源码剖析之epoll(2)

1. 源码剖析

本篇主要分析epoll_ctl以及相关函数

以下源码取自4.10

1.1 epoll_ctl

用于添加/调整/删除我们要监视的事件

fs/eventpoll.c

/* * The following function implements the controller interface for * the eventpoll file that enables the insertion/removal/change of * file descriptors inside the interest set. */SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,        struct epoll_event __user *, event){    int error;    int full_check = 0;    struct fd f, tf;    struct eventpoll *ep;    struct epitem *epi;    struct epoll_event epds;    struct eventpoll *tep = NULL;    error = -EFAULT;    /* 调用copy_from_user获得我们关注的事件 */    if (ep_op_has_event(op) &&        copy_from_user(&epds, event, sizeof(struct epoll_event)))        goto error_return;    error = -EBADF;    f = fdget(epfd);    if (!f.file)        goto error_return;    /* Get the "struct file *" for the target file */    tf = fdget(fd);    if (!tf.file)        goto error_fput;    /* The target file descriptor must support poll */    error = -EPERM;    if (!tf.file->f_op->poll)        goto error_tgt_fput;    /* Check if EPOLLWAKEUP is allowed */    if (ep_op_has_event(op))        ep_take_care_of_epollwakeup(&epds);    /*     * 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 (f.file == tf.file || !is_file_epoll(f.file))        goto error_tgt_fput;    /*     * epoll adds to the wakeup queue at EPOLL_CTL_ADD time only,     * so EPOLLEXCLUSIVE is not allowed for a EPOLL_CTL_MOD operation.     * Also, we do not currently supported nested exclusive wakeups.     */    if (epds.events & EPOLLEXCLUSIVE) {        if (op == EPOLL_CTL_MOD)            goto error_tgt_fput;        if (op == EPOLL_CTL_ADD && (is_file_epoll(tf.file) ||                (epds.events & ~EPOLLEXCLUSIVE_OK_BITS)))            goto error_tgt_fput;    }    /*     * At this point it is safe to assume that the "private_data" contains     * our own data structure.     */    ep = f.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 do not need to take the global 'epumutex' on EPOLL_CTL_ADD when     * the epoll file descriptor is attaching directly to a wakeup source,     * unless the epoll file descriptor is nested. The purpose of taking the     * 'epmutex' on add is to prevent complex toplogies such as loops and     * deep wakeup paths from forming in parallel through multiple     * EPOLL_CTL_ADD operations.     */    mutex_lock_nested(&ep->mtx, 0);    if (op == EPOLL_CTL_ADD) {        if (!list_empty(&f.file->f_ep_links) ||                        is_file_epoll(tf.file)) {            full_check = 1;            mutex_unlock(&ep->mtx);            mutex_lock(&epmutex);            if (is_file_epoll(tf.file)) {                error = -ELOOP;                if (ep_loop_check(ep, tf.file) != 0) {                    clear_tfile_check_list();                    goto error_tgt_fput;                }            } else                list_add(&tf.file->f_tfile_llink,                            &tfile_check_list);            mutex_lock_nested(&ep->mtx, 0);            if (is_file_epoll(tf.file)) {                tep = tf.file->private_data;                mutex_lock_nested(&tep->mtx, 1);            }        }    }    /*     * 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, tf.file, fd);    error = -EINVAL;    switch (op) {    case EPOLL_CTL_ADD:        if (!epi) {            epds.events |= POLLERR | POLLHUP;            error = ep_insert(ep, &epds, tf.file, fd, full_check);        } else            error = -EEXIST;        if (full_check)            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) {            if (!(epi->event.events & EPOLLEXCLUSIVE)) {                epds.events |= POLLERR | POLLHUP;                error = ep_modify(ep, epi, &epds);            }        } else            error = -ENOENT;        break;    }    if (tep != NULL)        mutex_unlock(&tep->mtx);    mutex_unlock(&ep->mtx);error_tgt_fput:    if (full_check)        mutex_unlock(&epmutex);    fdput(tf);error_fput:    fdput(f);error_return:    return error;}

