Index: sys/kern/subr_autoconf.c
===================================================================
RCS file: /cvsroot/src/sys/kern/subr_autoconf.c,v
retrieving revision 1.234
diff -p -u -r1.234 subr_autoconf.c
--- sys/kern/subr_autoconf.c	6 Mar 2015 09:28:15 -0000	1.234
+++ sys/kern/subr_autoconf.c	9 Mar 2015 03:13:23 -0000
@@ -221,6 +221,7 @@ static int config_finalize_done;
 /* list of all devices */
 static struct devicelist alldevs = TAILQ_HEAD_INITIALIZER(alldevs);
 static kmutex_t alldevs_mtx;
+static kcondvar_t device_refcnt_cv;
 static volatile bool alldevs_garbage = false;
 static volatile devgen_t alldevs_gen = 1;
 static volatile int alldevs_nread = 0;
@@ -341,6 +342,7 @@ config_init(void)
 	KASSERT(config_initialized == false);
 
 	mutex_init(&alldevs_mtx, MUTEX_DEFAULT, IPL_VM);
+	cv_init(&device_refcnt_cv, "dvrefcnt");
 
 	mutex_init(&config_misc_lock, MUTEX_DEFAULT, IPL_NONE);
 	cv_init(&config_misc_cv, "cfgmisc");
@@ -1243,6 +1245,10 @@ config_devunlink(device_t dev, struct de
 
 	KASSERT(mutex_owned(&alldevs_mtx));
 
+	/* Wait for users to drain.  */
+	while (dev->dv_refcnt)
+		cv_wait(&device_refcnt_cv, &alldevs_mtx);
+
  	/* Unlink from device list.  Link to garbage list. */
 	TAILQ_REMOVE(&alldevs, dev, dv_list);
 	TAILQ_INSERT_TAIL(garbage, dev, dv_list);
@@ -2215,6 +2221,77 @@ config_alldevs_unlock(int s)
 }
 
 /*
+ * device_hold:
+ *
+ *	Add a reference to a device, or return EBUSY if there are too
+ *	many references.
+ */
+int
+device_hold(device_t dv)
+{
+	unsigned refcnt;
+
+	do {
+		refcnt = dv->dv_refcnt;
+		if (refcnt == UINT_MAX)
+			return EBUSY;
+	} while (atomic_cas_uint(&dv->dv_refcnt, refcnt, (refcnt + 1))
+	    != refcnt);
+
+	return 0;
+}
+
+/*
+ * device_rele:
+ *
+ *	Release a reference to a device.  If it drops to zero, notify
+ *	anyone waiting for the device to be unreferenced.
+ */
+void
+device_rele(device_t dv)
+{
+	unsigned refcnt;
+	int s;
+
+	do {
+		refcnt = dv->dv_refcnt;
+		if (refcnt == 1) {
+			s = config_alldevs_lock();
+			KASSERT(mutex_owned(&alldevs_mtx));
+			if (atomic_dec_uint_nv(&dv->dv_refcnt) == 0)
+				cv_broadcast(&device_refcnt_cv);
+			config_alldevs_unlock(s);
+			return;
+		}
+	} while (atomic_cas_uint(&dv->dv_refcnt, refcnt, (refcnt - 1))
+	    != refcnt);
+}
+
+/*
+ * device_lookup_hold:
+ *
+ *	Look up a device instance for a given driver, and add a
+ *	reference to it.
+ */
+device_t
+device_lookup_hold(cfdriver_t cd, int unit)
+{
+	device_t dv;
+	int s;
+
+	s = config_alldevs_lock();
+	KASSERT(mutex_owned(&alldevs_mtx));
+	if (unit < 0 || unit >= cd->cd_ndevs)
+		dv = NULL;
+	else if ((dv = cd->cd_devs[unit]) != NULL &&
+	    (dv->dv_del_gen != 0 || device_hold(dv) != 0))
+		dv = NULL;
+	config_alldevs_unlock(s);
+
+	return dv;
+}
+
+/*
  * device_lookup:
  *
  *	Look up a device instance for a given driver.
Index: sys/sys/device.h
===================================================================
RCS file: /cvsroot/src/sys/sys/device.h,v
retrieving revision 1.147
diff -p -u -r1.147 device.h
--- sys/sys/device.h	6 Mar 2015 09:28:15 -0000	1.147
+++ sys/sys/device.h	9 Mar 2015 03:13:23 -0000
@@ -159,6 +159,7 @@ struct device {
 	void		*dv_private;	/* this device's private storage */
 	int		*dv_locators;	/* our actual locators (optional) */
 	prop_dictionary_t dv_properties;/* properties dictionary */
+	unsigned	dv_refcnt;	/* reference count */
 
 	size_t		dv_activity_count;
 	void		(**dv_activity_handlers)(device_t, devactive_t);
@@ -486,7 +487,10 @@ void	config_twiddle_fn(void *);
 
 void	null_childdetached(device_t, device_t);
 
-device_t	device_lookup(cfdriver_t, int);
+int		device_hold(device_t);
+void		device_rele(device_t);
+device_t	device_lookup_hold(cfdriver_t, int);
+device_t	device_lookup(cfdriver_t, int); /* deprecated */
 void		*device_lookup_private(cfdriver_t, int);
 void		device_register(device_t, void *);
 void		device_register_post_config(device_t, void *);