btrfs: add comments to barriers before waitqueue_active

Reduce number of undocumented barriers out there.

Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c
index d7e6baf1b205..03f8630dbaf2 100644
--- a/fs/btrfs/locking.c
+++ b/fs/btrfs/locking.c
@@ -280,6 +280,9 @@ void btrfs_tree_unlock(struct extent_buffer *eb)
         if (blockers) {
                 WARN_ON(atomic_read(&eb->spinning_writers));
                 atomic_dec(&eb->blocking_writers);
+                /*
+                 * Make sure counter is updated before we wake up waiters.
+                 */
                 smp_mb();
                 if (waitqueue_active(&eb->write_lock_wq))
                         wake_up(&eb->write_lock_wq);