其中有这样一段话：
This isn't due to 10.9's GCD being different - rather, it demonstrates the true asynchronous nature of GCD: The main thread has yet to return from requesting the worker (which it does by pthread_workqueue_addthreads_np, as I'll describe later), and already the worker thread has spawned and is mid execution, possibly on another CPU core. The exact state of the main thread with respect to the worker is largely unpredictable.
作者认为，GCD 申请到的线程有可能是一个正在处理其他任务的 thread，main thread 需要等待这个忙碌的线程返回才能继续执行，我对这种说法存疑。
最后求助无门的状况下，我决定使用一次宝贵的 TSL 机会，直接向 Apple 的工程师求教。这里不得不提下，向 Apple 寻求 technical support 是非常宝贵而且可行的方案，每个开发者账号每年都有 2 次机会，不用非常可惜。
我把问题抛过去后，得到一位 Apple 内核团队工程师的回复，我将精简过的回复以问答的形式展示和大家分享：
Q: looks like even if it's async dispatching, the main thread still has to wait for the other thread to return, during which time, the other thread happen to be in mid execution of sth. this confuses me, what exactly is the main thread waiting for?
为什么主线程需要等待 dispatch_async 返回，主线程到底在等待什么？
A: It's hard to say with just a user space backtrace. Frame 0 has clearly sent the current thread into the kernel, and this specific kernel call is /way/ too complex to analyse from outside [1].
