考察点

• 死锁如何形成

  死锁：串行队列+同步执行

• 由队列循环等待引起的死锁

• 同步提交的任务，也会在当前线程执行

串行队列 + 同步，会死锁，（主要针对在同一queue队列中）

- (void)test1 {
    dispatch_queue_t queue = dispatch_queue_create("serial", DISPATCH_QUEUE_SERIAL);
    NSLog(@"1");
    dispatch_async(queue, ^{
        NSLog(@"2");
        dispatch_sync(queue, ^{
            NSLog(@"3");
        });
        NSLog(@"4");
    });
    NSLog(@"5");
}
2020-12-26 15:15:30.063692+0800 dd[21630:5252182] 1
2020-12-26 15:15:30.063937+0800 dd[21630:5252182] 5
2020-12-26 15:15:30.063951+0800 dd[21630:5252264] 2
出现死锁奔溃现象：
---------------------------------------------------------------
Thread 3: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)

**
假设换成并发队列，依旧有同步任务，不会死锁

- (void)test1 {
    dispatch_queue_t queue = dispatch_queue_create("con", DISPATCH_QUEUE_CONCURRENT);
    NSLog(@"1");
    dispatch_async(queue, ^{
        NSLog(@"2");
        dispatch_sync(queue, ^{
            NSLog(@"3");
        });
        NSLog(@"4");
    });
    NSLog(@"5");
}

异步+并发队列，performSelector底层，线程与runloop