1.2 ep_insert

fs/eventpoll.c

static int ep_insert(struct eventpoll *ep, struct epoll_event *event,             struct file *tfile, int fd, int full_check){    int error, revents, pwake = 0;    unsigned long flags;    long user_watches;    struct epitem *epi;    struct ep_pqueue epq;    // 检测目前监视数量是否超过限制    user_watches = atomic_long_read(&ep->user->epoll_watches);    if (unlikely(user_watches >= max_user_watches))        return -ENOSPC;    // 从slab中分配空间用于存储epitem    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;    /* 设置epitem在RB-Tree中的key */    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);    }    /* Initialize the poll table using the queue callback */    epq.epi = epi;    /* 用ep_ptable_queue_proc作为poll的回调函数 */    /* 回调函数的作用是将该epitem加入的文件的等待队列中去 */    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.     * 获取文件当前就绪事件的掩码，并执行poll的回调函数ep_ptable_queue_proc      */    revents = ep_item_poll(epi, &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      * 文件会链接所有监听自己的epitem，把当前epitem加入到文件的链表中     */    spin_lock(&tfile->f_lock);    list_add_tail_rcu(&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.     * 将epitem插入到eventpoll的RB-Tree中     */    ep_rbtree_insert(ep, epi);    /* now check if we've created too many backpaths */    error = -EINVAL;    if (full_check && reverse_path_check())        goto error_remove_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 */    if ((revents & event->events) && !ep_is_linked(&epi->rdllink)) {        /* 将epitem加入到对应eventpoll的就绪链表中 */        list_add_tail(&epi->rdllink, &ep->rdllist);        ep_pm_stay_awake(epi);        /* Notify waiting tasks that events are available */        /* 唤醒所有阻塞与epoll_wait调用的所有进程/线程 */        if (waitqueue_active(&ep->wq))            wake_up_locked(&ep->wq);        /* 唤醒阻塞于epoll该eventpoll的所有进程/线程 */        if (waitqueue_active(&ep->poll_wait))            pwake++;    }    spin_unlock_irqrestore(&ep->lock, flags);    atomic_long_inc(&ep->user->epoll_watches);    /* We have to call this outside the lock */    if (pwake)        ep_poll_safewake(&ep->poll_wait);    return 0;error_remove_epi:    spin_lock(&tfile->f_lock);    list_del_rcu(&epi->fllink);    spin_unlock(&tfile->f_lock);    rb_erase(&epi->rbn, &ep->rbr);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);    wakeup_source_unregister(ep_wakeup_source(epi));error_create_wakeup_source:    kmem_cache_free(epi_cache, epi);    return error;}/**  * ffd.file->f_op->poll的warpper * 调用poll_table中指定的回调函数 * 如果文件当前发生了epitem的关心事件，返回事件掩码 */static inline unsigned int ep_item_poll(struct epitem *epi, poll_table *pt){    pt->_key = epi->event.events;    /**     * 调用被监视文件对应的poll函数     * 如果是tcp socket，那么调用tcp_poll     * 如果是udp socket，那么调用udp_poll     */    return epi->ffd.file->f_op->poll(epi->ffd.file, pt) & epi->event.events;}

1.3 tcp_poll

net/ipv4/tcp.c

unsigned int tcp_poll(struct file *file, struct socket *sock, poll_table *wait){    unsigned int mask;    struct sock *sk = sock->sk;    const struct tcp_sock *tp = tcp_sk(sk);    int state;    sock_rps_record_flow(sk);    /* 通过sk_sleep(sk)的得到该文件的等待队列作为第二个参数 */    /* 最终以同样的这三个参数调用poll_wait */    sock_poll_wait(file, sk_sleep(sk), wait);    // code omitted    return mask;}

1.4 sk_sleep

该函数返回socket文件的的等待队列

include/net/sock.h

static inline wait_queue_head_t *sk_sleep(struct sock *sk){    BUILD_BUG_ON(offsetof(struct socket_wq, wait) != 0);    return &rcu_dereference_raw(sk->sk_wq)->wait;}

1.5 poll_wait

该函数会调用poll_table->_qproc函数

include/linux/poll.h

static inline void poll_wait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p){    /* 如果poll_table中指定函数不为空，那么就调用该函数，否则什么也不做 */    if (p && p->_qproc && wait_address)        p->_qproc(filp, wait_address, p);}

1.6 ep_ptable_queue_proc

该函数是的作用是将包含epitem信息的等待队列项加入文件的等待队列

fs/eventpoll.c

/* * This is the callback that is used to add our wait queue to the * target file wakeup lists. */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);    /* 用于关联epitem/wait_queue_t/wait_queue_head_t三者，以便在回调的时候能找到 */    struct eppoll_entry *pwq;    if (epi->nwait >= 0 && (pwq = kmem_cache_alloc(pwq_cache, GFP_KERNEL))) {        /* 设置等待队列项的回调函数为ep_poll_callback */        init_waitqueue_func_entry(&pwq->wait, ep_poll_callback);        pwq->whead = whead;        pwq->base = epi;        /* 将配置好的等待队列项加入对应的等待队列中去 */        if (epi->event.events & EPOLLEXCLUSIVE)            add_wait_queue_exclusive(whead, &pwq->wait);        else            add_wait_queue(whead, &pwq->wait);        /* 关联epitem和epoll_entry */        list_add_tail(&pwq->llink, &epi->pwqlist);        epi->nwait++;    } else {        /* We have to signal that an error occurred */        epi->nwait = -1;    }}

