• 資料結構
• 函式
  • epoll_create
  • epoll_ctl
  • ep_insert
  • epoll_wait

資料結構

//   fs/eventpoll.c
struct eventpoll {
    /* Protect the access to this structure */
    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.
     */
    struct mutex mtx;

    /* Wait queue used by sys_epoll_wait() */
    wait_queue_head_t wq;

    /* Wait queue used by
 
 file->poll() */
    wait_queue_head_t poll_wait;

    /* List of ready file descriptors */
    struct list_head rdllist;

    /* RB tree root used to store monitored fd structs */
    struct rb_root rbr;

    /*
     * This is a single linked list that chains all the "struct epitem" that
     * happened while transferring ready events to userspace w/out
     * holding ->lock.
     */
    struct
 
 epitem *ovflist;

    /* wakeup_source used when ep_scan_ready_list is running */
    struct wakeup_source *ws;

    /* The user that created the eventpoll descriptor */
    struct user_struct *user;

    struct file *file;

    /* used to optimize loop detection check */
    int visited;
    struct list_head visited_list_link;
};

上面這個是epoll主要的資料結構，包含了epoll會用到的所有資訊：就緒的檔案描述符列表，使用epoll的使用者資訊，epoll fd這個檔案描述符的資訊等等。

struct epitem {
    /* RB tree node used to link this structure to the eventpoll RB tree */
    struct rb_node rbn;

    /* List header used to link this structure to the eventpoll ready list */
    struct list_head rdllink;

    /*
     * Works together "struct eventpoll"->ovflist in keeping the
     * single linked chain of items.
     */
    struct epitem *next;

    /* The file descriptor information this item refers to */
    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 */
    struct eventpoll *ep;

    /* List header used to link this item to the "struct file" items list */
    struct list_head fllink;

    /* wakeup_source used when EPOLLWAKEUP is set */
    struct wakeup_source __rcu *ws;

    /* The structure that describe the interested events and the source fd */
    struct epoll_event event;
};

上面這個資料結構可以看做是eventpoll裡面紅黑樹的節點，只是這個節點裡面還會維護自己的一些資訊，最熟悉的應該就是最後一個成員了：epoll_event：這個結構體裡面會包括我們感興趣的檔案描述符和其他一些東西。

函式

epoll_create

SYSCALL_DEFINE1(epoll_create1, int, flags)
{
    int error, fd;
    struct eventpoll *ep = NULL;
    struct file *file;

    /* Check the EPOLL_* constant for consistency.  */
    BUILD_BUG_ON(EPOLL_CLOEXEC != O_CLOEXEC);

    if (flags & ~EPOLL_CLOEXEC)
        return -EINVAL;
    /*
     * Create the internal data structure ("struct eventpoll").
     */
    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.
     */
    fd = get_unused_fd_flags(O_RDWR | (flags & O_CLOEXEC));
    if (fd < 0) {
        error = fd;
        goto out_free_ep;
    }
    file = anon_inode_getfile("[eventpoll]", &eventpoll_fops, ep,
                 O_RDWR | (flags & O_CLOEXEC));
    if (IS_ERR(file)) {
        error = PTR_ERR(file);
        goto out_free_fd;
    }
    ep->file = file;
    fd_install(fd, file);
    return fd;

out_free_fd:
    put_unused_fd(fd);
out_free_ep:
    ep_free(ep);
    return error;
}

static int ep_alloc(struct eventpoll **pep)
{
    int error;
    struct user_struct *user;
    struct eventpoll *ep;

    user = get_current_user();
    error = -ENOMEM;
    ep = kzalloc(sizeof(*ep), GFP_KERNEL);
    if (unlikely(!ep))
        goto free_uid;

    spin_lock_init(&ep->lock);
    mutex_init(&ep->mtx);
    init_waitqueue_head(&ep->wq);
    init_waitqueue_head(&ep->poll_wait);
    INIT_LIST_HEAD(&ep->rdllist);
    ep->rbr = RB_ROOT;
    ep->ovflist = EP_UNACTIVE_PTR;
    ep->user = user;

    *pep = ep;

    return 0;

free_uid:
    free_uid(user);
    return error;
}

可以看到epoll_create 會通過呼叫ep_alloc 函式來建立並初始化一個eventpoll結構體,之後會分配一個fd給這個eventpoll [這個就可以說明epfd就是一個普通的檔案描述符]，最後在檔案描述表裡面加入這個epfd，並返回這個epfd，所以我們就可以得到這個epfd並進行進一步的操作。

epoll_ctl

SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
        struct epoll_event __user *, event)
{
    int error;
    int did_lock_epmutex = 0;
    struct file *file, *tfile;
    struct eventpoll *ep;
    struct epitem *epi;
    struct epoll_event epds;

    error = -EFAULT;
    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 */
    error = -EBADF;
    file = fget(epfd);
    if (!file)
        goto error_return;

