Index: kern/subr_bus.c
===================================================================
--- kern/subr_bus.c	(revision 196686)
+++ kern/subr_bus.c	(working copy)
@@ -2576,10 +2576,16 @@
 {
 	int error;
 
+	if (dev->state >= DS_ATTACHING)
+		return (0);
 	device_sysctl_init(dev);
 	if (!device_is_quiet(dev))
 		device_print_child(dev->parent, dev);
+	dev->state = DS_ATTACHING;
 	if ((error = DEVICE_ATTACH(dev)) != 0) {
+		KASSERT(dev->state == DS_ATTACHING,
+		    ("%s: %p device state must not been changing", __func__,
+		    dev));
 		printf("device_attach: %s%d attach returned %d\n",
 		    dev->driver->name, dev->unit, error);
 		/* Unset the class; set in device_probe_child */
@@ -2590,6 +2596,8 @@
 		dev->state = DS_NOTPRESENT;
 		return (error);
 	}
+	KASSERT(dev->state == DS_ATTACHING,
+	    ("%s: %p device state must not been changing", __func__, dev));
 	device_sysctl_update(dev);
 	dev->state = DS_ATTACHED;
 	devadded(dev);
@@ -2625,8 +2633,16 @@
 	if (dev->state != DS_ATTACHED)
 		return (0);
 
-	if ((error = DEVICE_DETACH(dev)) != 0)
+	dev->state = DS_DETACHING;
+	if ((error = DEVICE_DETACH(dev)) != 0) {
+		KASSERT(dev->state == DS_DETACHING,
+		    ("%s: %p device state must not been changing", __func__,
+		    dev));
+		dev->state = DS_ATTACHED;
 		return (error);
+	}
+	KASSERT(dev->state == DS_DETACHING,
+	    ("%s: %p device state must not been changing", __func__, dev));
 	devremoved(dev);
 	if (!device_is_quiet(dev))
 		device_printf(dev, "detached\n");
@@ -2681,7 +2697,7 @@
 int
 device_shutdown(device_t dev)
 {
-	if (dev->state < DS_ATTACHED)
+	if (dev->state < DS_ATTACHED || dev->state == DS_DETACHING)
 		return (0);
 	return (DEVICE_SHUTDOWN(dev));
 }
Index: sys/bus.h
===================================================================
--- sys/bus.h	(revision 196686)
+++ sys/bus.h	(working copy)
@@ -52,8 +52,11 @@
 typedef enum device_state {
 	DS_NOTPRESENT,			/**< @brief not probed or probe failed */
 	DS_ALIVE,			/**< @brief probe succeeded */
+	DS_ATTACHING,			/**< @brief attaching is in progress */
 	DS_ATTACHED,			/**< @brief attach method called */
-	DS_BUSY				/**< @brief device is open */
+	DS_BUSY,			/**< @brief device is open */
+	DS_DETACHING			/**< @brief detaching is in progress */
+
 } device_state_t;
 
 /**