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;
}