源码剖析之epoll

操作系统

1. 源码剖析

本篇主要分析epoll_ctl以及相关函数

以下源码取自4.10

1.1 epoll_ctl

用于添加/调整/删除我们要监视的事件

fs/eventpoll.c

/*
 * The following function implements the controller interface for
 * the eventpoll file that enables the insertion/removal/change of
 * file descriptors inside the interest set.
 */
SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
		struct epoll_event __user *, event)
{
	int error;
	int full_check = 0;
	struct fd f, tf;
	struct eventpoll *ep;
	struct epitem *epi;
	struct epoll_event epds;
	struct eventpoll *tep = NULL;

	error = -EFAULT;
	/* 调用copy_from_user获得我们关注的事件 */
	if (ep_op_has_event(op) &&
	    copy_from_user(&epds, event, sizeof(struct epoll_event)))
		goto error_return;

	error = -EBADF;
	f = fdget(epfd);
	if (!f.file)
		goto error_return;

	/* Get the "struct file *" for the target file */
	tf = fdget(fd);
	if (!tf.file)
		goto error_fput;

	/* The target file descriptor must support poll */
	error = -EPERM;
	if (!tf.file->f_op->poll)
		goto error_tgt_fput;

	/* Check if EPOLLWAKEUP is allowed */
	if (ep_op_has_event(op))
		ep_take_care_of_epollwakeup(&epds);

	/*
	 * We have to check that the file structure underneath the file descriptor
	 * the user passed to us _is_ an eventpoll file. And also we do not permit
	 * adding an epoll file descriptor inside itself.
	 */
	error = -EINVAL;
	if (f.file == tf.file || !is_file_epoll(f.file))
		goto error_tgt_fput;

	/*
	 * epoll adds to the wakeup queue at EPOLL_CTL_ADD time only,
	 * so EPOLLEXCLUSIVE is not allowed for a EPOLL_CTL_MOD operation.
	 * Also, we do not currently supported nested exclusive wakeups.
	 */
	if (epds.events & EPOLLEXCLUSIVE) {
		if (op == EPOLL_CTL_MOD)
			goto error_tgt_fput;
		if (op == EPOLL_CTL_ADD && (is_file_epoll(tf.file) ||
				(epds.events & ~EPOLLEXCLUSIVE_OK_BITS)))
			goto error_tgt_fput;
	}

	/*
	 * At this point it is safe to assume that the "private_data" contains
	 * our own data structure.
	 */
	ep = f.file->private_data;

	/*
	 * When we insert an epoll file descriptor, inside another epoll file
	 * descriptor, there is the change of creating closed loops, which are
	 * better be handled here, than in more critical paths. While we are
	 * checking for loops we also determine the list of files reachable
	 * and hang them on the tfile_check_list, so we can check that we
	 * haven't created too many possible wakeup paths.
	 *
	 * We do not need to take the global 'epumutex' on EPOLL_CTL_ADD when
	 * the epoll file descriptor is attaching directly to a wakeup source,
	 * unless the epoll file descriptor is nested. The purpose of taking the
	 * 'epmutex' on add is to prevent complex toplogies such as loops and
	 * deep wakeup paths from forming in parallel through multiple
	 * EPOLL_CTL_ADD operations.
	 */
	mutex_lock_nested(&ep->mtx, 0);
	if (op == EPOLL_CTL_ADD) {
		if (!list_empty(&f.file->f_ep_links) ||
						is_file_epoll(tf.file)) {
			full_check = 1;
			mutex_unlock(&ep->mtx);
			mutex_lock(&epmutex);
			if (is_file_epoll(tf.file)) {
				error = -ELOOP;
				if (ep_loop_check(ep, tf.file) != 0) {
					clear_tfile_check_list();
					goto error_tgt_fput;
				}
			} else
				list_add(&tf.file->f_tfile_llink,
							&tfile_check_list);
			mutex_lock_nested(&ep->mtx, 0);
			if (is_file_epoll(tf.file)) {
				tep = tf.file->private_data;
				mutex_lock_nested(&tep->mtx, 1);
			}
		}
	}

