--- kernel/bsd/sys/vnode.h	2018/04/24 18:26:01	1.1
+++ kernel/bsd/sys/vnode.h	2018/04/24 18:28:37	1.1.1.2
@@ -276,7 +276,8 @@ extern __inline void vhold(vp)
 	struct vnode *vp;
 {
 	simple_lock(&vp->v_interlock);
-	vp->v_holdcnt++;
+	if (++vp->v_holdcnt <= 0)
+		panic("vhold: v_holdcnt");
 	simple_unlock(&vp->v_interlock);
 }
 #define	VREF(vp)	vref(vp)		/* increase reference */
@@ -284,7 +285,8 @@ extern __inline void vref(vp)
 	struct vnode *vp;
 {
 	simple_lock(&vp->v_interlock);
-	vp->v_usecount++;
+	if (++vp->v_usecount <= 0)
+		panic("vref: v_usecount");
 	simple_unlock(&vp->v_interlock);
 }
 #endif /* DIAGNOSTIC */