網上很多博客說epoll使用了共享內存,這個是完全錯誤的 ,可以閱讀源碼,會發現完全沒有使用共享內存的任何api，而是 使用了copy_from_user跟__put_user進行內核跟用戶虛擬空間數據交互.
/*
* 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 <[email protected]>
*
*/
/*
* 在深入瞭解epoll的實現之前, 先來了解內核的3個方面.
* 1. 等待隊列 waitqueue
* 我們簡單解釋一下等待隊列:
* 隊列頭(wait_queue_head_t)往往是資源生產者,
* 隊列成員(wait_queue_t)往往是資源消費者,
* 當頭的資源ready後, 會逐個執行每個成員指定的回調函數,
* 來通知它們資源已經ready了, 等待隊列大致就這個意思.
* 2. 內核的poll機制
* 被Poll的fd, 必須在實現上支持內核的Poll技術,
* 比如fd是某個字符設備,或者是個socket, 它必須實現
* file_operations中的poll操作, 給自己分配有一個等待隊列頭.
* 主動poll fd的某個進程必須分配一個等待隊列成員, 添加到
* fd的對待隊列裏面去, 並指定資源ready時的回調函數.
* 用socket做例子, 它必須有實現一個poll操作, 這個Poll是
* 發起輪詢的代碼必須主動調用的, 該函數中必須調用poll_wait(),
* poll_wait會將發起者作爲等待隊列成員加入到socket的等待隊列中去.
* 這樣socket發生狀態變化時可以通過隊列頭逐個通知所有關心它的進程.
* 這一點必須很清楚的理解, 否則會想不明白epoll是如何
* 得知fd的狀態發生變化的.
* 3. epollfd本身也是個fd, 所以它本身也可以被epoll,
* 可以猜測一下它是不是可以無限嵌套epoll下去...
*
* epoll基本上就是使用了上面的1,2點來完成.
* 可見epoll本身並沒有給內核引入什麼特別複雜或者高深的技術,
* 只不過是已有功能的重新組合, 達到了超過select的效果.
*/
/*
* 相關的其它內核知識:
* 1. fd我們知道是文件描述符, 在內核態, 與之對應的是struct file結構,
* 可以看作是內核態的文件描述符.
* 2. spinlock, 自旋鎖, 必須要非常小心使用的鎖,
* 尤其是調用spin_lock_irqsave()的時候, 中斷關閉, 不會發生進程調度,
* 被保護的資源其它CPU也無法訪問. 這個鎖是很強力的, 所以只能鎖一些
* 非常輕量級的操作.
* 3. 引用計數在內核中是非常重要的概念,
* 內核代碼裏面經常有些release, free釋放資源的函數幾乎不加任何鎖,
* 這是因爲這些函數往往是在對象的引用計數變成0時被調用,
* 既然沒有進程在使用在這些對象, 自然也不需要加鎖.
* struct file 是持有引用計數的.
*/
/* --- epoll相關的數據結構 --- */
/*
* This structure is stored inside the "private_data" member of the file
* structure and rapresent the main data sructure for the eventpoll
* interface.
*/
/* 每創建一個epollfd, 內核就會分配一個eventpoll與之對應, 可以說是
* 內核態的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, 修改或者刪除監聽fd的時候, 以及epoll_wait返回, 向用戶空間
* 傳遞數據時都會持有這個互斥鎖, 所以在用戶空間可以放心的在多個線程
* 中同時執行epoll相關的操作, 內核級已經做了保護. */
struct mutex mtx;
/* Wait queue used by sys_epoll_wait() */
/* 調用epoll_wait()時, 我們就是"睡"在了這個等待隊列上... */
wait_queue_head_t wq;
/* Wait queue used by file->poll() */
/* 這個用於epollfd本事被poll的時候... */
wait_queue_head_t poll_wait;
/* List of ready file descriptors */
/* 所有已經ready的epitem都在這個鏈表裏面 */
struct list_head rdllist;
/* RB tree root used to store monitored fd structs */
/* 所有要監聽的epitem都在這裏 */
struct rb_root rbr;
/*
這是一個單鏈錶鏈接着所有的struct epitem當event轉移到用戶空間時
*/
* 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 */
/* 這裏保存了一些用戶變量, 比如fd監聽數量的最大值等等 */
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 表示一個被監聽的fd */
struct epitem {
/* RB tree node used to link this structure to the eventpoll RB tree */
/* rb_node, 當使用epoll_ctl()將一批fds加入到某個epollfd時, 內核會分配
* 一批的epitem與fds們對應, 而且它們以rb_tree的形式組織起來, tree的root
* 保存在epollfd, 也就是struct eventpoll中.
