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documentation: atomic_add_unless() doesn't imply mb() on failure

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(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>
This commit is contained in:
Oleg Nesterov 2008-02-24 00:03:29 +03:00 committed by Linus Torvalds
parent 2dda81ca31
commit 02c608c1fe
2 changed files with 3 additions and 2 deletions
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3
Documentation/atomic_ops.txt
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@ -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)

2
Documentation/memory-barriers.txt
View 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();