From: Oleg Nesterov Date: Sat, 23 Feb 2008 21:03:29 +0000 (+0300) Subject: documentation: atomic_add_unless() doesn't imply mb() on failure X-Git-Tag: v2.6.25-rc3~15 X-Git-Url: https://err.no/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=02c608c1fefe821a4c6fc34c45a0dea3cebf4764;p=linux-2.6 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 Acked-by: Nick Piggin Signed-off-by: Linus Torvalds --- diff --git a/Documentation/atomic_ops.txt b/Documentation/atomic_ops.txt index f20c10c285..4ef2450104 100644 --- a/Documentation/atomic_ops.txt +++ b/Documentation/atomic_ops.txt @@ -186,7 +186,8 @@ If the atomic value v is not equal to u, this function adds a to v, and 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 --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index 4e17beba23..1f506f7830 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -1493,7 +1493,7 @@ explicit lock operations, described later). These include: 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();