diff --git a/sys/kern/subr_bus.c b/sys/kern/subr_bus.c
--- a/sys/kern/subr_bus.c
+++ b/sys/kern/subr_bus.c
@@ -5285,31 +5285,38 @@
 	kobj_class_t driver;
 	int error, pass;
 
-	bus_topo_lock();
-
 	dmd = (struct driver_module_data *)arg;
-	bus_devclass = devclass_find_internal(dmd->dmd_busname, NULL, TRUE);
 	error = 0;
 
 	switch (what) {
 	case MOD_LOAD:
+		bus_topo_lock();
+		bus_devclass = devclass_find_internal(dmd->dmd_busname, NULL,
+		    TRUE);
+		bus_topo_unlock();
 		if (dmd->dmd_chainevh)
-			error = dmd->dmd_chainevh(mod,what,dmd->dmd_chainarg);
+			error = dmd->dmd_chainevh(mod, what, dmd->dmd_chainarg);
 
 		pass = dmd->dmd_pass;
 		driver = dmd->dmd_driver;
 		PDEBUG(("Loading module: driver %s on bus %s (pass %d)",
 		    DRIVERNAME(driver), dmd->dmd_busname, pass));
+		bus_topo_lock();
 		error = devclass_add_driver(bus_devclass, driver, pass,
 		    dmd->dmd_devclass);
+		bus_topo_unlock();
 		break;
 
 	case MOD_UNLOAD:
 		PDEBUG(("Unloading module: driver %s from bus %s",
 		    DRIVERNAME(dmd->dmd_driver),
 		    dmd->dmd_busname));
+		bus_topo_lock();
+		bus_devclass = devclass_find_internal(dmd->dmd_busname, NULL,
+		    FALSE);
 		error = devclass_delete_driver(bus_devclass,
 		    dmd->dmd_driver);
+		bus_topo_unlock();
 
 		if (!error && dmd->dmd_chainevh)
 			error = dmd->dmd_chainevh(mod,what,dmd->dmd_chainarg);
@@ -5318,8 +5325,12 @@
 		PDEBUG(("Quiesce module: driver %s from bus %s",
 		    DRIVERNAME(dmd->dmd_driver),
 		    dmd->dmd_busname));
+		bus_topo_lock();
+		bus_devclass = devclass_find_internal(dmd->dmd_busname, NULL,
+		    FALSE);
 		error = devclass_quiesce_driver(bus_devclass,
 		    dmd->dmd_driver);
+		bus_topo_unlock();
 
 		if (!error && dmd->dmd_chainevh)
 			error = dmd->dmd_chainevh(mod,what,dmd->dmd_chainarg);
@@ -5329,8 +5340,6 @@
 		break;
 	}
 
-	bus_topo_unlock();
-
 	return (error);
 }