linux-kernel - [PATCH, 3rd resend] documentation: atomic_add

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Date:	Sun, 24 Feb 2008 00:03:29 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	a.p.zijlstra@...llo.nl, apw@...dowen.org,
	Ingo Molnar <mingo@...e.hu>, nickpiggin@...oo.com.au,
	paulmck@...ux.vnet.ibm.com, rusty@...tcorp.com.au,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-arch@...r.kernel.org
Subject: [PATCH, 3rd resend] documentation: atomic_add_unless() doesn't imply mb() on failure

(sorry for being offtpoic, but while experts are here...)

A "typical" implementation of atomic_add_unless() can return 0 immediately
after the first atomic_read() (before doing cmpxchg). In that case it doesn't
provide any barrier semantics. See include/asm-ia64/atomic.h as an example.

We should either change the implementation, or fix the docs.

Signed-off-by: Oleg Nesterov <oleg@...sign.ru>
Acked-by: Nick Piggin <npiggin@...e.de>

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

--- t/Documentation/atomic_ops.txt~doc_aau	2008-01-14 23:43:11.000000000 +0300
+++ t/Documentation/atomic_ops.txt	2008-02-23 23:53:12.000000000 +0300
@@ -186,7 +186,8 @@ 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)
 
--- t/Documentation/memory-barriers.txt~doc_aau	2008-01-14 23:43:11.000000000 +0300
+++ t/Documentation/memory-barriers.txt	2008-02-23 23:53:12.000000000 +0300
@@ -1493,7 +1493,7 @@ explicit lock operations, described late
 	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();

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