Index: subr_autoconf.c
===================================================================
RCS file: /cvs/src/sys/kern/subr_autoconf.c,v
retrieving revision 1.84
diff -u -p -r1.84 subr_autoconf.c
--- subr_autoconf.c	22 Jan 2015 01:19:51 -0000	1.84
+++ subr_autoconf.c	9 Feb 2015 11:24:17 -0000
@@ -922,7 +922,7 @@ device_mpath(void)
 void
 device_ref(struct device *dv)
 {
-	dv->dv_ref++;
+	atomic_inc_int(&dv->dv_ref);
 }
 
 /*
@@ -937,8 +937,7 @@ device_unref(struct device *dv)
 {
 	struct cfattach *ca;
 
-	dv->dv_ref--;
-	if (dv->dv_ref == 0) {
+	if (atomic_dec_int_nv(&dv->dv_ref) == 0) {
 		ca = dv->dv_cfdata->cf_attach;
 		free(dv, M_DEVBUF, ca->ca_devsize);
 	}