--- kernel/bsd/vfs/vfs_subr.c	2018/04/24 18:26:06	1.1.1.1
+++ kernel/bsd/vfs/vfs_subr.c	2018/04/24 18:31:24	1.1.1.2
@@ -872,7 +872,8 @@ vget(vp, flags, p)
 		freevnodes--;
 		simple_unlock(&vnode_free_list_slock);
 	}
-	vp->v_usecount++;
+        if (++vp->v_usecount <= 0)
+                panic("vget: v_usecount");                     
 	if (flags & LK_TYPE_MASK) {
 		if (error = vn_lock(vp, flags | LK_INTERLOCK, p))
 			vrele(vp);
@@ -992,11 +993,11 @@ void
 vref(vp)
 	struct vnode *vp;
 {
-
 	simple_lock(&vp->v_interlock);
 	if (vp->v_usecount <= 0)
 		panic("vref used where vget required");
-	vp->v_usecount++;
+        if (++vp->v_usecount <= 0)
+                panic("vref v_usecount");                     
 	simple_unlock(&vp->v_interlock);
 }
 
@@ -1009,7 +1010,7 @@ vput(vp)
 {
 	struct proc *p = current_proc();	/* XXX */
 
-#if DIGANOSTIC
+#if DIAGNOSTIC
 	if (vp == NULL)
 		panic("vput: null vp");
 #endif
@@ -1227,7 +1228,8 @@ vclean(vp, flags, p)
 	 * race against ourselves to recycle it.
 	 */
 	if (active = vp->v_usecount)
-		vp->v_usecount++;
+		if (++vp->v_usecount <= 0)
+			panic("vclean: v_usecount");
 	/*
 	 * Prevent the vnode from being recycled or
 	 * brought into use while we clean it out.