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 <[e-mail]>
Acked-by: Nick Piggin <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/atomic_ops.txt

···
 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)
 

Documentation/memory-barriers.txt

···
 	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();