Index: head/sys/geom/geom_dev.c
===================================================================
--- head/sys/geom/geom_dev.c
+++ head/sys/geom/geom_dev.c
@@ -401,6 +401,20 @@
 #else
 	e = 0;
 #endif
+
+	/*
+	 * The vgonel(9) - caused by eg. forced unmount of devfs - calls
+	 * VOP_CLOSE(9) on devfs vnode without any FREAD or FWRITE flags,
+	 * which would result in zero deltas, which in turn would cause
+	 * panic in g_access(9).
+	 *
+	 * Note that we cannot zero the counters (ie. do "r = cp->acr"
+	 * etc) instead, because the consumer might be opened in another
+	 * devfs instance.
+	 */
+	if (r + w + e == 0)
+		return (EINVAL);
+
 	sc = cp->private;
 	mtx_lock(&sc->sc_mtx);
 	sc->sc_open += r + w + e;