Index: subr_bus.c
===================================================================
--- subr_bus.c	(revision 196857)
+++ subr_bus.c	(working copy)
@@ -125,10 +125,11 @@
 #define	DF_FIXEDCLASS	2		/* devclass specified at create time */
 #define	DF_WILDCARD	4		/* unit was originally wildcard */
 #define	DF_DESCMALLOCED	8		/* description was malloced */
-#define	DF_QUIET	16		/* don't print verbose attach message */
-#define	DF_DONENOMATCH	32		/* don't execute DEVICE_NOMATCH again */
-#define	DF_EXTERNALSOFTC 64		/* softc not allocated by us */
-#define	DF_REBID	128		/* Can rebid after attach */
+#define	DF_QUIET	0x10		/* don't print verbose attach message */
+#define	DF_DONENOMATCH	0x20		/* don't execute DEVICE_NOMATCH again */
+#define	DF_EXTERNALSOFTC 0x40		/* softc not allocated by us */
+#define	DF_REBID	0x80		/* Can rebid after attach */
+#define DF_TRANSITION	0x100		/* attach or detach running */
 	u_char	order;			/**< order from device_add_child_ordered() */
 	u_char	pad;
 	void	*ivars;			/**< instance variables  */
@@ -2625,15 +2626,17 @@
 {
 	int error;
 
-	if (dev->state >= DS_ATTACHING)
+	GIANT_REQUIRED;
+
+	if (dev->flags & DF_TRANSITION)
 		return (0);
 	device_sysctl_init(dev);
 	if (!device_is_quiet(dev))
 		device_print_child(dev->parent, dev);
-	dev->state = DS_ATTACHING;
+	dev->flags |= DF_TRANSITION;
 	if ((error = DEVICE_ATTACH(dev)) != 0) {
-		KASSERT(dev->state == DS_ATTACHING,
-		    ("%s: %p device state must not been changing", __func__,
+		KASSERT(dev->flags & DF_TRANSITION,
+		    ("%s: %p device must not drop DF_TRANSITION", __func__,
 		    dev));
 		printf("device_attach: %s%d attach returned %d\n",
 		    dev->driver->name, dev->unit, error);
@@ -2643,12 +2646,15 @@
 		device_set_driver(dev, NULL);
 		device_sysctl_fini(dev);
 		dev->state = DS_NOTPRESENT;
+		dev->flags &= ~DF_TRANSITION;
 		return (error);
 	}
-	KASSERT(dev->state == DS_ATTACHING,
-	    ("%s: %p device state must not been changing", __func__, dev));
+	KASSERT(dev->flags & DF_TRANSITION,
+	    ("%s: %p devie must not drop DF_TRANSITION", __func__,
+	    dev));
 	device_sysctl_update(dev);
 	dev->state = DS_ATTACHED;
+	dev->flags &= ~DF_TRANSITION;
 	devadded(dev);
 	return (0);
 }
@@ -2681,17 +2687,20 @@
 		return (EBUSY);
 	if (dev->state != DS_ATTACHED)
 		return (0);
-
-	dev->state = DS_DETACHING;
+	if (dev->flags & DF_TRANSITION)
+		return (0);
+	dev->flags |= DF_TRANSITION;
 	if ((error = DEVICE_DETACH(dev)) != 0) {
-		KASSERT(dev->state == DS_DETACHING,
-		    ("%s: %p device state must not been changing", __func__,
+		KASSERT(dev->flags & DF_TRANSITION,
+		    ("%s: %p device must not drop DF_TRANSITION", __func__,
 		    dev));
 		dev->state = DS_ATTACHED;
+		dev->flags &= ~DF_TRANSITION;
 		return (error);
 	}
-	KASSERT(dev->state == DS_DETACHING,
-	    ("%s: %p device state must not been changing", __func__, dev));
+	KASSERT(dev->flags & DF_TRANSITION,
+	    ("%s: %p device must not drop DF_TRANSITION", __func__,
+	    dev));
 	devremoved(dev);
 	if (!device_is_quiet(dev))
 		device_printf(dev, "detached\n");
@@ -2700,7 +2709,7 @@
 
 	if (!(dev->flags & DF_FIXEDCLASS))
 		devclass_delete_device(dev->devclass, dev);
-
+	dev->flags &= ~DF_TRANSITION;
 	dev->state = DS_NOTPRESENT;
 	device_set_driver(dev, NULL);
 	device_set_desc(dev, NULL);
@@ -2746,7 +2755,7 @@
 int
 device_shutdown(device_t dev)
 {
-	if (dev->state < DS_ATTACHED || dev->state == DS_DETACHING)
+	if (dev->state < DS_ATTACHED || (dev->flags & DF_TRANSITION))
 		return (0);
 	return (DEVICE_SHUTDOWN(dev));
 }