	/*
	 * Try to lookup the file inside our RB tree, Since we grabbed "mtx"
	 * above, we can be sure to be able to use the item looked up by
	 * ep_find() till we release the mutex.
	 */
	epi = ep_find(ep, tf.file, fd);

	error = -EINVAL;
	switch (op) {
	case EPOLL_CTL_ADD:
		if (!epi) {
			epds.events |= POLLERR | POLLHUP;
			error = ep_insert(ep, &epds, tf.file, fd, full_check);
		} else
			error = -EEXIST;
		if (full_check)
			clear_tfile_check_list();
		break;
	case EPOLL_CTL_DEL:
		if (epi)
			error = ep_remove(ep, epi);
		else
			error = -ENOENT;
		break;
	case EPOLL_CTL_MOD:
		if (epi) {
			if (!(epi->event.events & EPOLLEXCLUSIVE)) {
				epds.events |= POLLERR | POLLHUP;
				error = ep_modify(ep, epi, &epds);
			}
		} else
			error = -ENOENT;
		break;
	}
	if (tep != NULL)
		mutex_unlock(&tep->mtx);
	mutex_unlock(&ep->mtx);

error_tgt_fput:
	if (full_check)
		mutex_unlock(&epmutex);

	fdput(tf);
error_fput:
	fdput(f);
error_return:

	return error;
}

1.2 ep_insert

fs/eventpoll.c

static int ep_insert(struct eventpoll *ep, struct epoll_event *event,
		     struct file *tfile, int fd, int full_check)
{
	int error, revents, pwake = 0;
	unsigned long flags;
	long user_watches;
	struct epitem *epi;
	struct ep_pqueue epq;

	// 检测目前监视数量是否超过限制
	user_watches = atomic_long_read(&ep->user->epoll_watches);
	if (unlikely(user_watches >= max_user_watches))
		return -ENOSPC;
		
	// 从slab中分配空间用于存储epitem
	if (!(epi = kmem_cache_alloc(epi_cache, GFP_KERNEL)))
		return -ENOMEM;

	/* Item initialization follow here ... */
	INIT_LIST_HEAD(&epi->rdllink);
	INIT_LIST_HEAD(&epi->fllink);
	INIT_LIST_HEAD(&epi->pwqlist);
	epi->ep = ep;
	
	/* 设置epitem在RB-Tree中的key */
	ep_set_ffd(&epi->ffd, tfile, fd);
	epi->event = *event;
	epi->nwait = 0;
	epi->next = EP_UNACTIVE_PTR;
	if (epi->event.events & EPOLLWAKEUP) {
		error = ep_create_wakeup_source(epi);
		if (error)
			goto error_create_wakeup_source;
	} else {
		RCU_INIT_POINTER(epi->ws, NULL);
	}

	/* Initialize the poll table using the queue callback */
	epq.epi = epi;
	/* 用ep_ptable_queue_proc作为poll的回调函数 */
	/* 回调函数的作用是将该epitem加入的文件的等待队列中去 */
	init_poll_funcptr(&epq.pt, ep_ptable_queue_proc);

	/*
	 * Attach the item to the poll hooks and get current event bits.
	 * We can safely use the file* here because its usage count has
	 * been increased by the caller of this function. Note that after
	 * this operation completes, the poll callback can start hitting
	 * the new item.
	 * 获取文件当前就绪事件的掩码,并执行poll的回调函数ep_ptable_queue_proc 
	 */
	revents = ep_item_poll(epi, &epq.pt);

	/*
	 * We have to check if something went wrong during the poll wait queue
	 * install process. Namely an allocation for a wait queue failed due
	 * high memory pressure.
	 */
	error = -ENOMEM;
	if (epi->nwait < 0)
		goto error_unregister;

	/**
	 * Add the current item to the list of active epoll hook for this file 
	 * 文件会链接所有监听自己的epitem,把当前epitem加入到文件的链表中
	 */
	spin_lock(&tfile->f_lock);
	list_add_tail_rcu(&epi->fllink, &tfile->f_ep_links);
	spin_unlock(&tfile->f_lock);

	/*
	 * Add the current item to the RB tree. All RB tree operations are
	 * protected by "mtx", and ep_insert() is called with "mtx" held.
	 * 将epitem插入到eventpoll的RB-Tree中
	 */
	ep_rbtree_insert(ep, epi);

	/* now check if we've created too many backpaths */
	error = -EINVAL;
	if (full_check && reverse_path_check())
		goto error_remove_epi;

	/* We have to drop the new item inside our item list to keep track of it */
	spin_lock_irqsave(&ep->lock, flags);

	/* If the file is already "ready" we drop it inside the ready list */
	if ((revents & event->events) && !ep_is_linked(&epi->rdllink)) {
		
		/* 将epitem加入到对应eventpoll的就绪链表中 */
		list_add_tail(&epi->rdllink, &ep->rdllist);
		ep_pm_stay_awake(epi);

		/* Notify waiting tasks that events are available */
		/* 唤醒所有阻塞与epoll_wait调用的所有进程/线程 */
		if (waitqueue_active(&ep->wq))
			wake_up_locked(&ep->wq);
			
