Merge branch 'fix/waitqueue-barriers' of git://git.kernel.org/pub/scm/linux/kernel/git/kdave/linux into for-linus-4.4

1 files changed, 12 insertions, 0 deletions

diff --git a/fs/btrfs/locking.c b/fs/btrfs/locking.c
index d7e6baf1b205..8077461fc56a 100644
--- a/fs/btrfs/locking.c
+++ b/fs/btrfs/locking.c
@@ -79,6 +79,9 @@ void btrfs_clear_lock_blocking_rw(struct extent_buffer *eb, int rw)
 		write_lock(&eb->lock);
 		WARN_ON(atomic_read(&eb->spinning_writers));
 		atomic_inc(&eb->spinning_writers);
+		/*
+		 * atomic_dec_and_test implies a barrier for waitqueue_active
+		 */
 		if (atomic_dec_and_test(&eb->blocking_writers) &&
 		    waitqueue_active(&eb->write_lock_wq))
 			wake_up(&eb->write_lock_wq);
@@ -86,6 +89,9 @@ void btrfs_clear_lock_blocking_rw(struct extent_buffer *eb, int rw)
 		BUG_ON(atomic_read(&eb->blocking_readers) == 0);
 		read_lock(&eb->lock);
 		atomic_inc(&eb->spinning_readers);
+		/*
+		 * atomic_dec_and_test implies a barrier for waitqueue_active
+		 */
 		if (atomic_dec_and_test(&eb->blocking_readers) &&
 		    waitqueue_active(&eb->read_lock_wq))
 			wake_up(&eb->read_lock_wq);
@@ -229,6 +235,9 @@ void btrfs_tree_read_unlock_blocking(struct extent_buffer *eb)
 	}
 	btrfs_assert_tree_read_locked(eb);
 	WARN_ON(atomic_read(&eb->blocking_readers) == 0);
+	/*
+	 * atomic_dec_and_test implies a barrier for waitqueue_active
+	 */
 	if (atomic_dec_and_test(&eb->blocking_readers) &&
 	    waitqueue_active(&eb->read_lock_wq))
 		wake_up(&eb->read_lock_wq);
@@ -280,6 +289,9 @@ 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);