diff --git a/fs/btrfs/fs.h b/fs/btrfs/fs.h
index 396e2a4634807..203d2a2678287 100644
--- a/fs/btrfs/fs.h
+++ b/fs/btrfs/fs.h
@@ -576,6 +576,7 @@ struct btrfs_fs_info {
 	s32 dirty_metadata_batch;
 	s32 delalloc_batch;
 
+	/* Protected by 'trans_lock'. */
 	struct list_head dirty_cowonly_roots;
 
 	struct btrfs_fs_devices *fs_devices;