    /* Get the "struct file *" for the target file */
    tfile = fget(fd);
    if (!tfile)
        goto error_fput;

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

    /* Check if EPOLLWAKEUP is allowed */
    if ((epds.events & EPOLLWAKEUP) && !capable(CAP_BLOCK_SUSPEND))
        epds.events &= ~EPOLLWAKEUP;

    /*
     * We have to check that the file structure underneath the file descriptor
     * the user passed to us _is_ an eventpoll file. And also we do not permit
     * adding an epoll file descriptor inside itself.
     */
    error = -EINVAL;
    if (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.
     */
    ep = file->private_data;

    /*
     * When we insert an epoll file descriptor, inside another epoll file
     * descriptor, there is the change of creating closed loops, which are
     * better be handled here, than in more critical paths. While we are
     * checking for loops we also determine the list of files reachable
     * and hang them on the tfile_check_list, so we can check that we
     * haven't created too many possible wakeup paths.
     *
     * We need to hold the epmutex across both ep_insert and ep_remove
     * b/c we want to make sure we are looking at a coherent view of
     * epoll network.
     */
    if (op == EPOLL_CTL_ADD || op == EPOLL_CTL_DEL) {
        mutex_lock(&epmutex);
        did_lock_epmutex = 1;
    }
    if (op == EPOLL_CTL_ADD) {
        if (is_file_epoll(tfile)) {
            error = -ELOOP;
            if (ep_loop_check(ep, tfile) != 0) {
                clear_tfile_check_list();
                goto error_tgt_fput;
            }
        } else
            list_add(&tfile->f_tfile_llink, &tfile_check_list);
    }

    mutex_lock_nested(&ep->mtx, 0);

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

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

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

    fput(tfile);
error_fput:
    fput(file);
error_return:

    return error;
}

先看下epoll_ctl 函式原型：

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

epoll_ctl 這個函式 首先會copy_from_user ：把函式原型的最後一個引數拷貝進來，再獲得epfd 和 fd 的詳細資訊，並且判斷他們兩個是否指向了同一個檔案。前面都是斷言判斷。

• 之後才是真正的對eventpoll資料結構進行操作：如果我們是對eventpoll內部的紅黑樹進行增加或者刪除操作的話，我們需要先對eventpoll進行上鎖保護，避免出現多個執行緒同時對同一個資料結構進行操作會破壞資料結構的這種問題。
• 上鎖之後，如果是EPOLL_CTL_ADD 那麼就把fd和對應的檔案insert到eventpoll內部的紅黑樹裡面。
• 這裡的EPOLL_CTL_ADD，EPOLL_CTL_DEL 和 EPOLL_CTL_MOD 都是對紅黑樹進行操作。
• 操作完成之後，釋放鎖並返回。

ep_insert

static int ep_insert(struct eventpoll *ep, struct epoll_event *event,
             struct file *tfile, int fd)

{
    int error, revents, pwake = 0;
    unsigned long flags;
    long user_watches;
    struct epitem *epi;
    struct ep_pqueue epq;

    user_watches = atomic_long_read(&ep->user->epoll_watches);
    if (unlikely(user_watches >= max_user_watches))
        return -ENOSPC;
    if (!(epi = kmem_cache_alloc(epi_cache, GFP_KERNEL)))
        return -ENOMEM;

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

    epq.epi = epi;
    init_poll_funcptr(&epq.pt, ep_ptable_queue_proc);

    revents = ep_item_poll(epi, &epq.pt);


    error = -ENOMEM;
    if (epi->nwait < 0)
        goto error_unregister;

    spin_lock(&tfile->f_lock);
    list_add_tail(&epi->fllink, &tfile->f_ep_links);
    spin_unlock(&tfile->f_lock);


    ep_rbtree_insert(ep, epi);


    error = -EINVAL;
    if (reverse_path_check())
        goto error_remove_epi;


    spin_lock_irqsave(&ep->lock, flags);


    if ((revents & event->events) && !ep_is_linked(&epi->rdllink)) {
        list_add_tail(&epi->rdllink, &ep->rdllist);
        ep_pm_stay_awake(epi);

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

    spin_unlock_irqrestore(&ep->lock, flags);

    atomic_long_inc(&ep->user->epoll_watches);
    if (pwake)
        ep_poll_safewake(&ep->poll_wait);

    return 0;

error_remove_epi:
    spin_lock(&tfile->f_lock);
    if (ep_is_linked(&epi->fllink))
        list_del_init(&epi->fllink);
    spin_unlock(&tfile->f_lock);

