When performing an unlock on an extent buffer we'd like to order the decrement of extent_buffer::blocking_writers with waking up any waiters. In such situations it's sufficient to use smp_mb__after_atomic rather than the heavy smp_mb. On architectures where atomic operations are fully ordered (such as x86 or s390) unconditionally executing a heavyweight smp_mb instruction causes a severe hit to performance while bringin no improvements in terms of correctness.
The better thing is to use the appropriate smp_mb__after_atomic routine which will do the correct thing (invoke a full smp_mb or in the case of ordered atomics insert a compiler barrier). Put another way, an RMW atomic op + smp_load__after_atomic equals, in terms of semantics, to a full smp_mb. This ensures that none of the problems described in the accompanying comment of waitqueue_active occur. No functional changes. Signed-off-by: Nikolay Borisov <nbori...@suse.com> --- fs/btrfs/locking.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c index d13128c70ddd..621083f8932c 100644 --- a/fs/btrfs/locking.c +++ b/fs/btrfs/locking.c @@ -290,7 +290,7 @@ void btrfs_tree_unlock(struct extent_buffer *eb) /* * Make sure counter is updated before we wake up waiters. */ - smp_mb(); + smp_mb__after_atomic(); if (waitqueue_active(&eb->write_lock_wq)) wake_up(&eb->write_lock_wq); } else { -- 2.7.4 -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html