Date: Sun, 26 Nov 2006 00:18:18 +0300
From: Oleg Nesterov <oleg@tv-sign.ru>
To: "David S. Miller" <davem@davemloft.net>,
       David Howells <dhowells@redhat.com>
Cc: Alan Stern <stern@rowland.harvard.edu>,
       "Paul E. McKenney" <paulmck@us.ibm.com>, linux-kernel@vger.kernel.org
Subject: atomic_add_unless() and mb()
Message-ID: <20061125211818.GA167@oleg>
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Both Documentation/memory-barriers.txt and Documentation/atomic_ops.txt state
that atomic_add_unless() implies smp_mb() on each side of the operation.

Is it true?

include/asm-most_of/atomic.h:

	#define atomic_add_unless(v, a, u)				\
	({								\
		int c, old;						\
		c = atomic_read(v);					\
		for (;;) {						\
			if (unlikely(c == (u)))				\
				break;					\
			old = atomic_cmpxchg((v), c, c + (a));		\
			if (likely(old == c))				\
				break;					\
			c = old;					\
		}							\
		c != (u);						\
	})

This looks like atomic_add_unless() implies mb() only if it returns 1.
Otherwise it could fail (return 0) before the first atomic_cmpxchg(),
but atomic_read() provides a compiler barrier only.

Could you clarify?

Oleg.

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