documentation: atomic_add_unless() doesn't imply mb() on failure
authorOleg Nesterov <oleg@tv-sign.ru>
Sat, 23 Feb 2008 21:03:29 +0000 (00:03 +0300)
committerLinus Torvalds <torvalds@woody.linux-foundation.org>
Sun, 24 Feb 2008 01:52:36 +0000 (17:52 -0800)
(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 <oleg@tv-sign.ru>
Acked-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Documentation/atomic_ops.txt
Documentation/memory-barriers.txt

index f20c10c2858fc7c60f3003a77c905bf108dfddd7..4ef245010457fc9301f21f6e78f2b1fc16c1d014 100644 (file)
@@ -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)
 
index 4e17beba237902908b33d38b764c285fec433724..1f506f7830ecf8e0589d5188dca6621f1b514f9d 100644 (file)
@@ -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();