mfence in rbtdb

lib/dns/rbtdb.c

@@ -2807,6 +2807,7 @@ findnodeintree(dns_rbtdb_t *rbtdb, dns_rbt_t *tree, const dns_name_t *name,
 		 * It would be nice to try to upgrade the lock instead of
 		 * unlocking then relocking.
 		 */
+		__sync_synchronize();
 		locktype = isc_rwlocktype_write;
 		RWLOCK(&rbtdb->tree_lock, locktype);
 		node = NULL;