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

error_unregister:
    ep_unregister_pollwait(ep, epi);
    spin_lock_irqsave(&ep->lock, flags);
    if (ep_is_linked(&epi->rdllink))
        list_del_init(&epi->rdllink);
    spin_unlock_irqrestore(&ep->lock, flags);

    wakeup_source_unregister(ep_wakeup_source(epi));

error_create_wakeup_source:
    kmem_cache_free(epi_cache, epi);

    return error;
}

這個是EPOLL_CTL_ADD會呼叫的函式

• 這個函式開始會先初始化epitem節點裡面的三個連結串列：就緒連結串列，等待連結串列，檔案連結串列。
• 緊接著會呼叫init_poll_funcptr 來設定回撥函式：ep_ptable_queue_proc

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))) {
        init_waitqueue_func_entry(&pwq->wait, ep_poll_callback);
        pwq->whead = whead;
        pwq->base = epi;
        add_wait_queue(whead, &pwq->wait);
        list_add_tail(&pwq->llink, &epi->pwqlist);
        epi->nwait++;
    } else {
        /* We have to signal that an error occurred */
        epi->nwait = -1;
    }
}

• ep_ptable_queue_proc這個函式裡面又會呼叫init_waitqueue_func_entry函式來註冊另外一個回撥函式： ep_poll_callback，這個回撥函式會啟用當前執行緒，讓當前程序從睡眠狀態變成active狀態。
• 接下來把當前的epitem加入eventpoll 監聽佇列，並插入紅黑樹。
• 如果當前item已經就緒，那麼我們直接把這個item加入到active佇列裡面。並喚醒當前epfd。

epoll_wait

SYSCALL_DEFINE4(epoll_wait, int, epfd, struct epoll_event __user *, events,
        int, maxevents, int, timeout)
{
    int error;
    struct fd f;
    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 */
    if (!access_ok(VERIFY_WRITE, events, maxevents * sizeof(struct epoll_event)))
        return -EFAULT;

    /* Get the "struct file *" for the eventpoll file */
    f = fdget(epfd);
    if (!f.file)
        return -EBADF;

    /*
     * We have to check that the file structure underneath the fd
     * the user passed to us _is_ an eventpoll file.
     */
    error = -EINVAL;
    if (!is_file_epoll(f.file))
        goto error_fput;

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

    /* Time to fish for events ... */
    error = ep_poll(ep, events, maxevents, timeout);

error_fput:
    fdput(f);
    return error;
}

可以看到epoll_wait會呼叫ep_poll函式

static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
           int maxevents, long timeout)
{
    int res = 0, eavail, timed_out = 0;
    unsigned long flags;
    long slack = 0;
    wait_queue_t wait;
    ktime_t expires, *to = NULL;

    if (timeout > 0) {
        struct timespec end_time = ep_set_mstimeout(timeout);

        slack = select_estimate_accuracy(&end_time);
        to = &expires;
        *to = timespec_to_ktime(end_time);
    } else if (timeout == 0) {
        timed_out = 1;
        spin_lock_irqsave(&ep->lock, flags);
        goto check_events;
    }

fetch_events:
    spin_lock_irqsave(&ep->lock, flags);

    if (!ep_events_available(ep)) {
        /*
         * 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.
         */
        init_waitqueue_entry(&wait, current);
        __add_wait_queue_exclusive(&ep->wq, &wait);

        for (;;) {
            set_current_state(TASK_INTERRUPTIBLE);
            if (ep_events_available(ep) || timed_out)
                break;
            if (signal_pending(current)) {
                res = -EINTR;
                break;
            }

            spin_unlock_irqrestore(&ep->lock, flags);
            if (!schedule_hrtimeout_range(to, slack, HRTIMER_MODE_ABS))
                timed_out = 1;

            spin_lock_irqsave(&ep->lock, flags);
        }
        __remove_wait_queue(&ep->wq, &wait);

        set_current_state(TASK_RUNNING);
    }
check_events:
    eavail = ep_events_available(ep);

    spin_unlock_irqrestore(&ep->lock, flags);

    if (!res && eavail &&
        !(res = ep_send_events(ep, events, maxevents)) && !timed_out)
        goto fetch_events;

    return res;
}

static inline int ep_events_available(struct eventpoll *ep)
{
    return !list_empty(&ep->rdllist) || ep->ovflist != EP_UNACTIVE_PTR;
}

fetch_event : 如果還沒有訊息就緒的話，那麼就阻塞在__add_wait_queue_exclusive這裡，當前執行緒會被ep_poll_callback啟用。
執行緒被重新啟用之後，會進入死迴圈：如果已經有訊息就緒的話 或者 超時 或者 出現錯誤的話，都會退出這個死迴圈。最後呼叫ep_send_events函式把ep就緒佇列裡面的時間拷貝到events——即我們呼叫epoll_wait傳入的最後一個引數裡面。

所以epoll的核心就是
1.紅黑樹：用來管理所有加入到epfd裡面的epoll_event
2.epitem這個是用來銜接核心和使用者態的資料結構。
3.回撥機制：當有事件發生時，會呼叫回撥函式把事件加入到ep->rdllist連結串列裡面，如果連結串列不為空，那麼就把就緒的事件拷貝到使用者態。