		/* 唤醒阻塞于epoll该eventpoll的所有进程/线程 */
		if (waitqueue_active(&ep->poll_wait))
			pwake++;
	}

	spin_unlock_irqrestore(&ep->lock, flags);

	atomic_long_inc(&ep->user->epoll_watches);

	/* We have to call this outside the lock */
	if (pwake)
		ep_poll_safewake(&ep->poll_wait);

	return 0;

error_remove_epi:
	spin_lock(&tfile->f_lock);
	list_del_rcu(&epi->fllink);
	spin_unlock(&tfile->f_lock);

	rb_erase(&epi->rbn, &ep->rbr);

error_unregister:
	ep_unregister_pollwait(ep, epi);

	/*
	 * We need to do this because an event could have been arrived on some
	 * allocated wait queue. Note that we don't care about the ep->ovflist
	 * list, since that is used/cleaned only inside a section bound by "mtx".
	 * And ep_insert() is called with "mtx" held.
	 */
	spin_lock_irqsave(&ep->lock, flags);
	if (ep_is_linked(&epi->rdllink))
		list_del_init(&epi->rdllink);
	spin_unlock_irqrestore(&ep->lock, flags);

	wakeup_source_unregister(ep_wakeup_source(epi));

error_create_wakeup_source:
	kmem_cache_free(epi_cache, epi);

	return error;
}

/** 
 * ffd.file->f_op->poll的warpper
 * 调用poll_table中指定的回调函数
 * 如果文件当前发生了epitem的关心事件,返回事件掩码
 */
static inline unsigned int ep_item_poll(struct epitem *epi, poll_table *pt)
{
	pt->_key = epi->event.events;

	/**
	 * 调用被监视文件对应的poll函数
	 * 如果是tcp socket,那么调用tcp_poll
	 * 如果是udp socket,那么调用udp_poll
	 */
	return epi->ffd.file->f_op->poll(epi->ffd.file, pt) & epi->event.events;
}

1.3 tcp_poll

net/ipv4/tcp.c

unsigned int tcp_poll(struct file *file, struct socket *sock, poll_table *wait)
{
	unsigned int mask;
	struct sock *sk = sock->sk;
	const struct tcp_sock *tp = tcp_sk(sk);
	int state;

	sock_rps_record_flow(sk);
	
	/* 通过sk_sleep(sk)的得到该文件的等待队列作为第二个参数 */
	/* 最终以同样的这三个参数调用poll_wait */
	sock_poll_wait(file, sk_sleep(sk), wait);
	// code omitted
	return mask;
}

1.4 sk_sleep

该函数返回socket文件的的等待队列

include/net/sock.h

static inline wait_queue_head_t *sk_sleep(struct sock *sk)
{
	BUILD_BUG_ON(offsetof(struct socket_wq, wait) != 0);
	return &rcu_dereference_raw(sk->sk_wq)->wait;
}

1.5 poll_wait

该函数会调用poll_table->_qproc函数

include/linux/poll.h

static inline void poll_wait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p)
{
	/* 如果poll_table中指定函数不为空,那么就调用该函数,否则什么也不做 */
	if (p && p->_qproc && wait_address)
		p->_qproc(filp, wait_address, p);
}

1.6 ep_ptable_queue_proc

该函数是的作用是将包含epitem信息的等待队列项加入文件的等待队列

fs/eventpoll.c

/*
 * This is the callback that is used to add our wait queue to the
 * target file wakeup lists.
 */
static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead,
				 poll_table *pt)
{
	struct epitem *epi = ep_item_from_epqueue(pt);
	/* 用于关联epitem/wait_queue_t/wait_queue_head_t三者,以便在回调的时候能找到 */
	struct eppoll_entry *pwq;

	if (epi->nwait >= 0 && (pwq = kmem_cache_alloc(pwq_cache, GFP_KERNEL))) {
		/* 设置等待队列项的回调函数为ep_poll_callback */
		init_waitqueue_func_entry(&pwq->wait, ep_poll_callback);
		pwq->whead = whead;
		pwq->base = epi;
		
		/* 将配置好的等待队列项加入对应的等待队列中去 */
		if (epi->event.events & EPOLLEXCLUSIVE)
			add_wait_queue_exclusive(whead, &pwq->wait);
		else
			add_wait_queue(whead, &pwq->wait);
			
		/* 关联epitem和epoll_entry */
		list_add_tail(&pwq->llink, &epi->pwqlist);
		epi->nwait++;
	} else {
		/* We have to signal that an error occurred */
		epi->nwait = -1;
	}
}

