--- x/fs/overlayfs/util.c
+++ y/fs/overlayfs/util.c
@@ -639,6 +639,8 @@ static int kernfs_fop_open(struct inode
 	 */
 	if (has_mmap)
 		mutex_init(&of->mutex);
+	else if (file->f_mode & FMODE_WRITE)
+		mutex_init(&of->mutex);
 	else
 		mutex_init(&of->mutex);