Index: kern/subr_bus.c
===================================================================
--- kern/subr_bus.c	(revision 196087)
+++ kern/subr_bus.c	(working copy)
@@ -1917,7 +1917,7 @@ device_probe_child(device_t dev, device_t child)
 	 * If the state is already probed, then return.  However, don't
 	 * return if we can rebid this object.
 	 */
-	if (child->state == DS_ALIVE && (child->flags & DF_REBID) == 0)
+	if (child->state >= DS_ALIVE && (child->flags & DF_REBID) == 0)
 		return (0);
 
 	for (; dc; dc = dc->parent) {
@@ -2440,7 +2440,7 @@ device_is_enabled(device_t dev)
 int
 device_is_alive(device_t dev)
 {
-	return (dev->state >= DS_ALIVE);
+	return (dev->state >= DS_ALIVE && dev->state < DS_DETACHING);
 }
 
 /**
@@ -2450,7 +2450,7 @@ device_is_alive(device_t dev)
 int
 device_is_attached(device_t dev)
 {
-	return (dev->state >= DS_ATTACHED);
+	return (dev->state >= DS_ATTACHED && dev->state < DS_DETACHING);
 }
 
 /**
@@ -2628,13 +2628,20 @@ device_attach(device_t dev)
 
 	NBL_ASSERT(SA_XLOCKED);
 
+	if (dev->state != DS_ALIVE)
+		return (0);
 	device_sysctl_init(dev);
 	if (!device_is_quiet(dev))
 		device_print_child(dev->parent, dev);
+	dev->state = DS_ATTACHING;
+	NBL_XUNLOCK();
 	if ((error = DEVICE_ATTACH(dev)) != 0) {
 		printf("device_attach: %s%d attach returned %d\n",
 		    dev->driver->name, dev->unit, error);
 		/* Unset the class; set in device_probe_child */
+		NBL_XLOCK();
+		KASSERT(dev->state == DS_ATTACHING,
+		    ("%s: device state changed unappropriately", __func__));
 		if (dev->devclass == NULL)
 			device_set_devclass(dev, NULL);
 		device_set_driver(dev, NULL);
@@ -2642,6 +2649,9 @@ device_attach(device_t dev)
 		dev->state = DS_NOTPRESENT;
 		return (error);
 	}
+	NBL_XLOCK();
+	KASSERT(dev->state == DS_ATTACHING,
+	    ("%s: device state changed unappropriately", __func__));
 	device_sysctl_update(dev);
 	dev->state = DS_ATTACHED;
 	devadded(dev);
@@ -2676,9 +2686,19 @@ device_detach(device_t dev)
 		return (EBUSY);
 	if (dev->state != DS_ATTACHED)
 		return (0);
+	dev->state = DS_DETACHING;
+	NBL_XUNLOCK();
 
-	if ((error = DEVICE_DETACH(dev)) != 0)
+	if ((error = DEVICE_DETACH(dev)) != 0) {
+		NBL_XLOCK();
+		KASSERT(dev->state == DS_DETACHING,
+		    ("%s: device state changed unappropriately", __func__));
+		dev->state = DS_ATTACHED;
 		return (error);
+	}
+	NBL_XLOCK();
+	KASSERT(dev->state == DS_DETACHING,
+	    ("%s: device state changed unappropriately", __func__));
 	devremoved(dev);
 	if (!device_is_quiet(dev))
 		device_printf(dev, "detached\n");
@@ -2735,10 +2755,28 @@ device_quiesce(device_t dev)
 int
 device_shutdown(device_t dev)
 {
+	device_state_t prev;
+	int error;
 
-	if (dev->state < DS_ATTACHED)
+	NBL_ASSERT(SA_XLOCKED);
+	if (dev->state != DS_ATTACHED && dev->state != DS_BUSY)
 		return (0);
-	return (DEVICE_SHUTDOWN(dev));
+	prev = dev->state;
+	dev->state = DS_SHUTTINGDOWN;
+	NBL_XUNLOCK();
+	error = DEVICE_SHUTDOWN(dev);
+	NBL_XLOCK();
+	KASSERT(dev->state == DS_SHUTTINGDOWN,
+	    ("%s: device state changed unappropriately", __func__));
+
+	/*
+	 * System is going to reboot so we don't care about returning
+	 * with the state on DS_SHUTTINGDOWN.  Probabilly, unwinding the
+	 * state in case of failed shutdown is paranoic.
+	 */
+	if (error != 0)
+		dev->state = prev;
+	return (error);
 }
 
 /**
@@ -4131,7 +4169,7 @@ driver_module_handler(module_t mod, int what, void
 		return (ENOMEM);
 	}
 
-		switch (what) {
+	switch (what) {
 	case MOD_LOAD:
 		if (dmd->dmd_chainevh)
 			error = dmd->dmd_chainevh(mod,what,dmd->dmd_chainarg);
Index: sys/bus.h
===================================================================
--- sys/bus.h	(revision 196087)
+++ sys/bus.h	(working copy)
@@ -52,8 +52,11 @@ struct u_businfo {
 typedef enum device_state {
 	DS_NOTPRESENT,			/**< @brief not probed or probe failed */
 	DS_ALIVE,			/**< @brief probe succeeded */
+	DS_ATTACHING,			/**< @brief attaching in progress */
 	DS_ATTACHED,			/**< @brief attach method called */
-	DS_BUSY				/**< @brief device is open */
+	DS_BUSY,			/**< @brief device is open */
+	DS_DETACHING,			/**< @brief detaching in progress */
+	DS_SHUTTINGDOWN,		/**< @brief shutdown in progress */
 } device_state_t;
 
 /**