GCD 死锁

1、  main_queue中追加block

 dispatch_apply(10, dispatch_get_main_queue(), ^(size_t index) {

        NSLog(@"index:%zu %@",index,[NSThread currentThread]);

   });

2、main_queue中添加同步任务

dispatch_sync(dispatch_get_main_queue(), ^{

})

3、main_queue添加同步barrier

dispatch_barrier_sync(dispatch_get_main_queue(), ^{

NSLog(@"2 %s",__func__);

});