您可以尝试以下代码：

dispatch_queue_t reentrantAvoidanceQueue = dispatch_queue_create("reentrantAvoidanceQueue", DISPATCH_QUEUE_SERIAL);
    dispatch_async(reentrantAvoidanceQueue, ^{
        STXMLParser2 *stXMLParser2 = [[STXMLParser2 alloc]init];    
        stXMLParser2.xmlParser = [[NSXMLParser alloc] initWithData:responseLoader.xmlData];
        [stXMLParser2.xmlParser setDelegate:self];
        [stXMLParser2.xmlParser setShouldResolveExternalEntities:YES];
        [stXMLParser2.xmlParser parse];
    });
    dispatch_sync(reentrantAvoidanceQueue, ^{ });