diff --git a/fs/btrfs/extent_map.c b/fs/btrfs/extent_map.c
index 9558d79..9d30acc 100644
--- a/fs/btrfs/extent_map.c
+++ b/fs/btrfs/extent_map.c
@@ -384,6 +384,8 @@ int add_extent_mapping(struct extent_map_tree *tree,
 {
 	int ret = 0;
 
+	lockdep_assert_held_write(&tree->lock);
+
 	ret = tree_insert(&tree->map, em);
 	if (ret)
 		goto out;