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

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

[Oleg Nesterov]

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@tv-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)
 

Re: [PATCH] doc: atomic_add_unless() doesn't imply mb() on failure

[David Howells]

Oleg Nesterov <oleg@tv-sign.ru> wrote:

> 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.

Ummm...  I wonder if we should instead change the behaviour of
atomic_add_unless() to include an explicit smp_rmb() between the read and the
conditional jump on all archs where this isn't already implied.  Otherwise,
what governs when the initial test going to be done by the CPU?

Note that not all archs do cmpxchg() here, though I don't think that affects
your argument.

David