epoll内核源码详解+自己总结的流程

发现自己发的一篇面经后,很多小伙伴向我索要epoll的内核源码实现,那我就在牛客网发下这源码还有自己总结的流程.另外网上很多博客说epoll使用了共享内存,这个是完全错误的 ,可以阅读源码,会发现完全没有使用共享内存的任何api,而是使用了copy_from_user跟__put_user进行内核跟用户虚拟空间数据交互.

/**  fs/eventpoll.c (Efficient event retrieval implementation)*  Copyright (C) 2001,...,2009  Davide Libenzi**  This program is free software; you can redistribute it and/or modify*  it under the terms of the GNU General Public License as published by*  the Free Software Foundation; either version 2 of the License, or*  (at your option) any later version.**  Davide Libenzi <davidel@xmailserver.org>**/
/** 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;/*这是一个单链表链接着所有的struct epitem当event转移到用户空间时*/* 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;
};
/* poll所用到的钩子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;//主描述符/* 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, 内核都有分配一个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);//初始化自己睡在的等待队列init_waitqueue_head(&ep->poll_wait);//初始化INIT_LIST_HEAD(&ep->rdllist);//初始化就绪链表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_***_alloc(epi_***, 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_***_free(epi_***, 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_***_alloc(pwq_***, 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);//从等待队列获取epitem.需要知道哪个进程挂载到这个设备struct eventpoll *ep = epi->ep;//获取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;//等待队列/** 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);//初始化等待队列,wait表示当前进程__add_wait_queue_exclusive(&ep->wq, &wait);//挂载到ep结构的等待队列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);//睡觉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 (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)) {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)) {/* 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... */

epoll_create
从slab缓存中创建一个eventpoll对象,并且创建一个匿名的fd跟fd对应的file对象,
而eventpoll对象保存在struct file结构的private指针中,并且返回,
该fd对应的file operations只是实现了poll跟release操作

创建eventpoll对象的初始化操作
获取当前用户信息,是不是root,最大监听fd数目等并且保存到eventpoll对象中
初始化等待队列,初始化就绪链表,初始化红黑树的头结点

epoll_ctl操作
将epoll_event结构拷贝到内核空间中
并且判断加入的fd是否支持poll结构(epoll,poll,selectI/O多路复用必须支持poll操作).
并且从epfd->file->privatedata获取event_poll对象,根据op区分是添加删除还是修改,
首先在eventpoll结构中的红黑树查找是否已经存在了相对应的fd,没找到就支持插入操作,否则报重复的错误.
相对应的修改,删除比较简单就不啰嗦了

插入操作时,会创建一个与fd对应的epitem结构,并且初始化相关成员,比如保存监听的fd跟file结构之类的
重要的是指定了调用poll_wait时的回调函数用于数据就绪时唤醒进程,(其内部,初始化设备的等待队列,将该进程注册到等待队列)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.
最后调用加入的fd的file operation->poll函数(最后会调用poll_wait操作)用于完成注册操作.
最后将epitem结构添加到红黑树中

epoll_wait操作
计算睡眠时间(如果有),判断eventpoll对象的链表是否为空,不为空那就干活不睡明.并且初始化一个等待队列,把自己挂上去,设置自己的进程状态
为可睡眠状态.判断是否有信号到来(有的话直接被中断醒来,),如果啥事都没有那就调用schedule_timeout进行睡眠,如果超时或者被唤醒,首先从自己初始化的等待队列删除
,然后开始拷贝资源给用户空间了
拷贝资源则是先把就绪事件链表转移到中间链表,然后挨个遍历拷贝到用户空间,
并且挨个判断其是否为水平触发,是的话再次插入到就绪链表

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