Index: sys/fs/unionfs/union_subr.c
===================================================================
--- sys/fs/unionfs/union_subr.c	(revision 231702)
+++ sys/fs/unionfs/union_subr.c	(working copy)
@@ -261,7 +261,9 @@ unionfs_nodeget(struct mount *mp, struct vnode *up
 		free(unp, M_UNIONFSNODE);
 		return (error);
 	}
+	vn_lock(vp, LK_EXCLUSIVE | LK_RETRY);
 	error = insmntque(vp, mp);	/* XXX: Too early for mpsafe fs */
+	VOP_UNLOCK(vp, 0);
 	if (error != 0) {
 		free(unp, M_UNIONFSNODE);
 		return (error);