* 在這裏使用rb_tree的原因我認爲是提高查找,插入以及刪除的速度.
* rb_tree對以上3個操作都具有O(lgN)的時間複雜度 */
struct rb_node rbn;
/* List header used to link this structure to the eventpoll ready list */
/* 鏈表節點, 所有已經ready的epitem都會被鏈到eventpoll的rdllist中 */
struct list_head rdllink;
/*
* Works together "struct eventpoll"->ovflist in keeping the
* single linked chain of items.
*/
/* 這個在代碼中再解釋... */
struct epitem *next;
/* The file descriptor information this item refers to */
/* epitem對應的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 */
/* 當前epitem屬於哪個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 */
/* 當前的epitem關係哪些events, 這個數據是調用epoll_ctl時從用戶態傳遞過來 */
struct epoll_event event;
};
struct epoll_filefd {
struct file *file;
int fd;
};
/* poll所用到的鉤子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;
};
/* --- 代碼註釋 --- */
/* 你沒看錯, 這就是epoll_create()的真身, 基本啥也不幹直接調用epoll_create1了,
* 另外你也可以發現, size這個參數其實是沒有任何用處的... */
SYSCALL_DEFINE1(epoll_create, int, size)
{
if (size <= 0)
return -EINVAL;
return sys_epoll_create1(0);
}
/* 這纔是真正的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. */
/* 這句沒啥用處... */
BUILD_BUG_ON(EPOLL_CLOEXEC != O_CLOEXEC);
/* 對於epoll來講, 目前唯一有效的FLAG就是CLOEXEC */
if (flags & ~EPOLL_CLOEXEC)
return -EINVAL;
/*
* Create the internal data structure ("struct eventpoll").
*/
/* 分配一個struct eventpoll, 分配和初始化細節我們隨後深聊~ */
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.
*/
/* 這裏是創建一個匿名fd, 說起來就話長了...長話短說:
* epollfd本身並不存在一個真正的文件與之對應, 所以內核需要創建一個
* "虛擬"的文件, 併爲之分配真正的struct file結構, 而且有真正的fd.
* 這裏2個參數比較關鍵:
* eventpoll_fops, fops就是file operations, 就是當你對這個文件(這裏是虛擬的)進行操作(比如讀)時,
* fops裏面的函數指針指向真正的操作實現, 類似C++裏面虛函數和子類的概念.
* epoll只實現了poll和release(就是close)操作, 其它文件系統操作都有VFS全權處理了.
* ep, ep就是struct epollevent, 它會作爲一個私有數據保存在struct file的private指針裏面.
* 其實說白了, 就是爲了能通過fd找到struct file, 通過struct file能找到eventpoll結構.
* 如果懂一點Linux下字符設備驅動開發, 這裏應該是很好理解的,
* 推薦閱讀 <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;
}
/*
* 創建好epollfd後, 接下來我們要往裏面添加fd咯
* 來看epoll_ctl
* epfd 就是epollfd
* op ADD,MOD,DEL
* fd 需要監聽的描述符
* event 我們關心的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;
/*
* 錯誤處理以及從用戶空間將epoll_event結構copy到內核空間.
