diff --git a/sys/dev/agp/agp.c b/sys/dev/agp/agp.c
--- a/sys/dev/agp/agp.c
+++ b/sys/dev/agp/agp.c
@@ -679,12 +679,14 @@
 	struct agp_memory *mem;
 
 	AGP_DPF("searching for memory block %d\n", id);
+	lockmgr(&sc->as_lock, LK_EXCLUSIVE);
 	TAILQ_FOREACH(mem, &sc->as_memory, am_link) {
 		AGP_DPF("considering memory block %d\n", mem->am_id);
 		if (mem->am_id == id)
-			return mem;
+			break;
 	}
-	return 0;
+	lockmgr(&sc->as_lock, LK_RELEASE);
+	return mem;
 }
 
 /* Implementation of the userland ioctl api */