深入理解epoll：高效I/O多路复用的核心技术（下）

/*
 *  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);
    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_***_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... */

int epoll_create(int size); //监听个数

int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event)

EPOLL_CTL_ADD：注册新的fd到epfd中；
EPOLL_CTL_MOD：修改已经注册的fd的监听事件；
EPOLL_CTL_DEL：从epfd中删除一个fd；

struct epoll_event {  
    __uint32_t events; /* Epoll events */
    epoll_data_t data; /* User data variable */
};

int epoll_wait(int epfd, struct epoll_event * events, int maxevents, int timeout)

#define LISTEN_MAX     5
#define EPOLL_FDSIZE   LISTEN_MAX
#define EPOLL_EVENTS   20
#define CLIENT_NUM     3
void EpollAddEvent(int epollfd, int fd, int event)
{
    PRINT("epollfd:%d add fd:%d(event:%d)\n", epollfd, fd, event);
    struct epoll_event ev;
    ev.events = event;
    ev.data.fd = fd;
    epoll_ctl(epollfd, EPOLL_CTL_ADD, fd, &ev);
}
void TcpServerThread()
{
  //------------socket
  int sockfd = socket(AF_UNIX, SOCK_STREAM, 0);
  if (sockfd < 0)
  {
    PRINT("create socket fail\n");
    return;
  }
  PRINT("create socketfd:%d\n", sockfd);
  struct sockaddr_un addr;
  memset (&addr, 0, sizeof(addr));
  addr.sun_family = AF_UNIX;
  strcpy(addr.sun_path, UNIX_TCP_SOCKET_ADDR);
  //------------bind
  if (bind(sockfd, (struct sockaddr *)&addr, sizeof(addr)))
  {
    PRINT("bind fail\n");
    return;
  }
  PRINT("bind ok\n");
  //------------listen
  if (listen(sockfd, LISTEN_MAX))
  {
    PRINT("listen fail\n");
    return;
  }
  PRINT("listen ok\n");
  //------------epoll---------------
  int epollfd = epoll_create(EPOLL_FDSIZE);
  if (epollfd < 0)
  {
    PRINT("epoll create fail\n");
    return;
  }
  PRINT("epoll create fd:%d\n", epollfd);
  EpollAddEvent(epollfd, sockfd, EPOLLIN);
  struct epoll_event events[EPOLL_EVENTS];
  while(1)
  {
    PRINT("epoll wait...\n");
    int num = epoll_wait(epollfd, events, EPOLL_EVENTS, -1);
    PRINT("epoll wait done, num:%d\n", num);
    for (int i = 0;i < num;i++)
    {
      int fd = events[i].data.fd;
      if (EPOLLIN == events[i].events)
      {
        //接受客户端的连接请求
        if (fd == sockfd)
        {
          //------------accept
          int clientfd = accept(sockfd, NULL, NULL);
          if (clientfd == -1)
          {
            PRINT("accpet error\n");
          }
          else
          {
            PRINT("=====> accept new clientfd:%d\n", clientfd);
            EpollAddEvent(epollfd, clientfd, EPOLLIN);
          }
        }
        //读取客户端发来的数据
        else
        {
          char buf[BUF_SIZE] = {0};
          //------------recv
          size_t size = recv(fd, buf, BUF_SIZE, 0);
          //size = read(clientfd, buf, BUF_SIZE);
          if (size > 0)
          {
            PRINT("recv from clientfd:%d, msg:%s\n", fd, buf);
          }
        }
      }
    }
  }
  PRINT("end\n");
}

int main()
{
  unlink(UNIX_TCP_SOCKET_ADDR);
  //创建一个服务端
  thread thServer(TcpServerThread);
  //创建多个客户端
  thread thClinet[CLIENT_NUM];
  for (int i=0; i<CLIENT_NUM; i++)
  {
    thClinet[i] = thread(TcpClientThread);
    sleep(1);
  }
  while(1)
  {
    sleep(5);
  }
}

[TcpServerThread] create socketfd:3
[TcpServerThread] bind ok
[TcpClientThread] create socketfd:4
[TcpServerThread] listen ok
[TcpServerThread] epoll create fd:5
[EpollAddEvent] epollfd:5 add fd:3(event:1)
[TcpServerThread] epoll wait...
[TcpClientThread] create socketfd:6
[TcpClientThread] connect ok
[TcpServerThread] epoll wait done, num:1
[TcpServerThread] =====> accept new clientfd:7
[EpollAddEvent] epollfd:5 add fd:7(event:1)
[TcpServerThread] epoll wait...
[TcpServerThread] epoll wait done, num:1
[TcpServerThread] recv from clientfd:7, msg:helloTCP(fd:4)1
[TcpServerThread] epoll wait...
[TcpClientThread] create socketfd:8
[TcpClientThread] connect ok
[TcpServerThread] epoll wait done, num:2
[TcpServerThread] recv from clientfd:7, msg:helloTCP(fd:4)2
[TcpServerThread] =====> accept new clientfd:9
[EpollAddEvent] epollfd:5 add fd:9(event:1)
[TcpServerThread] epoll wait...
[TcpServerThread] epoll wait done, num:1
[TcpServerThread] recv from clientfd:9, msg:helloTCP(fd:6)3
[TcpServerThread] epoll wait...
[TcpClientThread] connect ok
[TcpServerThread] epoll wait done, num:3
[TcpServerThread] =====> accept new clientfd:10
[EpollAddEvent] epollfd:5 add fd:10(event:1)
[TcpServerThread] recv from clientfd:7, msg:helloTCP(fd:4)5
[TcpServerThread] recv from clientfd:9, msg:helloTCP(fd:6)6
[TcpServerThread] epoll wait...
[TcpServerThread] epoll wait done, num:1
[TcpServerThread] recv from clientfd:10, msg:helloTCP(fd:8)4
[TcpServerThread] epoll wait...
[TcpServerThread] epoll wait done, num:3
[TcpServerThread] recv from clientfd:10, msg:helloTCP(fd:8)7
[TcpServerThread] recv from clientfd:7, msg:helloTCP(fd:4)8
[TcpServerThread] recv from clientfd:9, msg:helloTCP(fd:6)9
[TcpServerThread] epoll wait...
[TcpServerThread] epoll wait done, num:1
[TcpServerThread] recv from clientfd:7, msg:helloTCP(fd:4)10
[TcpServerThread] epoll wait...
[TcpServerThread] epoll wait done, num:2
[TcpServerThread] recv from clientfd:9, msg:helloTCP(fd:6)12
[TcpServerThread] recv from clientfd:10, msg:helloTCP(fd:8)11
[TcpServerThread] epoll wait...
[TcpServerThread] epoll wait done, num:3
[TcpServerThread] recv from clientfd:10, msg:helloTCP(fd:8)14
[TcpServerThread] recv from clientfd:7, msg:helloTCP(fd:4)13
[TcpServerThread] recv from clientfd:9, msg:helloTCP(fd:6)15
[TcpServerThread] epoll wait...
[TcpServerThread] epoll wait done, num:3
[TcpServerThread] recv from clientfd:10, msg:helloTCP(fd:8)16
[TcpServerThread] recv from clientfd:7, msg:helloTCP(fd:4)17
[TcpServerThread] recv from clientfd:9, msg:helloTCP(fd:6)18
[TcpServerThread] epoll wait...