Index: sys/net/bpf.c
===================================================================
--- sys/net/bpf.c
+++ sys/net/bpf.c
@@ -1515,18 +1515,20 @@
 	 * Put interface into promiscuous mode.
 	 */
 	case BIOCPROMISC:
+		BPF_LOCK();
 		if (d->bd_bif == NULL) {
 			/*
 			 * No interface attached yet.
 			 */
 			error = EINVAL;
-			break;
-		}
-		if (d->bd_promisc == 0) {
+		} else if (d->bd_promisc == 0) {
 			error = ifpromisc(d->bd_bif->bif_ifp, 1);
 			if (error == 0)
 				d->bd_promisc = 1;
+		} else {
+			error = 0;
 		}
+		BPF_UNLOCK();
 		break;
 
 	/*