*/
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結構, epfd既然是真正的fd, 那麼內核空間
* 就會有與之對於的一個struct file結構
* 這個結構在epoll_create1()中, 由函數anon_inode_getfd()分配 */
error = -EBADF;
file = fget(epfd);
if (!file)
goto error_return;
/* Get the "struct file *" for the target file */
/* 我們需要監聽的fd, 它當然也有個struct file結構, 上下2個不要搞混了哦 */
tfile = fget(fd);
if (!tfile)
goto error_fput;
/* The target file descriptor must support poll */
error = -EPERM;
/* 如果監聽的文件不支持poll, 那就沒轍了.
* 你知道什麼情況下, 文件會不支持poll嗎?
*/
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不能自己監聽自己... */
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.
*/
/* 取到我們的eventpoll結構, 來自與epoll_create1()中的分配 */
ep = file->private_data;
/* 接下來的操作有可能修改數據結構內容, 鎖之~ */
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.
*/
/* 對於每一個監聽的fd, 內核都有分配一個epitem結構,
* 而且我們也知道, epoll是不允許重複添加fd的,
* 所以我們首先查找該fd是不是已經存在了.
* ep_find()其實就是RBTREE查找, 跟C++STL的map差不多一回事, O(lgn)的時間複雜度.
*/
epi = ep_find(ep, tfile, fd);
error = -EINVAL;
switch (op) {
/* 首先我們關心添加 */
case EPOLL_CTL_ADD:
if (!epi) {
/* 之前的find沒有找到有效的epitem, 證明是第一次插入, 接受!
* 這裏我們可以知道, POLLERR和POLLHUP事件內核總是會關心的
* */
epds.events |= POLLERR | POLLHUP;
/* rbtree插入, 詳情見ep_insert()的分析
* 其實我覺得這裏有insert的話, 之前的find應該
* 是可以省掉的... */
error = ep_insert(ep, &epds, tfile, fd);
} else
/* 找到了!? 重複添加! */
error = -EEXIST;
break;
/* 刪除和修改操作都比較簡單 */
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;
}
/* 分配一個eventpoll結構 */
static int ep_alloc(struct eventpoll **pep)
{
int error;
struct user_struct *user;
struct eventpoll *ep;
/* 獲取當前用戶的一些信息, 比如是不是root啦, 最大監聽fd數目啦 */
user = get_current_user();
error = -ENOMEM;
ep = kzalloc(sizeof(*ep), GFP_KERNEL);
if (unlikely(!ep))
goto free_uid;
/* 這些都是初始化啦 */
spin_lock_init(&ep->lock);
mutex_init(&ep->mtx);
init_waitqueue_head(&ep->wq);//初始化自己睡在的等待隊列
init_waitqueue_head(&ep->poll_wait);//初始化
INIT_LIST_HEAD(&ep->rdllist);//初始化就緒鏈表
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()中被調用, 完成往epollfd裏面添加一個監聽fd的工作
* tfile是fd在內核態的struct file結構
*/
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;
/* 查看是否達到當前用戶的最大監聽數 */
if (unlikely(atomic_read(&ep->user->epoll_watches) >=
max_user_watches))
return -ENOSPC;
/* 從著名的slab中分配一個epitem */
if (!(epi = kmem_cache_alloc(epi_cache, GFP_KERNEL)))
return -ENOMEM;
/* Item initialization follow here ... */
/* 這些都是相關成員的初始化... */
INIT_LIST_HEAD(&epi->rdllink);
INIT_LIST_HEAD(&epi->fllink);
INIT_LIST_HEAD(&epi->pwqlist);
epi->ep = ep;
/* 這裏保存了我們需要監聽的文件fd和它的file結構 */
ep_set_ffd(&epi->ffd, tfile, fd);
epi->event = *event;
epi->nwait = 0;
/* 這個指針的初值不是NULL哦... */
epi->next = EP_UNACTIVE_PTR;
/* Initialize the poll table using the queue callback */
/* 好, 我們終於要進入到poll的正題了 */
epq.epi = epi;
/* 初始化一個poll_table
* 其實就是指定調用poll_wait(注意不是epoll_wait!!!)時的回調函數,和我們關心哪些events,
* ep_ptable_queue_proc()就是我們的回調啦, 初值是所有event都關心 */
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.
