当队列是空的时候，读线程现在并不抛出异常，而是在条件变量上阻塞自身。

Instead of throwing an exception when the queue is empty, the reader thread now blocks itself on the condition variable.

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第一个读线程锁住互斥锁，发现队列是空的，然后在 _cond 上阻塞自身，这会隐式地释放互斥锁。

The first reader thread locked the mutex, realized that the queue is empty, and blocked itself on _cond, which implicitly released the mutex.

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当线程的任务队列为空，它将尝试从另一个线程的deque的尾部窃取另一个任务。

In the event the thread's task queue is empty, it then tries to steal another task off the tail of another thread's deque.

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