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| author | Oleg Nesterov <oleg@tv-sign.ru> | 2008-02-24 00:03:29 +0300 |
|---|---|---|
| committer | Linus Torvalds <torvalds@woody.linux-foundation.org> | 2008-02-23 17:52:36 -0800 |
| commit | 02c608c1fefe821a4c6fc34c45a0dea3cebf4764 (patch) | |
| tree | 8fd360da8bc83dd45ecec5621791baa73ddfac70 /Documentation | |
| parent | 2dda81ca31dc73e695ff8b83351f7aaefbef192a (diff) | |
| download | linux-02c608c1fefe821a4c6fc34c45a0dea3cebf4764.tar.gz linux-02c608c1fefe821a4c6fc34c45a0dea3cebf4764.tar.bz2 linux-02c608c1fefe821a4c6fc34c45a0dea3cebf4764.zip | |
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.txt | 3 | ||||
| -rw-r--r-- | 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 f20c10c2858f..4ef245010457 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 4e17beba2379..1f506f7830ec 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(); |