*/
/* 這一部很關鍵, 也比較難懂, 完全是內核的poll機制導致的...
* 首先, f_op->poll()一般來說只是個wrapper, 它會調用真正的poll實現,
* 拿UDP的socket來舉例, 這裏就是這樣的調用流程: f_op->poll(), sock_poll(),
* udp_poll(), datagram_poll(), sock_poll_wait(), 最後調用到我們上面指定的
* ep_ptable_queue_proc()這個回調函數...(好深的調用路徑...).
* 完成這一步, 我們的epitem就跟這個socket關聯起來了, 當它有狀態變化時,
* 會通過ep_poll_callback()來通知.
* 最後, 這個函數還會查詢當前的fd是不是已經有啥event已經ready了, 有的話
* 會將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 */
/* 這個就是每個文件會將所有監聽自己的epitem鏈起來 */
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.
*/
/* 都搞定後, 將epitem插入到對應的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 */
/* 到達這裏後, 如果我們監聽的fd已經有事件發生, 那就要處理一下 */
if ((revents & event->events) && !ep_is_linked(&epi->rdllink)) {
/* 將當前的epitem加入到ready list中去 */
list_add_tail(&epi->rdllink, &ep->rdllist);
/* Notify waiting tasks that events are available */
/* 誰在epoll_wait, 就喚醒它... */
if (waitqueue_active(&ep->wq))
wake_up_locked(&ep->wq);
/* 誰在epoll當前的epollfd, 也喚醒它... */
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.
*/
/*
* 該函數在調用f_op->poll()時會被調用.
* 也就是epoll主動poll某個fd時, 用來將epitem與指定的fd關聯起來的.
* 關聯的辦法就是使用等待隊列(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))) {
/* 初始化等待隊列, 指定ep_poll_callback爲喚醒時的回調函數,
* 當我們監聽的fd發生狀態改變時, 也就是隊列頭被喚醒時,
* 指定的回調函數將會被調用. */
init_waitqueue_func_entry(&pwq->wait, ep_poll_callback);
pwq->whead = whead;
pwq->base = epi;
/* 將剛分配的等待隊列成員加入到頭中, 頭是由fd持有的 */
add_wait_queue(whead, &pwq->wait);
list_add_tail(&pwq->llink, &epi->pwqlist);
/* nwait記錄了當前epitem加入到了多少個等待隊列中,
* 我認爲這個值最大也只會是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.
*/
/*
* 這個是關鍵性的回調函數, 當我們監聽的fd發生狀態改變時, 它會被調用.
* 參數key被當作一個unsigned long整數使用, 攜帶的是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);//從等待隊列獲取epitem.需要知道哪個進程掛載到這個設備
struct eventpoll *ep = epi->ep;//獲取
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.
*/
/* 沒有我們關心的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.
*/
/*
* 這裏看起來可能有點費解, 其實幹的事情比較簡單:
* 如果該callback被調用的同時, epoll_wait()已經返回了,
* 也就是說, 此刻應用程序有可能已經在循環獲取events,
* 這種情況下, 內核將此刻發生event的epitem用一個單獨的鏈表
* 鏈起來, 不發給應用程序, 也不丟棄, 而是在下一次epoll_wait
* 時返回給用戶.
*/
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 */
/* 將當前的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.
*/
/* 喚醒epoll_wait... */
if (waitqueue_active(&ep->wq))
wake_up_locked(&ep->wq);
/* 如果epollfd也在被poll, 那就喚醒隊列裏面的所有成員. */
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 */
/* 這個地方有必要說明一下:
* 內核對應用程序採取的策略是"絕對不信任",
* 所以內核跟應用程序之間的數據交互大都是copy, 不允許(也時候也是不能...)指針引用.
* epoll_wait()需要內核返回數據給用戶空間, 內存由用戶程序提供,
* 所以內核會用一些手段來驗證這一段內存空間是不是有效的.
*/
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;
/* 獲取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;
/* 檢查一下它是不是一個真正的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.
