From: 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 Cc: David Howells Signed-off-by: Andrew Morton --- Documentation/atomic_ops.txt | 3 ++- Documentation/memory-barriers.txt | 2 +- 2 files changed, 3 insertions(+), 2 deletions(-) diff -puN Documentation/atomic_ops.txt~doc-atomic_add_unless-doesnt-imply-mb-on-failure Documentation/atomic_ops.txt --- a/Documentation/atomic_ops.txt~doc-atomic_add_unless-doesnt-imply-mb-on-failure +++ a/Documentation/atomic_ops.txt @@ -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) diff -puN Documentation/memory-barriers.txt~doc-atomic_add_unless-doesnt-imply-mb-on-failure Documentation/memory-barriers.txt --- a/Documentation/memory-barriers.txt~doc-atomic_add_unless-doesnt-imply-mb-on-failure +++ a/Documentation/memory-barriers.txt @@ -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(); _