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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@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)

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