(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@xxxxxxxxxx> Acked-by: Nick Piggin <npiggin@xxxxxxx> Documentation/atomic_ops.txt | 3 ++- Documentation/memory-barriers.txt | 2 +- 2 files changed, 3 insertions(+), 2 deletions(-) --- t/Documentation/atomic_ops.txt~doc_aau 2008-01-14 23:43:11.000000000 +0300 +++ t/Documentation/atomic_ops.txt 2008-02-23 23:53:12.000000000 +0300 @@ -186,7 +186,8 @@ If the atomic value v is not equal to u, 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) --- t/Documentation/memory-barriers.txt~doc_aau 2008-01-14 23:43:11.000000000 +0300 +++ t/Documentation/memory-barriers.txt 2008-02-23 23:53:12.000000000 +0300 @@ -1493,7 +1493,7 @@ explicit lock operations, described late 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(); - To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
