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发表于 2011-12-26 16:11 |只看该作者 |倒序浏览

本帖最后由 juruqiang 于 2011-12-26 16:11 编辑

APUE2中文讲pthread_cond_wait说到，

“函数把调用线程放到等待条件的线程列表上，然后对互斥变量解锁，这两个操作是原子操作。 ---(1)
  这样就关闭了条件检查和线程进入休眠状态等待条件改变这两个操作之间的时间通道......” ---(2)

我查看源码，大概过程是：
条件检查->enqueue->解锁->suspend(阻塞引起休眠)......    （理解对否？？？）

问题：
1、pthread_cond_wait中的处理顺序？
2、（1）（2）怎么理解？

附源码：
int __pthread_cond_wait(pthread_cond_t *cond, pthread_mutex_t *mutex)
{
  volatile pthread_descr self = thread_self();
  pthread_extricate_if extr;
  int already_canceled = 0;
  int spurious_wakeup_count;

  /* Check whether the mutex is locked and owned by this thread.  */
  if (mutex->__m_kind != PTHREAD_MUTEX_TIMED_NP
   && mutex->__m_kind != PTHREAD_MUTEX_ADAPTIVE_NP
   && mutex->__m_owner != self)
return EINVAL;

  /* Set up extrication interface */
  extr.pu_object = cond;
  extr.pu_extricate_func = cond_extricate_func;

  /* Register extrication interface */
  THREAD_SETMEM(self, p_condvar_avail, 0);
  __pthread_set_own_extricate_if(self, &extr);

  /* Atomically enqueue thread for waiting, but only if it is not
   canceled. If the thread is canceled, then it will fall through the
   suspend call below, and then call pthread_exit without
   having to worry about whether it is still on the condition variable queue.
   This depends on pthread_cancel setting p_canceled before calling the
   extricate function. */

  __pthread_lock(&cond->__c_lock, self);
  if (!(THREAD_GETMEM(self, p_canceled)
   && THREAD_GETMEM(self, p_cancelstate) == PTHREAD_CANCEL_ENABLE))  //条件检查
enqueue(&cond->__c_waiting, self);
  else
already_canceled = 1;
  __pthread_unlock(&cond->__c_lock);

  if (already_canceled) {
__pthread_set_own_extricate_if(self, 0);
__pthread_do_exit(PTHREAD_CANCELED, CURRENT_STACK_FRAME);
  }

  pthread_mutex_unlock(mutex);

  spurious_wakeup_count = 0;
  while (1)
{
   suspend(self);
   if (THREAD_GETMEM(self, p_condvar_avail) == 0
   && (THREAD_GETMEM(self, p_woken_by_cancel) == 0
      || THREAD_GETMEM(self, p_cancelstate) != PTHREAD_CANCEL_ENABLE))
{
   /* Count resumes that don't belong to us. */
   spurious_wakeup_count++;
   continue;
}
   break;
}

  __pthread_set_own_extricate_if(self, 0);

  /* Check for cancellation again, to provide correct cancellation
   point behavior */

  if (THREAD_GETMEM(self, p_woken_by_cancel)
   && THREAD_GETMEM(self, p_cancelstate) == PTHREAD_CANCEL_ENABLE) {
THREAD_SETMEM(self, p_woken_by_cancel, 0);
pthread_mutex_lock(mutex);
__pthread_do_exit(PTHREAD_CANCELED, CURRENT_STACK_FRAME);
  }

  /* Put back any resumes we caught that don't belong to us. */
  while (spurious_wakeup_count--)
restart(self);

  pthread_mutex_lock(mutex);
  return 0;
}