*/
/* 獲取eventpoll結構 */
ep = file->private_data;
/* Time to fish for events ... */
/* OK, 睡覺, 等待事件到來~~ */
error = ep_poll(ep, events, maxevents, timeout);
error_fput:
fput(file);
error_return:
return error;
}
/* 這個函數真正將執行epoll_wait的進程帶入睡眠狀態... */
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;//等待隊列
/*
* 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.
*/
/* 計算睡覺時間, 毫秒要轉換爲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不爲空, 就不睡了, 直接幹活... */
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, 初始化一個等待隊列, 準備直接把自己掛起,
* 注意current是一個宏, 代表當前進程 */
init_waitqueue_entry(&wait, current);//初始化等待隊列,wait表示當前進程
__add_wait_queue_exclusive(&ep->wq, &wait);//掛載到ep結構的等待隊列
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.
*/
/* 將當前進程設置位睡眠, 但是可以被信號喚醒的狀態,
* 注意這個設置是"將來時", 我們此刻還沒睡! */
set_current_state(TASK_INTERRUPTIBLE);
/* 如果這個時候, ready list裏面有成員了,
* 或者睡眠時間已經過了, 就直接不睡了... */
if (!list_empty(&ep->rdllist) || !jtimeout)
break;
/* 如果有信號產生, 也起牀... */
if (signal_pending(current)) {
res = -EINTR;
break;
}
/* 啥事都沒有,解鎖, 睡覺... */
spin_unlock_irqrestore(&ep->lock, flags);
/* jtimeout這個時間後, 會被喚醒,
* ep_poll_callback()如果此時被調用,
* 那麼我們就會直接被喚醒, 不用等時間了...
* 再次強調一下ep_poll_callback()的調用時機是由被監聽的fd
* 的具體實現, 比如socket或者某個設備驅動來決定的,
* 因爲等待隊列頭是他們持有的, epoll和當前進程
* 只是單純的等待...
**/
jtimeout = schedule_timeout(jtimeout);//睡覺
spin_lock_irqsave(&ep->lock, flags);
}
__remove_wait_queue(&ep->wq, &wait);
/* OK 我們醒來了... */
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發生, 就開始準備數據copy給用戶空間了... */
if (!res && eavail &&
!(res = ep_send_events(ep, events, maxevents)) && jtimeout)
goto retry;
return res;
}
/* 這個簡單, 我們直奔下一個... */
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);
/* 這一步要注意, 首先, 所有監聽到events的epitem都鏈到rdllist上了,
* 但是這一步之後, 所有的epitem都轉移到了txlist上, 而rdllist被清空了,
* 要注意哦, rdllist已經被清空了! */
list_splice_init(&ep->rdllist, &txlist);
/* ovflist, 在ep_poll_callback()裏面我解釋過, 此時此刻我們不希望
* 有新的event加入到ready list中了, 保存後下次再處理... */
ep->ovflist = NULL;
spin_unlock_irqrestore(&ep->lock, flags);
/*
* Now call the callback function.
*/
/* 在這個回調函數裏面處理每個epitem
* sproc 就是 ep_send_events_proc, 下面會註釋到. */
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.
*/
/* 現在我們來處理ovflist, 這些epitem都是我們在傳遞數據給用戶空間時
* 監聽到了事件. */
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.
*/
/* 將這些直接放入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".
*/
/* 上一次沒有處理完的epitem, 重新插入到ready list */
list_splice(&txlist, &ep->rdllist);
/* ready list不爲空, 直接喚醒... */
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;
}
/* 該函數作爲callbakc在ep_scan_ready_list()中被調用
* head是一個鏈表, 包含了已經ready的epitem,
* 這個不是eventpoll裏面的ready list, 而是上面函數中的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.
*/
/* 掃描整個鏈表... */
for (eventcnt = 0, uevent = esed->events;
!list_empty(head) && eventcnt < esed->maxevents;) {
/* 取出第一個成員 */
epi = list_first_entry(head, struct epitem, rdllink);
/* 然後從鏈表裏面移除 */
list_del_init(&epi->rdllink);
/* 讀取events,
* 注意events我們ep_poll_callback()裏面已經取過一次了, 爲啥還要再取?
