首先，看看如下代码的输出是什么？
- (void)viewDidLoad { [superviewDidLoad];NSLog(@"Hello");dispatch_sync(dispatch_get_main_queue(), ^{NSLog(@"World"); });}
首先答案是会发生死锁，我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.
如果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()会遇到跟我们在pthread中使用mutex锁一样的死锁问题。
话是这么说，我们看看究竟是怎么做的？先放码：
source/queue.c
voiddispatch_sync(dispatch_queue_t dq,void(^work)(void)){structBlock_basic *bb = (void*)work; dispatch_sync_f(dq, work, (dispatch_function_t)bb->Block_invoke);}DISPATCH_NOINLINEvoiddispatch_sync_f(dispatch_queue_t dq,void*ctxt, dispatch_function_t func){ typeof(dq->dq_running) prev_cnt;dispatch_queue_told_dq;if(dq->dq_width ==1) {returndispatch_barrier_sync_f(dq, ctxt, func); }// 1) ensure that this thread hasn't enqueued anything ahead of this call// 2) the queue is not suspendedif(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. 可以看到dispatch_sync将我们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），由于我们传入的main queue为串行队列，队列宽度为1，所有接下来会调用dispatch_barrier_sync_f，传入3个参数，dispatch_sync中的目标queue、上下文信息和由我们block函数指针转化过后的func结构体。
接下来我们看看dispatch_barrier_sync_f的实现
source/queue.c
voiddispatch_barrier_sync_f(dispatch_queue_t dq, void *ctxt, dispatch_function_tfunc){ 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 weirdif(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首先做了做了3个判断：
队列存在尾部节点状态（判断当前是不是处于队列尾部）
队列不为暂停状态
使用_dispatch_queue_trylock检查队列能被正常加锁。
满足所有条件则不执行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。
直接执行我们block函数指针的具体内容。
然后释放锁，整个调用结束。
然后在我们例子中，很显然当前队列中还有其他viewController的任务，我们的流程跑到_dispatch_barrier_aync_f_slow()函数体中。
刨根问底，让我们看看这个函数。
source/queue.c
staticvoid_dispatch_barrier_sync_f_slow(dispatch_queue_tdq,void*ctxt,dispatch_function_tfunc){// 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 threadstructdispatch_barrier_sync_slow2_s dbss2 = { .dbss2_dq = dq,#ifDISPATCH_COCOA_COMPAT.dbss2_func = func, .dbss2_ctxt = ctxt,#endif.dbss2_sema = _dispatch_get_thread_semaphore(), };structdispatch_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);#ifDISPATCH_COCOA_COMPAT// Main queue bound to main threadif(dbss2.dbss2_func ==NULL) {return; }#endifdispatch_queue_told_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. 既然我们上面已经判断了，main queue中还有其他任务，现在不能直接执行这个block，跳入到_dispatch_barrier_sync_f_slow函数体，那它怎么处理我们加入的block呢？
在_dispatch_barrier_sync_f_slow中，使用_dispatch_queue_push将我们的block压入main queue的FIFO队列中，然后等待信号量，ready后被唤醒。
然后dispatch_semaphore_wait返回_dispatch_semaphore_wait_slow(dsema, timeout)函数，持续轮训并等待，直到条件满足。
所以在此过程中，我们最初调用的dispatch_sync函数一直得不到返回，main queue被阻塞，而我们的block又需要等待main queue来执行它。死锁愉快的产生了。
最后：
我们绘制上张图来轻松的描述一下这个问题：
转载：文／jackjhu 原文链接：http://www.jianshu.com/p/44369c02b62a
