Index: if_vtnet.c
===================================================================
--- if_vtnet.c	(revision 299234)
+++ if_vtnet.c	(working copy)
@@ -308,27 +308,36 @@
 MODULE_DEPEND(vtnet, netmap, 1, 1, 1);
 #endif /* DEV_NETMAP */
 
+static struct sx mlu;
+SX_SYSINIT(vtnet_mlu, &mlu, "vtnet mod load/unload");
+
 static int
 vtnet_modevent(module_t mod, int type, void *unused)
 {
-	int error;
+	int error = 0;
+	static int loaded = 0;
 
-	error = 0;
-
 	switch (type) {
 	case MOD_LOAD:
-		vtnet_tx_header_zone = uma_zcreate("vtnet_tx_hdr",
-		    sizeof(struct vtnet_tx_header),
-		    NULL, NULL, NULL, NULL, 0, 0);
+		sx_xlock(&mlu);
+		if (loaded++ == 0)
+			vtnet_tx_header_zone = uma_zcreate("vtnet_tx_hdr",
+				sizeof(struct vtnet_tx_header),
+				NULL, NULL, NULL, NULL, 0, 0);
+		sx_xunlock(&mlu);
 		break;
 	case MOD_QUIESCE:
 	case MOD_UNLOAD:
-		if (uma_zone_get_cur(vtnet_tx_header_zone) > 0)
-			error = EBUSY;
-		else if (type == MOD_UNLOAD) {
-			uma_zdestroy(vtnet_tx_header_zone);
-			vtnet_tx_header_zone = NULL;
+		sx_xlock(&mlu);
+		if (--loaded == 0) {
+			if (uma_zone_get_cur(vtnet_tx_header_zone) > 0)
+				error = EBUSY;
+			else if (type == MOD_UNLOAD) {
+				uma_zdestroy(vtnet_tx_header_zone);
+				vtnet_tx_header_zone = NULL;
+			}
 		}
+		sx_xunlock(&mlu);
 		break;
 	case MOD_SHUTDOWN:
 		break;