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GCD什么情况下会死锁?

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本文字数： 6.4k 阅读时长 ≈ 6 分钟

A serial dispatch queue runs only one task at a time, waiting until that task is complete before dequeuing and starting a new one. By contrast, a concurrent dispatch queue starts as many tasks as it can without waiting for already started tasks to finish.

串行队列-同步-同步

运行结果：死锁。

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/**
条件：串行队列，同步执行，同步执行。
结果：死锁。
*/
- (void) testMethod2 {
    dispatch_queue_t queue = dispatch_queue_create("com.serial", DISPATCH_QUEUE_SERIAL);
    NSLog(@"11");
    dispatch_sync(queue, ^{
        NSLog(@"22");
        dispatch_sync(queue, ^{
           NSLog(@"33");
        });
        NSLog(@"44");
    });
    NSLog(@"55");
}

运行截图：

串行队列-同步-异步

运行结果：不死锁，同步操作在主线程中执行，异步操作在子线程执行。为什么第一个同步操作没有死锁，因为第一个同步操作在“com.serial”这个队列里，而testMethod10在主队列里，虽然是同步操作，但是二者不属于同一个队列，不会互相阻塞。

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/**
条件：串行队列，同步执行，异步执行。
结果：死锁。
*/
- (void) testMethod10 {
    dispatch_queue_t queue = dispatch_queue_create("com.serial", DISPATCH_QUEUE_SERIAL);
    NSLog(@"11, %@", [NSThread currentThread]);
    dispatch_sync(queue, ^{
        NSLog(@"22, %@", [NSThread currentThread]);
        dispatch_async(queue, ^{
           NSLog(@"33, %@", [NSThread currentThread]);
        });
        NSLog(@"44, %@", [NSThread currentThread]);
    });
    NSLog(@"55, %@", [NSThread currentThread]);
}

运行截图：

串行队列-异步-同步

运行结果：死锁。打印“22”、“44”的操作已经在串行队列上执行了，此时要求插入打印“33”的操作，这个同步操作造成“com.serial”串行队列死锁。

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/**
条件：串行队列，异步执行，同步执行。
结果：死锁。
*/
- (void) testMethod5 {
    dispatch_queue_t queue = dispatch_queue_create("com.serial", DISPATCH_QUEUE_SERIAL);
    NSLog(@"11, %@", [NSThread currentThread]);
    dispatch_async(queue, ^{
        NSLog(@"22, %@", [NSThread currentThread]);
        dispatch_sync(queue, ^{
           NSLog(@"33, %@", [NSThread currentThread]);
        });
        NSLog(@"44, %@", [NSThread currentThread]);
    });
    NSLog(@"55, %@", [NSThread currentThread]);
}

运行截图：

串行队列-异步-异步

运行结果：不死锁。

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/**
条件：串行队列，异步执行，异步执行。
结果：不死锁。
线程：创建了一个线程。
*/
- (void) testMethod8 {
    dispatch_queue_t queue = dispatch_queue_create("com.serial", DISPATCH_QUEUE_SERIAL);
    NSLog(@"11, %@", [NSThread currentThread]);
    dispatch_async(queue, ^{
        NSLog(@"22, %@", [NSThread currentThread]);
        dispatch_async(queue, ^{
           NSLog(@"33, %@", [NSThread currentThread]);
        });
        NSLog(@"44, %@", [NSThread currentThread]);
    });
    NSLog(@"55, %@", [NSThread currentThread]);
}

运行截图：

并发队列-同步-同步

运行结果：不死锁。

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/**
条件：并发队列，同步执行，同步执行。
结果：不死锁。
*/
- (void) testMethod3 {
    dispatch_queue_t queue = dispatch_queue_create("com.serial.11", DISPATCH_QUEUE_CONCURRENT);
    NSLog(@"11, %@", [NSThread currentThread]);
    dispatch_sync(queue, ^{
        NSLog(@"22, %@", [NSThread currentThread]);
        dispatch_sync(queue, ^{
           NSLog(@"33, %@", [NSThread currentThread]);
        });
        NSLog(@"44, %@", [NSThread currentThread]);
    });
    NSLog(@"55, %@", [NSThread currentThread]);
}

运行截图：

并发队列-同步-异步

运行结果：不死锁。

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/**
条件：并发队列，同步执行，异步执行。
结果：不死锁。
*/
- (void) testMethod11 {
    dispatch_queue_t queue = dispatch_queue_create("com.serial.11", DISPATCH_QUEUE_CONCURRENT);
    NSLog(@"11, %@", [NSThread currentThread]);
    dispatch_sync(queue, ^{
        NSLog(@"22, %@", [NSThread currentThread]);
        dispatch_async(queue, ^{
           NSLog(@"33, %@", [NSThread currentThread]);
        });
        NSLog(@"44, %@", [NSThread currentThread]);
    });
    NSLog(@"55, %@", [NSThread currentThread]);
}

