From 02c608c1fefe821a4c6fc34c45a0dea3cebf4764 Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Sun, 24 Feb 2008 00:03:29 +0300 Subject: [PATCH] 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 --- Documentation/atomic_ops.txt | 3 ++- Documentation/memory-barriers.txt | 2 +- 2 files changed, 3 insertions(+), 2 deletions(-) diff --git a/Documentation/atomic_ops.txt b/Documentation/atomic_ops.txt index f20c10c2858..4ef24501045 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 4e17beba237..1f506f7830e 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(); -- 2.20.1