Epoll的实现代码浅析

这篇文章写的通俗易懂，特定转来。附上原文地址：https://raw.githubusercontent.com/lijie/kernel-doc/master/comment/eventpoll.c/* * 在深入了解epoll的实现之前, 先来了解内核的3个方面. * 1. 等待队列 waitqueue * 我们简单解释一下等待队列: * 队列头(wait_queue_head_t)往往是资源生产者, * 队列成员(wait_queue_t)往往是资源消费者, * 当头的资源ready后, 会逐个执行每个成员指定的回调函数, * 来通知它们资源已经ready了, 等待队列大致就这个意思. * 2. 内核的poll机制 * 被Poll的fd, 必须在实现上支持内核的Poll技术, * 比如fd是某个字符设备,或者是个socket, 它必须实现 * file_operations中的poll操作, 给自己分配有一个等待队列头. * 主动poll fd的某个进程必须分配一个等待队列成员, 添加到 * fd的对待队列里面去, 并指定资源ready时的回调函数. * 用socket做例子, 它必须有实现一个poll操作, 这个Poll是 * 发起轮询的代码必须主动调用的, 该函数中必须调用poll_wait(), * poll_wait会将发起者作为等待队列成员加入到socket的等待队列中去. * 这样socket发生状态变化时可以通过队列头逐个通知所有关心它的进程. * 这一点必须很清楚的理解, 否则会想不明白epoll是如何 * 得知fd的状态发生变化的. * 3. epollfd本身也是个fd, 所以它本身也可以被epoll, * 可以猜测一下它是不是可以无限嵌套epoll下去...  * * epoll基本上就是使用了上面的1,2点来完成. * 可见epoll本身并没有给内核引入什么特别复杂或者高深的技术, * 只不过是已有功能的重新组合, 达到了超过select的效果. *//*  * 相关的其它内核知识: * 1. fd我们知道是文件描述符, 在内核态, 与之对应的是struct file结构, * 可以看作是内核态的文件描述符. * 2. spinlock, 自旋锁, 必须要非常小心使用的锁, * 尤其是调用spin_lock_irqsave()的时候, 中断关闭, 不会发生进程调度, * 被保护的资源其它CPU也无法访问. 这个锁是很强力的, 所以只能锁一些 * 非常轻量级的操作. * 3. 引用计数在内核中是非常重要的概念, * 内核代码里面经常有些release, free释放资源的函数几乎不加任何锁, * 这是因为这些函数往往是在对象的引用计数变成0时被调用, * 既然没有进程在使用在这些对象, 自然也不需要加锁. * struct file 是持有引用计数的. *//* --- epoll相关的数据结构 --- *//* * This structure is stored inside the "private_data" member of the file * structure and rapresent the main data sructure for the eventpoll * interface. *//* 每创建一个epollfd, 内核就会分配一个eventpoll与之对应, 可以说是 * 内核态的epollfd. */struct eventpoll {/* Protect the this structure access */spinlock_t lock;/* * This mutex is used to ensure that files are not removed * while epoll is using them. This is held during the event * collection loop, the file cleanup path, the epoll file exit * code and the ctl operations. *//* 添加, 修改或者删除监听fd的时候, 以及epoll_wait返回, 向用户空间 * 传递数据时都会持有这个互斥锁, 所以在用户空间可以放心的在多个线程 * 中同时执行epoll相关的操作, 内核级已经做了保护. */struct mutex mtx;/* Wait queue used by sys_epoll_wait() *//* 调用epoll_wait()时, 我们就是"睡"在了这个等待队列上... */wait_queue_head_t wq;/* Wait queue used by file->poll() *//* 这个用于epollfd本事被poll的时候... */wait_queue_head_t poll_wait;/* List of ready file descriptors *//* 所有已经ready的epitem都在这个链表里面 */struct list_head rdllist;/* RB tree root used to store monitored fd structs *//* 所有要监听的epitem都在这里 */struct rb_root rbr;/* * This is a single linked list that chains all the "struct epitem" that * happened while transfering ready events to userspace w/out * holding ->lock. */struct epitem *ovflist;/* The user that created the eventpoll descriptor *//* 这里保存了一些用户变量, 比如fd监听数量的最大值等等 */struct user_struct *user;};/* * Each file descriptor added to the eventpoll interface will * have an entry of this type linked to the "rbr" RB tree. *//* epitem 表示一个被监听的fd */struct epitem {/* RB tree node used to link this structure to the eventpoll RB tree *//* rb_node, 当使用epoll_ctl()将一批fds加入到某个epollfd时, 内核会分配 * 一批的epitem与fds们对应, 而且它们以rb_tree的形式组织起来, tree的root * 保存在epollfd, 也就是struct eventpoll中.  * 在这里使用rb_tree的原因我认为是提高查找,插入以及删除的速度. * rb_tree对以上3个操作都具有O(lgN)的时间复杂度 */struct rb_node rbn;/* List header used to link this structure to the eventpoll ready list *//* 链表节点, 所有已经ready的epitem都会被链到eventpoll的rdllist中 */struct list_head rdllink;/* * Works together "struct eventpoll"->ovflist in keeping the * single linked chain of items. *//* 这个在代码中再解释... */struct epitem *next;/* The file descriptor information this item refers to *//* epitem对应的fd和struct file */struct epoll_filefd ffd;/* Number of active wait queue attached to poll operations */int nwait;/* List containing poll wait queues */struct list_head pwqlist;/* The "container" of this item *//* 当前epitem属于哪个eventpoll */struct eventpoll *ep;/* List header used to link this item to the "struct file" items list */struct list_head fllink;/* The structure that describe the interested events and the source fd *//* 当前的epitem关系哪些events, 这个数据是调用epoll_ctl时从用户态传递过来 */struct epoll_event event;};struct epoll_filefd {struct file *file;int fd;};/* Wait structure used by the poll hooks */struct eppoll_entry {/* List header used to link this structure to the "struct epitem" */struct list_head llink;/* The "base" pointer is set to the container "struct epitem" */struct epitem *base;/* * Wait queue item that will be linked to the target file wait * queue head. */wait_queue_t wait;/* The wait queue head that linked the "wait" wait queue item */wait_queue_head_t *whead;};/* Wrapper struct used by poll queueing */struct ep_pqueue {poll_table pt;struct epitem *epi;};/* Used by the ep_send_events() function as callback private data */struct ep_send_events_data {int maxevents;struct epoll_event __user *events;};/* --- 代码注释 --- *//* 你没看错, 这就是epoll_create()的真身, 基本啥也不干直接调用epoll_create1了, * 另外你也可以发现, size这个参数其实是没有任何用处的... */SYSCALL_DEFINE1(epoll_create, int, size){        if (size <= 0)                return -EINVAL;        return sys_epoll_create1(0);}/* 这才是真正的epoll_create啊~~ */SYSCALL_DEFINE1(epoll_create1, int, flags){int error;struct eventpoll *ep = NULL;/* Check the EPOLL_* constant for consistency.  *//* 这句没啥用处... */BUILD_BUG_ON(EPOLL_CLOEXEC != O_CLOEXEC);/* 对于epoll来讲, 目前唯一有效的FLAG就是CLOEXEC */if (flags & ~EPOLL_CLOEXEC)return -EINVAL;/* * Create the internal data structure ("struct eventpoll"). *//* 分配一个struct eventpoll, 分配和初始化细节我们随后深聊~ */error = ep_alloc(&ep);if (error < 0)return error;/* * Creates all the items needed to setup an eventpoll file. That is, * a file structure and a free file descriptor. *//* 这里是创建一个匿名fd, 说起来就话长了...长话短说: * epollfd本身并不存在一个真正的文件与之对应, 所以内核需要创建一个 * "虚拟"的文件, 并为之分配真正的struct file结构, 而且有真正的fd. * 这里2个参数比较关键: * eventpoll_fops, fops就是file operations, 就是当你对这个文件(这里是虚拟的)进行操作(比如读)时, * fops里面的函数指针指向真正的操作实现, 类似C++里面虚函数和子类的概念. * epoll只实现了poll和release(就是close)操作, 其它文件系统操作都有VFS全权处理了. * ep, ep就是struct epollevent, 它会作为一个私有数据保存在struct file的private指针里面. * 其实说白了, 就是为了能通过fd找到struct file, 通过struct file能找到eventpoll结构. * 如果懂一点Linux下字符设备驱动开发, 这里应该是很好理解的, * 推荐阅读 <Linux device driver 3rd> */error = anon_inode_getfd("[eventpoll]", &eventpoll_fops, ep, O_RDWR | (flags & O_CLOEXEC));if (error < 