首先，看看如下代碼的輸出是什么？
- (void)viewDidLoad {
[super viewDidLoad];
NSLog(@"Hello");
dispatch_sync(dispatch_get_main_queue(), ^{
NSLog(@"World");
});
}
首先答案是會發生死鎖，我們看看官方文檔關于dispatch_sync的解釋：
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()的目標queue為當前queue，會發生死鎖(并行queue并不會)。使用dispatch_sync()會遇到跟我們在pthread中使用mutex鎖一樣的死鎖問題。
話是這么說，我們看看究竟是怎么做的？先放碼：
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. 可以看到dispatch_sync將我們block函數指針進行了一些轉換后，直接傳給了dispatch_sync_f()去處理。
Step2. dispatch_sync_f首先檢查傳入的隊列寬度（dq_width），由于我們傳入的main queue為串行隊列，隊列寬度為1，所有接下來會調用dispatch_barrier_sync_f，傳入3個參數，dispatch_sync中的目標queue、上下文信息和由我們block函數指針轉化過后的func結構體。
接下來我們看看dispatch_barrier_sync_f的實現
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首先做了做了3個判斷：
- 隊列存在尾部節點狀態（判斷當前是不是處于隊列尾部）
- 隊列不為暫停狀態
- 使用_dispatch_queue_trylock檢查隊列能被正常加鎖。
滿足所有條件則不執行if語句內的內容，執行下面代碼，簡單解釋為：
- 使用mutex鎖，獲取到當前進程資源鎖。
- 直接執行我們block函數指針的具體內容。
- 然后釋放鎖，整個調用結束。
然后在我們例子中，很顯然當前隊列中還有其他viewController的任務，我們的流程跑到_dispatch_barrier_aync_f_slow()函數體中。
刨根問底，讓我們看看這個函數。
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,
};
//---------------重點是這里---------------
_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. 既然我們上面已經判斷了，main queue中還有其他任務，現在不能直接執行這個block，跳入到_dispatch_barrier_sync_f_slow函數體，那它怎么處理我們加入的block呢？
在_dispatch_barrier_sync_f_slow中，使用_dispatch_queue_push將我們的block壓入main queue的FIFO隊列中，然后等待信號量，ready后被喚醒。
然后dispatch_semaphore_wait返回_dispatch_semaphore_wait_slow(dsema, timeout)函數，持續輪訓并等待，直到條件滿足。
所以在此過程中，我們最初調用的dispatch_sync函數一直得不到返回，main queue被阻塞，而我們的block又需要等待main queue來執行它。死鎖愉快的產生了。
最后：
我們繪制上張圖來輕松的描述一下這個問題：
