@...l.org>
Cc:	David Howells <dhowells@...hat.com>,
	Alan Stern <stern@...land.harvard.edu>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH] doc: atomic_add_unless() doesn't imply mb() on failure

Most implementations of atomic_add_unless() can fail (return 0) after the first
atomic_read() (before cmpxchg). In that case we have a compiler barrier only.

Signed-off-by: Oleg Nesterov <oleg@...sign.ru>

 Documentation/atomic_ops.txt      |    3 ++-
 Documentation/memory-barriers.txt |    2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

--- 19-rc6/Documentation/memory-barriers.txt~doc	2006-11-27 21:20:20.000000000 +0300
+++ 19-rc6/Documentation/memory-barriers.txt	2006-11-30 03:32:06.000000000 +0300
@@ -1492,7 +1492,7 @@ about the state (old or new) implies an 
 	atomic_dec_and_test();
 	atomic_sub_and_test();
 	atomic_add_negative();
-	atomic_add_unless();
+	atomic_add_unless();	/* when succeeds (returns 1) */
 	test_and_set_bit();
 	test_and_clear_bit();
 	test_and_change_bit();
--- 19-rc6/Documentation/atomic_ops.txt~doc	2006-07-29 05:05:33.000000000 +0400
+++ 19-rc6/Documentation/atomic_ops.txt	2006-11-30 03:22:58.000000000 +0300
@@ -137,7 +137,8 @@ If the atomic value v is not equal to u,
 returns non zero. If v is equal to u then it returns zero. This is done as
 an atomic operation.
 
-atomic_add_unless requires explicit memory barriers around the operation.
+atomic_add_unless requires explicit memory barriers around the operation
+unless it fails (returns 0).
 
 atomic_inc_not_zero, equivalent to atomic_add_unless(v, 1, 0)
 

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