[PATCH,RESEND] documentation: atomic_add_unless() doesn't imply mb() on failure

[Oleg Nesterov]

(the explicit ack/nack from maintainers is wanted).

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>

 Documentation/atomic_ops.txt      |    3 ++-
 Documentation/memory-barriers.txt |    2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff -puN Documentation/atomic_ops.txt~doc-atomic_add_unless-doesnt-imply-mb-on-failure Documentation/atomic_ops.txt
--- a/Documentation/atomic_ops.txt~doc-atomic_add_unless-doesnt-imply-mb-on-failure
+++ a/Documentation/atomic_ops.txt
@@ -137,7 +137,8 @@ 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)
 
diff -puN Documentation/memory-barriers.txt~doc-atomic_add_unless-doesnt-imply-mb-on-failure Documentation/memory-barriers.txt
--- a/Documentation/memory-barriers.txt~doc-atomic_add_unless-doesnt-imply-mb-on-failure
+++ a/Documentation/memory-barriers.txt
@@ -1492,7 +1492,7 @@ about the state (old or new) implies an 
 	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();
_

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