串行队列的死锁

Posted 2021-05-01 AC自动机

问题

这个死锁是会发生在 dispatch_sync 这个GCD接口调用上。

Summary

Submits a block object for execution on a dispatch queue and waits until that block completes.

1void dispatch_sync(dispatch_queue_t queue, dispatch_block_t block);

Discussion

Submits a block to a dispatch queue for synchronous execution. Unlike dispatch_async, this function does not return until the block has finished. Calling this function and targeting the current queue results in deadlock.
Unlike with dispatch_async, no retain is performed on the target queue. Because calls to this function are synchronous, it "borrows" the reference of the caller. Moreover, no Block_copy is performed on the block.
As an optimization, this function invokes the block on the current thread when possible.

正如 接口文档中所说的那样： Calling this function and targeting the current queue results in deadlock 在当前线程队列中使用本接口是会导致死锁的。

这个死锁情况仅限在 DISPATCH_QUEUE_SERIAL的线程队列中。串行队列同步调用会死锁等待。

如何解决

使用串行队列可以避免加锁操作，像一些著名的三方库GCDAsyncSocket,FMDB都有这种使用场景：

GCDAsyncSocket

 1if (sq)
 2{
 3    NSAssert(sq != dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_LOW, 0),
 4                @"The given socketQueue parameter must not be a concurrent queue.");
 5    NSAssert(sq != dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0),
 6                @"The given socketQueue parameter must not be a concurrent queue.");
 7    NSAssert(sq != dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0),
 8                @"The given socketQueue parameter must not be a concurrent queue.");
 9
10    socketQueue = sq;
11    #if !OS_OBJECT_USE_OBJC
12    dispatch_retain(sq);
13    #endif
14} else {
15    socketQueue = dispatch_queue_create([GCDAsyncSocketQueueName UTF8String], NULL);
16}

1IsOnSocketQueueOrTargetQueueKey = &IsOnSocketQueueOrTargetQueueKey;        
2void *nonNullUnusedPointer = (__bridge void *)self;
3dispatch_queue_set_specific(socketQueue, IsOnSocketQueueOrTargetQueueKey, nonNullUnusedPointer, NULL);

再看一下他的sync逻辑是如何控制：

1if (dispatch_get_specific(IsOnSocketQueueOrTargetQueueKey)) {
2    [self closeWithError:nil];
3} else {
4    dispatch_sync(socketQueue, ^{
5        [self closeWithError:nil];
6    });
7}

用意就很明确了，在sync可能会导致死锁的情况下，先判断一下是否已经处于当前队列中。

还有一个不推荐的做法来自GPUImage：

 1#if !OS_OBJECT_USE_OBJC
 2#pragma clang diagnostic push
 3#pragma clang diagnostic ignored "-Wdeprecated-declarations"
 4    if (dispatch_get_current_queue() == videoProcessingQueue)
 5#pragma clang diagnostic pop
 6#else
 7    if (dispatch_get_specific([GPUImageContext contextKey]))
 8#endif
 9    {
10        block();
11    }else {
12        dispatch_sync(videoProcessingQueue, block);
13    }

这里他还使用dispatch_get_current_queue()API, 但苹果的文档中已经标注为废弃API了。

This function is deprecated and will be removed in a future release.
不会在未来的版本中保证该API是否还会存在。