epoll内核源码详解

牛客网上找到的一篇博客，仅供自己学习使用，

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的等待队列里面去,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

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.

作者：赛罗奥特曼~
链接：https://www.nowcoder.com/discuss/26226?type=0&order=0&pos=27&page=1
来源：牛客网

/*
 * 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;
};

作者：赛罗奥特曼~
链接：https://www.nowcoder.com/discuss/26226?type=0&order=0&pos=27&page=1
来源：牛客网

/*
 * 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);
        else
            error = -ENOENT;
        break;
    case EPOLL_CTL_MOD:
        if (epi) {
            epds.events |= POLLERR | POLLHUP;
            error = ep_modify(ep, epi, &epds);
        } else
            error = -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_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);//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