0)ep_free(ep);return error;}/* * 创建好epollfd后, 接下来我们要往里面添加fd咯* 来看epoll_ctl* epfd 就是epollfd* op ADD,MOD,DEL* fd 需要监听的描述符* event 我们关心的events*/SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,struct epoll_event __user *, event){int error;struct file *file, *tfile;struct eventpoll *ep;struct epitem *epi;struct epoll_event epds;error = -EFAULT;/*  * 错误处理以及从用户空间将epoll_event结构copy到内核空间. */if (ep_op_has_event(op) &&    copy_from_user(&epds, event, sizeof(struct epoll_event)))goto error_return;/* Get the "struct file *" for the eventpoll file *//* 取得struct file结构, epfd既然是真正的fd, 那么内核空间 * 就会有与之对于的一个struct file结构 * 这个结构在epoll_create1()中, 由函数anon_inode_getfd()分配 */error = -EBADF;file = fget(epfd);if (!file)goto error_return;/* Get the "struct file *" for the target file *//* 我们需要监听的fd, 它当然也有个struct file结构, 上下2个不要搞混了哦 */tfile = fget(fd);if (!tfile)goto error_fput;/* The target file descriptor must support poll */error = -EPERM;/* 如果监听的文件不支持poll, 那就没辙了. * 你知道什么情况下, 文件会不支持poll吗? */if (!tfile->f_op || !tfile->f_op->poll)goto error_tgt_fput;/* * 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;/* epoll不能自己监听自己... */if (file == tfile || !is_file_epoll(file))goto error_tgt_fput;/* * At this point it is safe to assume that the "private_data" contains * our own data structure. *//* 取到我们的eventpoll结构, 来自与epoll_create1()中的分配 */ep = file->private_data;/* 接下来的操作有可能修改数据结构内容, 锁之~ */mutex_lock(&ep->mtx);/* * 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. *//* 对于每一个监听的fd, 内核都有分片一个epitem结构, * 而且我们也知道, epoll是不允许重复添加fd的, * 所以我们首先查找该fd是不是已经存在了. * ep_find()其实就是RBTREE查找, 跟C++STL的map差不多一回事, O(lgn)的时间复杂度. */epi = ep_find(ep, tfile, fd);error = -EINVAL;switch (op) {/* 首先我们关心添加 */case EPOLL_CTL_ADD:if (!epi) {/* 之前的find没有找到有效的epitem, 证明是第一次插入, 接受! * 这里我们可以知道, POLLERR和POLLHUP事件内核总是会关心的 * */epds.events |= POLLERR | POLLHUP;/* rbtree插入, 详情见ep_insert()的分析 * 其实我觉得这里有insert的话, 之前的find应该 * 是可以省掉的... */error = ep_insert(ep, &epds, tfile, fd);} else/* 找到了!? 重复添加! */error = -EEXIST;break;/* 删除和修改操作都比较简单 */case EPOLL_CTL_DEL:if (epi)error = ep_remove(ep, epi);elseerror = -ENOENT;break;case EPOLL_CTL_MOD:if (epi) {epds.events |= POLLERR | POLLHUP;error = ep_modify(ep, epi, &epds);} elseerror = -ENOENT;break;}mutex_unlock(&ep->mtx);error_tgt_fput:fput(tfile);error_fput:fput(file);error_return:return error;}/* 分配一个eventpoll结构 */static int ep_alloc(struct eventpoll **pep){int error;struct user_struct *user;struct eventpoll *ep;/* 获取当前用户的一些信息, 比如是不是root啦, 最大监听fd数目啦 */user = get_current_user();error = -ENOMEM;ep = kzalloc(sizeof(*ep), GFP_KERNEL);if (unlikely(!ep))goto free_uid;/* 这些都是初始化啦 */spin_lock_init(&ep->lock);mutex_init(&ep->mtx);init_waitqueue_head(&ep->wq);init_waitqueue_head(&ep->poll_wait);INIT_LIST_HEAD(&ep->rdllist);ep->rbr = RB_ROOT;ep->ovflist = EP_UNACTIVE_PTR;ep->user = user;*pep = ep;return 0;free_uid:free_uid(user);return error;}/* * Must be called with "mtx" held. *//*  * ep_insert()在epoll_ctl()中被调用, 完成往epollfd里面添加一个监听fd的工作 * tfile是fd在内核态的struct file结构 */static int ep_insert(struct eventpoll *ep, struct epoll_event *event,     struct file *tfile, int fd){int error, revents, pwake = 0;unsigned long flags;struct epitem *epi;struct ep_pqueue epq;/* 查看是否达到当前用户的最大监听数 */if (unlikely(atomic_read(&ep->user->epoll_watches) >=     max_user_watches))return -ENOSPC;/* 从著名的slab中分配一个epitem */if (!