1.7 ep_poll_callback

该函数是当文件有事件发生时的回调函数

fs/eventpoll.c

/* * This is the callback that is passed to the wait queue wakeup * mechanism. It is called by the stored file descriptors when they * have events to report. */static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *key){    int pwake = 0;    unsigned long flags;    struct epitem *epi = ep_item_from_wait(wait);    struct eventpoll *ep = epi->ep;    int ewake = 0;    if ((unsigned long)key & POLLFREE) {        ep_pwq_from_wait(wait)->whead = NULL;        /*         * whead = NULL above can race with ep_remove_wait_queue()         * which can do another remove_wait_queue() after us, so we         * can't use __remove_wait_queue(). whead->lock is held by         * the caller.         */        list_del_init(&wait->task_list);    }    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.     */    if (key && !((unsigned long) key & epi->event.events))        goto out_unlock;    /*     * If we are transferring 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 happen during that period of time are     * chained in ep->ovflist and requeued later on.     */    if (unlikely(ep->ovflist != EP_UNACTIVE_PTR)) {        if (epi->next == EP_UNACTIVE_PTR) {            epi->next = ep->ovflist;            ep->ovflist = epi;            if (epi->ws) {                /*                 * Activate ep->ws since epi->ws may get                 * deactivated at any time.                 */                __pm_stay_awake(ep->ws);            }        }        goto out_unlock;    }    /* If this file is already in the ready list we exit soon */    if (!ep_is_linked(&epi->rdllink)) {        list_add_tail(&epi->rdllink, &ep->rdllist);        ep_pm_stay_awake_rcu(epi);    }    /*     * Wake up ( if active ) both the eventpoll wait list and the ->poll()     * wait list.     */    if (waitqueue_active(&ep->wq)) {        if ((epi->event.events & EPOLLEXCLUSIVE) &&                    !((unsigned long)key & POLLFREE)) {            switch ((unsigned long)key & EPOLLINOUT_BITS) {            case POLLIN:                if (epi->event.events & POLLIN)                    ewake = 1;                break;            case POLLOUT:                if (epi->event.events & POLLOUT)                    ewake = 1;                break;            case 0:                ewake = 1;                break;            }        }        /* 唤醒阻塞于epoll_wait和epoll_pwait调用的进程/线程 */        wake_up_locked(&ep->wq);    }    /* 唤醒阻塞于poll该eventpoll的进程/线程 */    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);    if (epi->event.events & EPOLLEXCLUSIVE)        return ewake;    return 1;}

1.8 ep_modify

由于ep_modify和ep_insert类似，所以不具体分析了

fs/eventpoll.c

/* * Modify the interest event mask by dropping an event if the new mask * has a match in the current file status. Must be called with "mtx" held. */static int ep_modify(struct eventpoll *ep, struct epitem *epi, struct epoll_event *event){    int pwake = 0;    unsigned int revents;    poll_table pt;    init_poll_funcptr(&pt, NULL);    /*     * Set the new event interest mask before calling f_op->poll();     * otherwise we might miss an event that happens between the     * f_op->poll() call and the new event set registering.     */    epi->event.events = event->events; /* need barrier below */    epi->event.data = event->data; /* protected by mtx */    if (epi->event.events & EPOLLWAKEUP) {        if (!ep_has_wakeup_source(epi))            ep_create_wakeup_source(epi);    } else if (ep_has_wakeup_source(epi)) {        ep_destroy_wakeup_source(epi);    }    /*     * The following barrier has two effects:     *     * 1) Flush epi changes above to other CPUs.  This ensures     *    we do not miss events from ep_poll_callback if an     *    event occurs immediately after we call f_op->poll().     *    We need this because we did not take ep->lock while     *    changing epi above (but ep_poll_callback does take     *    ep->lock).     *     * 2) We also need to ensure we do not miss _past_ events     *    when calling f_op->poll().  This barrier also     *    pairs with the barrier in wq_has_sleeper (see     *    comments for wq_has_sleeper).     *     * This barrier will now guarantee ep_poll_callback or f_op->poll     * (or both) will notice the readiness of an item.     */    smp_mb();    /*     * Get current event bits. We can safely use the file* here because     * its usage count has been increased by the caller of this function.     */    revents = ep_item_poll(epi, &pt);    /*     * If the item is "hot" and it is not registered inside the ready     * list, push it inside.     */    if (revents & event->events) {        spin_lock_irq(&ep->lock);        if (!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_irq(&ep->lock);    }    /* We have to call this outside the lock */    if (pwake)        ep_poll_safewake(&ep->poll_wait);    return 0;}