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上文书说到，epoll是如何加到每个监控描述符的wait queue中，这只是第一步。上次也提过，epoll实际上也是一个阻塞操作，只不过是可以同时监控多个文件描述符。下面看一下epoll_wait->ep_poll的实现。
epoll既然是阻塞的，必然需要wait queue。但是这个不能使用监控的文件描述符的wait queue，epoll自己本身也是一个虚拟的文件系统。epoll_create的返回值也是一个文件描述符。Unix下，一切皆是文件嘛。
所以epoll的实现代码如下：
- init_waitqueue_entry(&wait, current);
- __add_wait_queue_exclusive(&ep->wq, &wait);
-
- for (;;) {
- /*
- * We don't want to sleep if the ep_poll_callback() sends us
- * a wakeup in between. That's why we set the task state
- * to TASK_INTERRUPTIBLE before doing the checks.
- */
- set_current_state(TASK_INTERRUPTIBLE);
- if (ep_events_available(ep) || timed_out)
- break;
- if (signal_pending(current)) {
- res = -EINTR;
- break;
- }
-
- spin_unlock_irqrestore(&ep->lock, flags);
- if (!schedule_hrtimeout_range(to, slack, HRTIMER_MODE_ABS))
- timed_out = 1;
-
- spin_lock_irqsave(&ep->lock, flags);
- }
- __remove_wait_queue(&ep->wq, &wait);
回答这个问题，需要我们再跳回ep_ptable_queue_proc——不记得这个函数的同学，请翻看前面的文章。这个函数调用init_waitqueue_func_entry(&pwq->wait, ep_poll_callback);，将epoll当前进程的wait queue节点的回调函数设置为ep_poll_callback。对比epoll调用的init_waitqueue_entry函数，这个函数设置wait queue节点的回调函数为default_wake_function。
那么当监控文件描述符执行wakeup动作时，比如一个socket收到数据时，调用sk_data_ready->sock_def_readable->wake_up_interruptible_sync_poll->....最终会执行wait_queue节点的回调函数。对于epoll来说，即ep_poll_callback。
- static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *key)
- {
- int pwake = 0;
- unsigned long flags;
- struct epitem *epi = ep_item_from_wait(wait);
- struct eventpoll *ep = epi->ep;
-
- spin_lock_irqsave(&ep->lock, flags);
-
- /*
- * If the event mask does not contain any poll(2) event, we consider the
- * descriptor to be disabled. This condition is likely the effect of the
- * EPOLLONESHOT bit that disables the descriptor when an event is received,
- * until the next EPOLL_CTL_MOD will be issued.
- */
- if (!(epi->event.events & ~EP_PRIVATE_BITS))
- goto out_unlock;
-
- /*
- * Check the events coming with the callback. At this stage, not
- * every device reports the events in the "key" parameter of the
- * callback. We need to be able to handle both cases here, hence the
- * test for "key" != NULL before the event match test.
- */
- if (key && !((unsigned long) key & epi->event.events))
- goto out_unlock;
-
- /*
- * If we are transferring events to userspace, we can hold no locks
- * (because we're accessing user memory, and because of linux f_op->poll()
- * semantics). All the events that happen during that period of time are
- * chained in ep->ovflist and requeued later on.
- */
- if (unlikely(ep->ovflist != EP_UNACTIVE_PTR)) {
- if (epi->next == EP_UNACTIVE_PTR) {
- epi->next = ep->ovflist;
- ep->ovflist = epi;
- }
- goto out_unlock;
- }
-
- /* If this file is already in the ready list we exit soon */
- if (!ep_is_linked(&epi->rdllink))
- list_add_tail(&epi->rdllink, &ep->rdllist);
-
- /*
- * Wake up ( if active ) both the eventpoll wait list and the ->poll()
- * wait list.
- */
- if (waitqueue_active(&ep->wq))
- wake_up_locked(&ep->wq);
- if (waitqueue_active(&ep->poll_wait))
- pwake++;
-
- out_unlock:
- spin_unlock_irqrestore(&ep->lock, flags);
-
- /* We have to call this outside the lock */
- if (pwake)
- ep_poll_safewake(&ep->poll_wait);
-
- return 1;
- }
- if (waitqueue_active(&ep->wq))
- wake_up_locked(&ep->wq);
这两篇文章基本上理清了epoll如何监控多个描述符及如何获得通知的过程。对于如何监控来说，还欠缺了epoll内部结构，如何保存的各个描述符，如何维护的信息等。不过这样的文章网上已经有了很多。也许以后我会针对这个问题，再写两篇文章吧。
