Index: kern/subr_bus.c
===================================================================
--- kern/subr_bus.c	(revision 234057)
+++ kern/subr_bus.c	(working copy)
@@ -2472,12 +2472,13 @@
 void
 device_busy(device_t dev)
 {
-	if (dev->state < DS_ATTACHED)
+	if (dev->state < DS_ATTACHING)
 		panic("device_busy: called for unattached device");
 	if (dev->busy == 0 && dev->parent)
 		device_busy(dev->parent);
 	dev->busy++;
-	dev->state = DS_BUSY;
+	if (dev->state == DS_ATTACHED)
+		dev->state = DS_BUSY;
 }
 
 /**
@@ -2486,14 +2487,16 @@
 void
 device_unbusy(device_t dev)
 {
-	if (dev->state != DS_BUSY)
+	if (dev->busy != 0 && dev->state != DS_BUSY &&
+	    dev->state != DS_ATTACHING)
 		panic("device_unbusy: called for non-busy device %s",
 		    device_get_nameunit(dev));
 	dev->busy--;
 	if (dev->busy == 0) {
 		if (dev->parent)
 			device_unbusy(dev->parent);
-		dev->state = DS_ATTACHED;
+		if (dev->state == DS_BUSY)
+			dev->state = DS_ATTACHED;
 	}
 }
 
@@ -2729,6 +2732,7 @@
 	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) {
 		printf("device_attach: %s%d attach returned %d\n",
 		    dev->driver->name, dev->unit, error);
@@ -2736,11 +2740,15 @@
 			devclass_delete_device(dev->devclass, dev);
 		(void)device_set_driver(dev, NULL);
 		device_sysctl_fini(dev);
+		KASSERT(dev->busy == 0, ("attach failed but busy"));
 		dev->state = DS_NOTPRESENT;
 		return (error);
 	}
 	device_sysctl_update(dev);
-	dev->state = DS_ATTACHED;
+	if (dev->busy)
+		dev->state = DS_BUSY;
+	else
+		dev->state = DS_ATTACHED;
 	dev->flags &= ~DF_DONENOMATCH;
 	devadded(dev);
 	return (0);
Index: sys/bus.h
===================================================================
--- sys/bus.h	(revision 234057)
+++ sys/bus.h	(working copy)
@@ -52,6 +52,7 @@
 typedef enum device_state {
 	DS_NOTPRESENT = 10,		/**< @brief not probed or probe failed */
 	DS_ALIVE = 20,			/**< @brief probe succeeded */
+	DS_ATTACHING = 25,		/**< @brief currently attaching */
 	DS_ATTACHED = 30,		/**< @brief attach method called */
 	DS_BUSY = 40			/**< @brief device is open */
 } device_state_t;