* 1. 我們當然希望能拿到此刻的最新數據, events是會變的~
* 2. 不是所有的poll實現, 都通過等待隊列傳遞了events, 有可能某些驅動壓根沒傳
* 必須主動去讀取. */
revents = epi->ffd.file->f_op->poll(epi->ffd.file, NULL) &
epi->event.events;
if (revents) {
/* 將當前的事件和用戶傳入的數據都copy給用戶空間,
* 就是epoll_wait()後應用程序能讀到的那一堆數據. */
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的區別就在這一步之差呀~
* 如果是ET, epitem是不會再進入到readly list,
* 除非fd再次發生了狀態改變, ep_poll_callback被調用.
* 如果是非ET, 不管你還有沒有有效的事件或者數據,
* 都會被重新插入到ready list, 再下一次epoll_wait
* 時, 會立即返回, 並通知給用戶空間. 當然如果這個
* 被監聽的fds確實沒事件也沒數據了, epoll_wait會返回一個0,
* 空轉一次.
*/
list_add_tail(&epi->rdllink, &ep->rdllist);
}
}
}
return eventcnt;
}
/* ep_free在epollfd被close時調用,
* 釋放一些資源而已, 比較簡單 */
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".
*/
/* 之所以在關閉epollfd之前不需要調用epoll_ctl移除已經添加的fd,
* 是因爲這裏已經做了... */
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我認爲比較重要的函數都註釋完了... */
epoll_create
從slab緩存中創建一個eventpoll對象,並且創建一個匿名的fd跟fd對應的file對象,
而eventpoll對象保存在struct file結構的private指針中,並且返回,
該fd對應的file operations只是實現了poll跟release操作
創建eventpoll對象的初始化操作
獲取當前用戶信息,是不是root,最大監聽fd數目等並且保存到eventpoll對象中
初始化等待隊列,初始化就緒鏈表,初始化紅黑樹的頭結點
epoll_ctl操作
將epoll_event結構拷貝到內核空間中
並且判斷加入的fd是否支持poll結構(epoll,poll,selectI/O多路複用必須支持poll操作).
並且從epfd->file->privatedata獲取event_poll對象,根據op區分是添加刪除還是修改,
首先在eventpoll結構中的紅黑樹查找是否已經存在了相對應的fd,沒找到就支持插入操作,否則報重複的錯誤.
相對應的修改,刪除比較簡單就不囉嗦了
插入操作時,會創建一個與fd對應的epitem結構,並且初始化相關成員,比如保存監聽的fd跟file結構之類的
重要的是指定了調用poll_wait時的回調函數用於數據就緒時喚醒進程,(其內部,初始化設備的等待隊列,將該進程註冊到等待隊列)完成這一步, 我們的epitem就跟這個socket關聯起來了, 當它有狀態變化時,
會通過ep_poll_callback()來通知.
最後調用加入的fd的file operation->poll函數(最後會調用poll_wait操作)用於完成註冊操作.
最後將epitem結構添加到紅黑樹中
epoll_wait操作
計算睡眠時間(如果有),判斷eventpoll對象的鏈表是否爲空,不爲空那就幹活不睡明.並且初始化一個等待隊列,把自己掛上去,設置自己的進程狀態
爲可睡眠狀態.判斷是否有信號到來(有的話直接被中斷醒來,),如果啥事都沒有那就調用schedule_timeout進行睡眠,如果超時或者被喚醒,首先從自己初始化的等待隊列刪除
,然後開始拷貝資源給用戶空間了
拷貝資源則是先把就緒事件鏈表轉移到中間鏈表,然後挨個遍歷拷貝到用戶空間,
並且挨個判斷其是否爲水平觸發,是的話再次插入到就緒鏈表
作者：賽羅奧特曼~
鏈接：https://www.nowcoder.com/discuss/26226
來源：牛客網