(epi = kmem_cache_alloc(epi_cache, GFP_KERNEL)))return -ENOMEM;/* Item initialization follow here ... *//* 这些都是相关成员的初始化... */INIT_LIST_HEAD(&epi->rdllink);INIT_LIST_HEAD(&epi->fllink);INIT_LIST_HEAD(&epi->pwqlist);epi->ep = ep;/* 这里保存了我们需要监听的文件fd和它的file结构 */ep_set_ffd(&epi->ffd, tfile, fd);epi->event = *event;epi->nwait = 0;/* 这个指针的初值不是NULL哦... */epi->next = EP_UNACTIVE_PTR;/* Initialize the poll table using the queue callback *//* 好, 我们终于要进入到poll的正题了 */epq.epi = epi;/* 初始化一个poll_table * 其实就是指定调用poll_wait(注意不是epoll_wait!!!)时的回调函数,和我们关心哪些events, * ep_ptable_queue_proc()就是我们的回调啦, 初值是所有event都关心 */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. *//* 这一部很关键, 也比较难懂, 完全是内核的poll机制导致的... * 首先, f_op->poll()一般来说只是个wrapper, 它会调用真正的poll实现, * 拿UDP的socket来举例, 这里就是这样的调用流程: f_op->poll(), sock_poll(), * udp_poll(), datagram_poll(), sock_poll_wait(), 最后调用到我们上面指定的 * ep_ptable_queue_proc()这个回调函数...(好深的调用路径...). * 完成这一步, 我们的epitem就跟这个socket关联起来了, 当它有状态变化时, * 会通过ep_poll_callback()来通知. * 最后, 这个函数还会查询当前的fd是不是已经有啥event已经ready了, 有的话 * 会将event返回. */revents = tfile->f_op->poll(tfile, &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 *//* 这个就是每个文件会将所有监听自己的epitem链起来 */spin_lock(&tfile->f_lock);list_add_tail(&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. *//* 都搞定后, 将epitem插入到对应的eventpoll中去 */ep_rbtree_insert(ep, 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 *//* 到达这里后, 如果我们监听的fd已经有事件发生, 那就要处理一下 */if ((revents & event->events) && !ep_is_linked(&epi->rdllink)) {/* 将当前的epitem加入到ready list中去 */list_add_tail(&epi->rdllink, &ep->rdllist);/* Notify waiting tasks that events are available *//* 谁在epoll_wait, 就唤醒它... */if (waitqueue_active(&ep->wq))wake_up_locked(&ep->wq);/* 谁在epoll当前的epollfd, 也唤醒它... */if (waitqueue_active(&ep->poll_wait))pwake++;}spin_unlock_irqrestore(&ep->lock, flags);atomic_inc(&ep->user->epoll_watches);/* We have to call this outside the lock */if (pwake)ep_poll_safewake(&ep->poll_wait);return 0;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);kmem_cache_free(epi_cache, epi);return error;}/* * This is the callback that is used to add our wait queue to the * target file wakeup lists. *//*  * 该函数在调用f_op->poll()时会被调用. * 也就是epoll主动poll某个fd时, 用来将epitem与指定的fd关联起来的. * 关联的办法就是使用等待队列(waitqueue) */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);struct eppoll_entry *pwq;if (epi->nwait >= 0 && (pwq = kmem_cache_alloc(pwq_cache, GFP_KERNEL))) {/* 初始化等待队列, 指定ep_poll_callback为唤醒时的回调函数, * 当我们监听的fd发生状态改变时, 也就是队列头被唤醒时, * 指定的回调函数将会被调用. */init_waitqueue_func_entry(&pwq->wait, ep_poll_callback);pwq->whead = whead;pwq->base = epi;/* 将刚分配的等待队列成员加入到头中, 头是由fd持有的 */add_wait_queue(whead, &pwq->wait);list_add_tail(&pwq->llink, &epi->pwqlist);/* nwait记录了当前epitem加入到了多少个等待队列中, * 