1.7 ep_poll_callback

该函数是当文件有事件发生时的回调函数

fs/eventpoll.c

/*
 * This is the callback that is passed to the wait queue wakeup
 * mechanism. It is called by the stored file descriptors when they
 * have events to report.
 */
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;
	int ewake = 0;

	if ((unsigned long)key & POLLFREE) {
		ep_pwq_from_wait(wait)->whead = NULL;
		/*
		 * whead = NULL above can race with ep_remove_wait_queue()
		 * which can do another remove_wait_queue() after us, so we
		 * can't use __remove_wait_queue(). whead->lock is held by
		 * the caller.
		 */
		list_del_init(&wait->task_list);
	}

	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;
			if (epi->ws) {
				/*
				 * Activate ep->ws since epi->ws may get
				 * deactivated at any time.
				 */
				__pm_stay_awake(ep->ws);
			}

		}
		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);
		ep_pm_stay_awake_rcu(epi);
	}

	/*
	 * Wake up ( if active ) both the eventpoll wait list and the ->poll()
	 * wait list.
	 */
	if (waitqueue_active(&ep->wq)) {
		if ((epi->event.events & EPOLLEXCLUSIVE) &&
					!((unsigned long)key & POLLFREE)) {
			switch ((unsigned long)key & EPOLLINOUT_BITS) {
			case POLLIN:
				if (epi->event.events & POLLIN)
					ewake = 1;
				break;
			case POLLOUT:
				if (epi->event.events & POLLOUT)
					ewake = 1;
				break;
			case 0:
				ewake = 1;
				break;
			}
		}
		/* 唤醒阻塞于epoll_wait和epoll_pwait调用的进程/线程 */
		wake_up_locked(&ep->wq);
	}
	/* 唤醒阻塞于poll该eventpoll的进程/线程 */
	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);

	if (epi->event.events & EPOLLEXCLUSIVE)
		return ewake;

	return 1;
}

1.8 ep_modify

由于ep_modifyep_insert类似,所以不具体分析了

fs/eventpoll.c

/*
 * Modify the interest event mask by dropping an event if the new mask
 * has a match in the current file status. Must be called with "mtx" held.
 */
static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_event *event)
{
	int pwake = 0;
	unsigned int revents;
	poll_table pt;

	init_poll_funcptr(&pt, NULL);

	/*
	 * Set the new event interest mask before calling f_op->poll();
	 * otherwise we might miss an event that happens between the
	 * f_op->poll() call and the new event set registering.
	 */
	epi->event.events = event->events; /* need barrier below */
	epi->event.data = event->data; /* protected by mtx */
	if (epi->event.events & EPOLLWAKEUP) {
		if (!ep_has_wakeup_source(epi))
			ep_create_wakeup_source(epi);
	} else if (ep_has_wakeup_source(epi)) {
		ep_destroy_wakeup_source(epi);
	}

	/*
	 * The following barrier has two effects:
	 *
	 * 1) Flush epi changes above to other CPUs.  This ensures
	 *    we do not miss events from ep_poll_callback if an
	 *    event occurs immediately after we call f_op->poll().
	 *    We need this because we did not take ep->lock while
	 *    changing epi above (but ep_poll_callback does take
	 *    ep->lock).
	 *
	 * 2) We also need to ensure we do not miss _past_ events
	 *    when calling f_op->poll().  This barrier also
	 *    pairs with the barrier in wq_has_sleeper (see
	 *    comments for wq_has_sleeper).
	 *
	 * This barrier will now guarantee ep_poll_callback or f_op->poll
	 * (or both) will notice the readiness of an item.
	 */
	smp_mb();

	/*
	 * Get current event bits. We can safely use the file* here because
	 * its usage count has been increased by the caller of this function.
	 */
	revents = ep_item_poll(epi, &pt);

	/*
	 * If the item is "hot" and it is not registered inside the ready
	 * list, push it inside.
	 */
	if (revents & event->events) {
		spin_lock_irq(&ep->lock);
		if (!ep_is_linked(&epi->rdllink)) {
			list_add_tail(&epi->rdllink, &ep->rdllist);
			ep_pm_stay_awake(epi);

			/* Notify waiting tasks that events are available */
			if (waitqueue_active(&ep->wq))
				wake_up_locked(&ep->wq);
			if (waitqueue_active(&ep->poll_wait))
				pwake++;
		}
		spin_unlock_irq(&ep->lock);
	}

	/* We have to call this outside the lock */
	if (pwake)
		ep_poll_safewake(&ep->poll_wait);
                

	return 0;
}

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