dispatch_sync死锁问题

首先，看看以下代码的输出是什么？

- (void)viewDidLoad { [super viewDidLoad]; NSLog(@"Hello"); dispatch_sync(dispatch_get_main_queue(), ^{ NSLog(@"World"); }); }

首先答案是会发生死锁，咱们看看官方文档关于dispatch_sync的解释：

Submits a block to a dispatch queue like dispatch_async(), however
dispatch_sync() will not return until the block has finished.

Calls to dispatch_sync() targeting the current queue will result
in dead-lock. Use of dispatch_sync() is also subject to the same
multi-party dead-lock problems that may result from the use of a mutex.
Use of dispatch_async() is preferred.

Unlike dispatch_async(), no retain is performed on the target queue. Because
calls to this function are synchronous, the dispatch_sync() "borrows" the
reference of the caller.

As an optimization, dispatch_sync() invokes the block on the current
thread when possible.

若是dispatch_sync()的目标queue为当前queue，会发生死锁(并行queue并不会)。使用dispatch_sync()会遇到跟咱们在pthread中使用mutex锁同样的死锁问题。

话是这么说，咱们看看到底是怎么作的？先放码：

source/queue.c

void dispatch_sync(dispatch_queue_t dq, void (^work)(void)) { struct Block_basic *bb = (void *)work; dispatch_sync_f(dq, work, (dispatch_function_t)bb->Block_invoke); } DISPATCH_NOINLINE void dispatch_sync_f(dispatch_queue_t dq, void *ctxt, dispatch_function_t func) { typeof(dq->dq_running) prev_cnt; dispatch_queue_t old_dq; if (dq->dq_width == 1) { return dispatch_barrier_sync_f(dq, ctxt, func); } // 1) ensure that this thread hasn't enqueued anything ahead of this call // 2) the queue is not suspended if (slowpath(dq->dq_items_tail) || slowpath(DISPATCH_OBJECT_SUSPENDED(dq))) { _dispatch_sync_f_slow(dq); } else { prev_cnt = dispatch_atomic_add(&dq->dq_running, 2) - 2; if (slowpath(prev_cnt & 1)) { if (dispatch_atomic_sub(&dq->dq_running, 2) == 0) { _dispatch_wakeup(dq); } _dispatch_sync_f_slow(dq); } } old_dq = _dispatch_thread_getspecific(dispatch_queue_key); _dispatch_thread_setspecific(dispatch_queue_key, dq); func(ctxt); _dispatch_workitem_inc(); _dispatch_thread_setspecific(dispatch_queue_key, old_dq); if (slowpath(dispatch_atomic_sub(&dq->dq_running, 2) == 0)) { _dispatch_wakeup(dq); } }

Step1. 能够看到dispatch_sync将咱们block函数指针进行了一些转换后，直接传给了dispatch_sync_f()去处理。

Step2. dispatch_sync_f首先检查传入的队列宽度（dq_width），因为咱们传入的main queue为串行队列，队列宽度为1，全部接下来会调用dispatch_barrier_sync_f，传入3个参数，dispatch_sync中的目标queue、上下文信息和由咱们block函数指针转化事后的func结构体。

接下来咱们看看dispatch_barrier_sync_f的实现

source/queue.c

void
dispatch_barrier_sync_f(dispatch_queue_t dq, void *ctxt, dispatch_function_t func) { dispatch_queue_t old_dq = _dispatch_thread_getspecific(dispatch_queue_key); // 1) ensure that this thread hasn't enqueued anything ahead of this call // 2) the queue is not suspended // 3) the queue is not weird if (slowpath(dq->dq_items_tail) || slowpath(DISPATCH_OBJECT_SUSPENDED(dq)) || slowpath(!_dispatch_queue_trylock(dq))) { return _dispatch_barrier_sync_f_slow(dq, ctxt, func); } _dispatch_thread_setspecific(dispatch_queue_key, dq); func(ctxt); _dispatch_workitem_inc(); _dispatch_thread_setspecific(dispatch_queue_key, old_dq); _dispatch_queue_unlock(dq); }

Step3. disptach_barrier_sync_f首先作了作了3个判断：

• 队列存在尾部节点状态（判断当前是否是处于队列尾部）
• 队列不为暂停状态
• 使用_dispatch_queue_trylock检查队列能被正常加锁。

知足全部条件则不执行if语句内的内容，执行下面代码，简单解释为：

• 使用mutex锁，获取到当前进程资源锁。
• 直接执行咱们block函数指针的具体内容。
• 而后释放锁，整个调用结束。

而后在咱们例子中，很显然当前队列中还有其余viewController的任务，咱们的流程跑到_dispatch_barrier_aync_f_slow()函数体中。

刨根问底，让咱们看看这个函数。

source/queue.c

static void _dispatch_barrier_sync_f_slow(dispatch_queue_t dq, void *ctxt, dispatch_function_t func) { // It's preferred to execute synchronous blocks on the current thread // due to thread-local side effects, garbage collection, etc. However, // blocks submitted to the main thread MUST be run on the main thread struct dispatch_barrier_sync_slow2_s dbss2 = { .dbss2_dq = dq, #if DISPATCH_COCOA_COMPAT .dbss2_func = func, .dbss2_ctxt = ctxt, #endif .dbss2_sema = _dispatch_get_thread_semaphore(), }; struct dispatch_barrier_sync_slow_s { DISPATCH_CONTINUATION_HEADER(dispatch_barrier_sync_slow_s); } dbss = { .do_vtable = (void *)DISPATCH_OBJ_BARRIER_BIT, .dc_func = _dispatch_barrier_sync_f_slow_invoke, .dc_ctxt = &dbss2, }; //---------------重点是这里--------------- _dispatch_queue_push(dq, (void *)&dbss); dispatch_semaphore_wait(dbss2.dbss2_sema, DISPATCH_TIME_FOREVER); _dispatch_put_thread_semaphore(dbss2.dbss2_sema); #if DISPATCH_COCOA_COMPAT // Main queue bound to main thread if (dbss2.dbss2_func == NULL) { return; } #endif dispatch_queue_t old_dq = _dispatch_thread_getspecific(dispatch_queue_key); _dispatch_thread_setspecific(dispatch_queue_key, dq); func(ctxt); _dispatch_workitem_inc(); _dispatch_thread_setspecific(dispatch_queue_key, old_dq); dispatch_resume(dq); }

Step4. 既然咱们上面已经判断了，main queue中还有其余任务，如今不能直接执行这个block，跳入到_dispatch_barrier_sync_f_slow函数体，那它怎么处理咱们加入的block呢？

在_dispatch_barrier_sync_f_slow中，使用_dispatch_queue_push将咱们的block压入main queue的FIFO队列中，而后等待信号量，ready后被唤醒。

而后dispatch_semaphore_wait返回_dispatch_semaphore_wait_slow(dsema, timeout)函数，持续轮训并等待，直到条件知足。

因此在此过程当中，咱们最初调用的dispatch_sync函数一直得不到返回，main queue被阻塞，而咱们的block又须要等待main queue来执行它。死锁愉快的产生了。

最后：

咱们绘制上张图来轻松的描述一下这个问题：

 

转http://www.jianshu.com/p/44369c02b62a