线程屏障（基于linuxthreads-2.3）

线程屏障是线程同步的一种方式. 线程执行操作后，可能需要等待其他线程完成操作. 这时，当前线程将被挂起，直到另一个线程也完成一个操作，最后所有线程都被唤醒. 屏障具有三个主要功能.

int
pthread_barrier_wait(pthread_barrier_t *barrier)
{
  pthread_descr self = thread_self();
  pthread_descr temp_wake_queue, th;
  int result = 0;
  __pthread_lock(&barrier->__ba_lock, self);
  /* If the required number of threads have achieved rendezvous... */
  // pthread_barrier_wait被调用的次数达到阈值，__ba_present + 1 == __ba_required 
  if (barrier->__ba_present >= barrier->__ba_required - 1)
    {
      /* ... then this last caller shall be the serial thread */
      result = PTHREAD_BARRIER_SERIAL_THREAD;
      /* Copy and clear wait queue and reset barrier. */
      // 被阻塞的线程队列
      temp_wake_queue = barrier->__ba_waiting;
      // 重置字段
      barrier->__ba_waiting = NULL;
      barrier->__ba_present = 0;
    }
  else
    {
      result = 0;
      // 执行pthread_barrier_wait一次，加一
      barrier->__ba_present++;
      // 插入等待队列
      enqueue(&barrier->__ba_waiting, self);
    }
  __pthread_unlock(&barrier->__ba_lock);
  // 调用pthread_barrier_wait的次数还不够
  if (result == 0)
    {
      /* Non-serial threads have to suspend */
      // 挂起当前线程
      suspend(self);
      /* We dont bother dealing with cancellation because the POSIX
         spec for barriers doesnt mention that pthread_barrier_wait
         is a cancellation point. */
    }
  else
    {
      /* Serial thread wakes up all others. */
      // 唤醒其他的线程
      while ((th = dequeue(&temp_wake_queue)) != NULL)
  restart(th);
    }
  return result;
}
int
pthread_barrier_init(pthread_barrier_t *barrier,
        const pthread_barrierattr_t *attr,
        unsigned int count)
{
  if (count == 0)
     return EINVAL;
  __pthread_init_lock(&barrier->__ba_lock);
  // 需要执行pthread_barrier_wait的次数
  barrier->__ba_required = count;
  // 已经调用pthread_barrier_wait的次数
  barrier->__ba_present = 0;
  // 调用pthread_barrier_wait被阻塞的线程队列
  barrier->__ba_waiting = NULL;
  return 0;
}
int
pthread_barrier_destroy(pthread_barrier_t *barrier)
{
  // 有线程在等待
  if (barrier->__ba_waiting != NULL) return EBUSY;
  return 0;
}

从代码中我们知道，障碍的本质是计数. 当尚未达到一定数量时pthread 线程挂起，当前线程被阻止. 当最后一个线程执行pthread_barrier_wait函数并获得一定数目时pthread 线程挂起，所有线程都被唤醒.

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