我认为这个值最大也只会是1... */epi->nwait++;} else {/* We have to signal that an error occurred */epi->nwait = -1;}}/* * This is the callback that is passed to the wait queue wakeup * machanism. It is called by the stored file descriptors when they * have events to report. *//*  * 这个是关键性的回调函数, 当我们监听的fd发生状态改变时, 它会被调用. * 参数key被当作一个unsigned long整数使用, 携带的是events. */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. *//* 没有我们关心的event... */if (key && !((unsigned long) key & epi->event.events))goto out_unlock;/* * If we are trasfering 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 happens during that period of time are * chained in ep->ovflist and requeued later on. *//*  * 这里看起来可能有点费解, 其实干的事情比较简单: * 如果该callback被调用的同时, epoll_wait()已经返回了, * 也就是说, 此刻应用程序有可能已经在循环获取events, * 这种情况下, 内核将此刻发生event的epitem用一个单独的链表 * 链起来, 不发给应用程序, 也不丢弃, 而是在下一次epoll_wait * 时返回给用户. */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 *//* 将当前的epitem放入ready list */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. *//* 唤醒epoll_wait... */if (waitqueue_active(&ep->wq))wake_up_locked(&ep->wq);/* 如果epollfd也在被poll, 那就唤醒队列里面的所有成员. */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;}/* * Implement the event wait interface for the eventpoll file. It is the kernel * part of the user space epoll_wait(2). */SYSCALL_DEFINE4(epoll_wait, int, epfd, struct epoll_event __user *, events,int, maxevents, int, timeout){int error;struct file *file;struct eventpoll *ep;/* The maximum number of event must be greater than zero */if (maxevents <= 0 || maxevents > EP_MAX_EVENTS)return -EINVAL;/* Verify that the area passed by the user is writeable *//* 这个地方有必要说明一下: * 内核对应用程序采取的策略是"绝对不信任", * 所以内核跟应用程序之间的数据交互大都是copy, 不允许(也时候也是不能...)指针引用. * epoll_wait()需要内核返回数据给用户空间, 内存由用户程序提供, * 所以内核会用一些手段来验证这一段内存空间是不是有效的. */if (!access_ok(VERIFY_WRITE, events, maxevents * sizeof(struct epoll_event))) {error = -EFAULT;goto error_return;}/* Get the "struct file *" for the eventpoll file */error = -EBADF;/* 获取epollfd的struct file, epollfd也是文件嘛 */file = fget(epfd);if (!file)goto error_return;/* * We have to check that the file structure underneath the fd * the user passed to us _is_ an eventpoll file. */error = -EINVAL;/* 检查一下它是不是一个真正的epollfd... */if (!is_file_epoll(file))goto error_fput;/* * At this point it is safe to assume that the "private_data" contains * our own data structure. *//* 获取eventpoll结构 */ep = file->private_data;/* Time to fish for events ... *//* OK, 睡觉, 等待事件到来~~ */error = ep_poll(ep, events, maxevents, timeout);error_fput:fput(file);error_return:return error;}/* 这个函数真正将执行epoll_wait的进程带入睡眠状态... */static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,   int maxevents, long timeout){int res, eavail;unsigned long flags;long jtimeout;wait_queue_t wait;/* * Calculate the timeout by checking for the "infinite" value (-1) * and the overflow condition. The passed timeout is in milliseconds, * that why (t * HZ) / 1000. *//* 计算睡觉时间, 毫秒要转换为HZ */jtimeout = (timeout < 0 || timeout >= EP_MAX_MSTIMEO) ?MAX_SCHEDULE_TIMEOUT : (timeout * HZ + 999) / 1000;retry:spin_lock_irqsave(&ep->lock, flags);res = 0;/* 如果ready list不为空, 就不睡了, 直接干活... */if (list_empty(&ep->rdllist)) {/* * We don't have any available event to return to the caller. * We need to sleep here, and we will be wake up by * ep_poll_callback() when events will become available. *//* OK, 初始化一个等待队列, 准备直接把自己挂起, * 注意current是一个宏, 代表当前进程 */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. *//* 将当前进程设置位睡眠, 但是可以被信号唤醒的状态, * 注意这个设置是"将来时", 我们此刻还没睡! */set_current_state(TASK_INTERRUPTIBLE);/* 如果这个时候, ready list里面有成员了, * 或者睡眠时间已经过了, 就直接不睡了... */if (!list_empty(&ep->rdllist) || !jtimeout)break;/* 如果有信号产生, 也起床... */if (signal_pending(current)) {res = -EINTR;break;}/* 啥事都没有,解锁, 睡觉... */spin_unlock_irqrestore(&ep->lock, flags);/* jtimeout这个时间后, 会被唤醒, * ep_poll_callback()如果此时被调用, * 那么我们就会直接被唤醒, 不用等时间了...  * 再次强调一下ep_poll_callback()的调用时机是由被监听的fd * 的具体实现, 比如socket或者某个设备驱动来决定的, * 因为等待队列头是他们持有的, epoll和当前进程 * 只是单纯的等待... **/jtimeout = schedule_timeout(jtimeout);spin_lock_irqsave(&ep->lock, flags);}__remove_wait_queue(&ep->wq, &wait);/* OK 我们醒来了... */set_current_state(TASK_RUNNING);}/* Is it worth to try to dig for events ? */eavail = !list_empty(&ep->rdllist) || ep->ovflist != EP_UNACTIVE_PTR;spin_unlock_irqrestore(&ep->lock, flags);/* * Try to transfer events to user space. In case we get 0 events and * there's still timeout left over, we go trying again in search of * more luck. *//* 如果一切正常, 有event发生, 就开始准备数据copy给用户空间了... */if (!res && eavail &&    !(res = ep_send_events(ep, events, maxevents)) && jtimeout)goto retry;return res;}/* 这个简单, 我们直奔下一个... */static int ep_send_events(struct eventpoll *ep,  struct epoll_event __user *events, int maxevents){struct ep_send_events_data esed;esed.maxevents = maxevents;esed.events = events;return ep_scan_ready_list(ep, ep_send_events_proc, &esed);}/** * ep_scan_ready_list - Scans the ready list in a way that makes possible for *                      the scan code, to call f_op->poll(). Also allows for *                      O(NumReady) performance. * * @ep: Pointer to the epoll private data structure. * @sproc: Pointer to the scan callback. * @priv: Private opaque data passed to the @sproc callback. * * Returns: The same integer error code returned by the @sproc callback. */static int ep_scan_ready_list(struct eventpoll *ep,      int (*sproc)(struct eventpoll *,   struct list_head *, void *),      void *priv){int error, pwake = 0;unsigned long flags;struct epitem *epi, *nepi;LIST_HEAD(txlist);/* * We need to lock this because we could be hit by * eventpoll_release_file() and epoll_ctl(). */mutex_lock(&ep->mtx);/* * Steal the ready list, and re-init the original one to the * empty list. Also, set ep->ovflist to NULL so that events * happening while looping w/out locks, are not lost. We cannot * have the poll callback to queue directly on ep->rdllist, * because we want the "sproc" callback to be able to do it * in a lockless way. */spin_lock_irqsave(&ep->lock, flags);/* 