pthread-based event waking up only waiting threads

Question

In my C++ program I have a class CEvent with trigger and wait member functions based on pthreads (running on Linux). Implementation is quite obvious (i.e. many examples online) if there is one waiting process. However now I need to satisfy the requirement that multiple threads are waiting on the event and should ALL wake up reliably when trigger() is called. As a second condition, only threads that were waiting when trigger() was called should wake up.

My current code:

void CEvent::trigger() {
    pthread_mutex_lock(&mutex);
    wakeUp = true;
    pthread_cond_broadcast(&condition)
    pthread_mutex_unlock(&mutex);
    wakeUp = false;
}

void CEvent::wait() {
    pthread_mutex_lock(&mutex);
    while (!wakeUp)
        pthread_cond_wait(&condition, &mutex)

    pthread_mutex_unlock(&mutex);
}

这似乎几乎可以工作，因为在我将wakeUp 设置回false 之前等待唤醒的所有线程。然而，在广播和唤醒唤醒之间，调用 wait() 的其他（或相同）线程也将立即唤醒，这是不可接受的。在 mutext 解锁之前放置 wakeUp = false 可防止线程唤醒。

我的问题： * pthread_cond_broadcast 何时返回？即是否有保证它只会在所有线程都唤醒后才返回，还是可以在之前返回？* 是否有针对此问题的推荐解决方案？

Answer 1

请无视我之前的虚假回答。触发线程解锁互斥锁（从而释放等待线程）和设置唤醒值之间存在竞争。这意味着另一个（不等待的）线程可以进入，获取互斥体，并在其中看到真值wakeUp并退出，而无需等待。另一个错误是正在等待的线程在wakeUp重置后会被唤醒并立即恢复等待。

解决此问题的一种方法是使用计数 - 每个正在等待的线程都会增加计数，然后触发器将等到许多线程唤醒后再恢复。然后，您必须确保不允许非等待线程开始等待，直到发生这种情况。

// wake up "waiters" count of waiting threads
void CEvent::trigger()
{
    pthread_mutex_lock(&mutex);

    // wakey wakey
    wakeUp = true;
    pthread_cond_broadcast(&condition);

    // wait for them to awake
    while (waiters>0)
      pthread_cond_wait(&condition, &mutex);

    // stop waking threads up
    wakeUp = false;

    // let any "other" threads which were ready to start waiting, do so
    pthread_cond_broadcast(&condition);
    pthread_mutex_unlock(&mutex);
}

// wait for the condition to be notified for us
void CEvent::wait()
{
    pthread_mutex_lock(&mutex);

    // wait for us to be allowed to start waiting
    // we have to wait until any currrently being woken threads have gone
    while (wakeUp)
        pthread_cond_wait(&condition, &mutex);

    // our turn to start waiting
    waiters ++;

    // waiting
    while (!wakeUp)
        pthread_cond_wait(&condition, &mutex);

    // finished waiting, we were triggered
    waiters --;

    // let the trigger thread know we're done
    pthread_cond_broadcast(&condition);
    pthread_mutex_unlock(&mutex);
}