If necessary, a waiting thread is signaled to pick up the newly enqueued object.
如果需要，则通知等待线程选取刚刚进入队列的对象。
When the waiting thread is released, the state is reset to non-signaled state.
当正在等待的线程被释放时，其状态就被设置为无信号的状态。
When the mutex becomes signaled, only one waiting thread is released to access the resource.
当线程锁的被标记，只有一个等待线程被释放去访问这些资源。
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