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authorOleg Nesterov <oleg@tv-sign.ru>2008-02-24 00:03:29 +0300
committerLinus Torvalds <torvalds@woody.linux-foundation.org>2008-02-23 17:52:36 -0800
commit02c608c1fefe821a4c6fc34c45a0dea3cebf4764 (patch)
tree8fd360da8bc83dd45ecec5621791baa73ddfac70 /Documentation
parent2dda81ca31dc73e695ff8b83351f7aaefbef192a (diff)
downloadlinux-02c608c1fefe821a4c6fc34c45a0dea3cebf4764.tar.gz
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 <oleg@tv-sign.ru> Acked-by: Nick Piggin <npiggin@suse.de> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/atomic_ops.txt3
-rw-r--r--Documentation/memory-barriers.txt2
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();