运行截图：

并发队列-异步-同步

运行结果：不死锁。

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/**
条件：并发队列，异步执行，同步执行。
结果：不死锁。
*/
- (void) testMethod6 {
    dispatch_queue_t queue = dispatch_queue_create("com.serial.11", DISPATCH_QUEUE_CONCURRENT);
    NSLog(@"11, %@", [NSThread currentThread]);
    dispatch_async(queue, ^{
        NSLog(@"22, %@", [NSThread currentThread]);
        dispatch_sync(queue, ^{
           NSLog(@"33, %@", [NSThread currentThread]);
        });
        NSLog(@"44, %@", [NSThread currentThread]);
    });
    NSLog(@"55, %@", [NSThread currentThread]);
}

运行截图：

并发队列-异步-异步

运行结果：不死锁。

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/**
条件：并发队列，异步执行，异步执行。
结果：不死锁。
线程：创建了一个线程。
*/
- (void) testMethod9 {
    dispatch_queue_t queue = dispatch_queue_create("com.serial.11", DISPATCH_QUEUE_CONCURRENT);
    NSLog(@"11, %@", [NSThread currentThread]);
    dispatch_async(queue, ^{
        NSLog(@"22, %@", [NSThread currentThread]);
        dispatch_async(queue, ^{
           NSLog(@"33, %@", [NSThread currentThread]);
        });
        NSLog(@"44, %@", [NSThread currentThread]);
    });
    NSLog(@"55, %@", [NSThread currentThread]);
}

运行截图：

主队列-同步-同步

运行结果：死锁。

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/**
 条件：主队列，同步执行，同步执行。
 结果：死锁。
 */
- (void) testMethod1 {
    dispatch_queue_t queue = dispatch_get_main_queue();
    NSLog(@"11, %@", [NSThread currentThread]);
    dispatch_sync(queue, ^{
        NSLog(@"22, %@", [NSThread currentThread]);
        dispatch_sync(queue, ^{
           NSLog(@"33, %@", [NSThread currentThread]);
        });
        NSLog(@"44, %@", [NSThread currentThread]);
    });
    NSLog(@"55, %@", [NSThread currentThread]);
}

运行截图：

主队列-同步-异步

运行结果：死锁。

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/**
条件：主队列，同步执行，异步执行。
结果：死锁。
*/
- (void) testMethod12 {
    dispatch_queue_t queue = dispatch_get_main_queue();
    NSLog(@"11, %@", [NSThread currentThread]);
    dispatch_sync(queue, ^{
        NSLog(@"22, %@", [NSThread currentThread]);
        dispatch_async(queue, ^{
           NSLog(@"33, %@", [NSThread currentThread]);
        });
        NSLog(@"44, %@", [NSThread currentThread]);
    });
    NSLog(@"55, %@", [NSThread currentThread]);
}

运行截图：

主队列-异步-同步

运行结果：死锁。

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/**
条件：主队列，异步执行，同步执行。
结果：死锁。
*/
- (void) testMethod4 {
    dispatch_queue_t queue = dispatch_get_main_queue();
    NSLog(@"11, %@", [NSThread currentThread]);
    dispatch_async(queue, ^{
        NSLog(@"22, %@", [NSThread currentThread]);
        dispatch_sync(queue, ^{
           NSLog(@"33, %@", [NSThread currentThread]);
        });
        NSLog(@"44, %@", [NSThread currentThread]);
    });
    NSLog(@"55, %@", [NSThread currentThread]);
}

运行截图：

主队列-异步-异步

运行结果：不死锁。

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/**
条件：主队列，异步执行，异步执行。
结果：不死锁。
线程：主线程执行。
*/
- (void) testMethod7 {
    dispatch_queue_t queue = dispatch_get_main_queue();
    NSLog(@"11, %@", [NSThread currentThread]);
    dispatch_async(queue, ^{
        NSLog(@"22, %@", [NSThread currentThread]);
        dispatch_async(queue, ^{
           NSLog(@"33, %@", [NSThread currentThread]);
        });
        NSLog(@"44, %@", [NSThread currentThread]);
    });
    NSLog(@"55, %@", [NSThread currentThread]);
}

运行截图：

总结：在并发队列上，无论是同步还是异步都不会发生死锁；主队列上只有异步执行的时候才不会发生死锁；串行队列是否发生死锁需要具体分析。

参考链接

官方文档