这一步要注意, 首先, 所有监听到events的epitem都链到rdllist上了, * 但是这一步之后, 所有的epitem都转移到了txlist上, 而rdllist被清空了, * 要注意哦, rdllist已经被清空了! */list_splice_init(&ep->rdllist, &txlist);/* ovflist, 在ep_poll_callback()里面我解释过, 此时此刻我们不希望 * 有新的event加入到ready list中了, 保存后下次再处理... */ep->ovflist = NULL;spin_unlock_irqrestore(&ep->lock, flags);/* * Now call the callback function. *//* 在这个回调函数里面处理每个epitem * sproc 就是 ep_send_events_proc, 下面会注释到. */error = (*sproc)(ep, &txlist, priv);spin_lock_irqsave(&ep->lock, flags);/* * During the time we spent inside the "sproc" callback, some * other events might have been queued by the poll callback. * We re-insert them inside the main ready-list here. *//* 现在我们来处理ovflist, 这些epitem都是我们在传递数据给用户空间时 * 监听到了事件. */for (nepi = ep->ovflist; (epi = nepi) != NULL;     nepi = epi->next, epi->next = EP_UNACTIVE_PTR) {/* * We need to check if the item is already in the list. * During the "sproc" callback execution time, items are * queued into ->ovflist but the "txlist" might already * contain them, and the list_splice() below takes care of them. *//* 将这些直接放入readylist */if (!ep_is_linked(&epi->rdllink))list_add_tail(&epi->rdllink, &ep->rdllist);}/* * We need to set back ep->ovflist to EP_UNACTIVE_PTR, so that after * releasing the lock, events will be queued in the normal way inside * ep->rdllist. */ep->ovflist = EP_UNACTIVE_PTR;/* * Quickly re-inject items left on "txlist". *//* 上一次没有处理完的epitem, 重新插入到ready list */list_splice(&txlist, &ep->rdllist);/* ready list不为空, 直接唤醒... */if (!list_empty(&ep->rdllist)) {/* * Wake up (if active) both the eventpoll wait list and * the ->poll() wait list (delayed after we release the lock). */if (waitqueue_active(&ep->wq))wake_up_locked(&ep->wq);if (waitqueue_active(&ep->poll_wait))pwake++;}spin_unlock_irqrestore(&ep->lock, flags);mutex_unlock(&ep->mtx);/* We have to call this outside the lock */if (pwake)ep_poll_safewake(&ep->poll_wait);return error;}/* 该函数作为callbakc在ep_scan_ready_list()中被调用 * head是一个链表, 包含了已经ready的epitem, * 这个不是eventpoll里面的ready list, 而是上面函数中的txlist. */static int ep_send_events_proc(struct eventpoll *ep, struct list_head *head,       void *priv){struct ep_send_events_data *esed = priv;int eventcnt;unsigned int revents;struct epitem *epi;struct epoll_event __user *uevent;/* * We can loop without lock because we are passed a task private list. * Items cannot vanish during the loop because ep_scan_ready_list() is * holding "mtx" during this call. *//* 扫描整个链表... */for (eventcnt = 0, uevent = esed->events;     !list_empty(head) && eventcnt < esed->maxevents;) {/* 取出第一个成员 */epi = list_first_entry(head, struct epitem, rdllink);/* 然后从链表里面移除 */list_del_init(&epi->rdllink);/* 读取events,  * 注意events我们ep_poll_callback()里面已经取过一次了, 为啥还要再取? * 1. 我们当然希望能拿到此刻的最新数据, events是会变的~ * 2. 