wait_on_bit: add an acquire memory barrier

There are several places in the kernel where wait_on_bit is not followed
by a memory barrier (for example, in drivers/md/dm-bufio.c:new_read).

On architectures with weak memory ordering, it may happen that memory
accesses that follow wait_on_bit are reordered before wait_on_bit and
they may return invalid data.

Fix this class of bugs by introducing a new function "test_bit_acquire"
that works like test_bit, but has acquire memory ordering semantics.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Acked-by: Will Deacon <will@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 6 deletions

diff --git a/Documentation/atomic_bitops.txt b/Documentation/atomic_bitops.txt
index d8b101c97031..edea4656c5c0 100644
--- a/Documentation/atomic_bitops.txt
+++ b/Documentation/atomic_bitops.txt
@@ -58,13 +58,11 @@ Like with atomic_t, the rule of thumb is:
 
  - RMW operations that have a return value are fully ordered.
 
- - RMW operations that are conditional are unordered on FAILURE,
-   otherwise the above rules apply. In the case of test_and_set_bit_lock(),
-   if the bit in memory is unchanged by the operation then it is deemed to have
-   failed.
+ - RMW operations that are conditional are fully ordered.
 
-Except for a successful test_and_set_bit_lock() which has ACQUIRE semantics and
-clear_bit_unlock() which has RELEASE semantics.
+Except for a successful test_and_set_bit_lock() which has ACQUIRE semantics,
+clear_bit_unlock() which has RELEASE semantics and test_bit_acquire which has
+ACQUIRE semantics.
 
 Since a platform only has a single means of achieving atomic operations
 the same barriers as for atomic_t are used, see atomic_t.txt.