不是所有的poll实现, 都通过等待队列传递了events, 有可能某些驱动压根没传 * 必须主动去读取. */revents = epi->ffd.file->f_op->poll(epi->ffd.file, NULL) &epi->event.events;/* * If the event mask intersect the caller-requested one, * deliver the event to userspace. Again, ep_scan_ready_list() * is holding "mtx", so no operations coming from userspace * can change the item. */if (revents) {/* 将当前的事件和用户传入的数据都copy给用户空间, * 就是epoll_wait()后应用程序能读到的那一堆数据. */if (__put_user(revents, &uevent->events) ||    __put_user(epi->event.data, &uevent->data)) {/* 如果copy过程中发生错误, 会中断链表的扫描, * 并把当前发生错误的epitem重新插入到ready list. * 剩下的没处理的epitem也不会丢弃, 在ep_scan_ready_list() * 中它们也会被重新插入到ready list */list_add(&epi->rdllink, head);return eventcnt ? eventcnt : -EFAULT;}eventcnt++;uevent++;if (epi->event.events & EPOLLONESHOT)epi->event.events &= EP_PRIVATE_BITS;else if (!(epi->event.events & EPOLLET)) {/* * If this file has been added with Level * Trigger mode, we need to insert back inside * the ready list, so that the next call to * epoll_wait() will check again the events * availability. At this point, noone can insert * into ep->rdllist besides us. The epoll_ctl() * callers are locked out by * ep_scan_ready_list() holding "mtx" and the * poll callback will queue them in ep->ovflist. *//* 嘿嘿, EPOLLET和非ET的区别就在这一步之差呀~ * 如果是ET, epitem是不会再进入到readly list, * 除非fd再次发生了状态改变, ep_poll_callback被调用. * 如果是非ET, 不管你还有没有有效的事件或者数据, * 都会被重新插入到ready list, 再下一次epoll_wait * 时, 会立即返回, 并通知给用户空间. 当然如果这个 * 被监听的fds确实没事件也没数据了, epoll_wait会返回一个0, * 空转一次. */list_add_tail(&epi->rdllink, &ep->rdllist);}}}return eventcnt;}/* ep_free在epollfd被close时调用, * 释放一些资源而已, 比较简单 */static void ep_free(struct eventpoll *ep){struct rb_node *rbp;struct epitem *epi;/* We need to release all tasks waiting for these file */if (waitqueue_active(&ep->poll_wait))ep_poll_safewake(&ep->poll_wait);/* * We need to lock this because we could be hit by * eventpoll_release_file() while we're freeing the "struct eventpoll". * We do not need to hold "ep->mtx" here because the epoll file * is on the way to be removed and no one has references to it * anymore. The only hit might come from eventpoll_release_file() but * holding "epmutex" is sufficent here. */mutex_lock(&epmutex);/* * Walks through the whole tree by unregistering poll callbacks. */for (rbp = rb_first(&ep->rbr); rbp; rbp = rb_next(rbp)) {epi = rb_entry(rbp, struct epitem, rbn);ep_unregister_pollwait(ep, epi);}/* * Walks through the whole tree by freeing each "struct epitem". At this * point we are sure no poll callbacks will be lingering around, and also by * holding "epmutex" we can be sure that no file cleanup code will hit * us during this operation. So we can avoid the lock on "ep->lock". *//* 之所以在关闭epollfd之前不需要调用epoll_ctl移除已经添加的fd, * 是因为这里已经做了... */while ((rbp = rb_first(&ep->rbr)) != NULL) {epi = rb_entry(rbp, struct epitem, rbn);ep_remove(ep, epi);}mutex_unlock(&epmutex);mutex_destroy(&ep->mtx);free_uid(ep->user);kfree(ep);}/* File callbacks that implement the eventpoll file behaviour */static const struct file_operations eventpoll_fops = {.release= ep_eventpoll_release,.poll= ep_eventpoll_poll};/* Fast test to see if the file is an evenpoll file */static inline int is_file_epoll(struct file *f){return f->f_op == &eventpoll_fops;}/* OK, eventpoll我认为比较重要的函数都注释完了... */