作者:通北开徒 来源:原创 时间:2019-10-19 阅读:8403 次

台湾缓蹿步科技有限公司_Linux epoll源码注释

Linux系统运行源码剖析-epoll代码注释

理解了中断、等待队列、调度,你就能懂Linux的80%。

--老子

转发的话,请注明出处哦:http://www.cnblogs.com/stonehat/
Linux系统内核提供了三个系统调用:

include/linux/syscalls.h

// epoll_create,创建epoll描述符
asmlinkage long sys_epoll_create(int size);
// epoll_ctl, 操作epoll描述符,增删改
asmlinkage long sys_epoll_ctl(int epfd, int op, int fd,
                struct epoll_event __user *event);
// epoll_wait, 你懂的
asmlinkage long sys_epoll_wait(int epfd, struct epoll_event __user *events,
                int maxevents, int timeout);

其函数实现在fs/eventpoll.c

eventpoll 本身也是一个支持poll操作的文件,所以可以把eventpoll组成一个树形关系。

下面分别按照sys_epoll_create,sys_epoll_ctl,sys_epoll_wait的顺序分析三个系统调用。

重要的结构体

// eventpoll结构体
struct eventpoll {
    /* Protect the this structure access */
    rwlock_t lock;

    /*
     * 同步用的内核信号量
     */
    struct rw_semaphore sem;

    /**
     * 等待队列,epoll_wait()使用,将调用线程挂在这个队列上。
     */
    wait_queue_head_t wq;

    /* 等待队列,file->poll()会使用,在epoll中函数为ep_eventpoll_poll */
    wait_queue_head_t poll_wait;

    /* 就绪列表*/
    struct list_head rdllist;

    /* 红黑树,维护了 */
    struct rb_root rbr;
};
// 内核中文件
struct file {
    struct list_head    f_list;
    struct dentry       *f_dentry;
    struct vfsmount         *f_vfsmnt;
    //文件操作指针
    struct file_operations  *f_op;
    atomic_t        f_count;
    unsigned int        f_flags;
    mode_t          f_mode;
    int         f_error;
    loff_t          f_pos;
    struct fown_struct  f_owner;
    unsigned int        f_uid, f_gid;
    struct file_ra_state    f_ra;

    unsigned long       f_version;
    void            *f_security;

    /* file中的私有自定义数据 */
    void            *private_data;

#ifdef CONFIG_EPOLL
    /* Used by fs/eventpoll.c to link all the hooks to this file */
    struct list_head    f_ep_links;
    spinlock_t      f_ep_lock;
#endif /* #ifdef CONFIG_EPOLL */
    struct address_space    *f_mapping;
};

struct file_operations {
    struct module *owner;
    loff_t (*llseek) (struct file *, loff_t, int);
    ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
    ssize_t (*aio_read) (struct kiocb *, char __user *, size_t, loff_t);
    ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
    ssize_t (*aio_write) (struct kiocb *, const char __user *, size_t, loff_t);
    int (*readdir) (struct file *, void *, filldir_t);
    // 不阻塞,检测file状态(可读、可写等),如果条件不满足,pt将会被加到等待队列中。(一般是这种逻辑,最终如何实现还是要看设备驱动)
    unsigned int (*poll) (struct file *f, struct poll_table_struct *pt);
    int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned long);
    int (*mmap) (struct file *, struct vm_area_struct *);
    int (*open) (struct inode *, struct file *);
    int (*flush) (struct file *);
    int (*release) (struct inode *, struct file *);
    int (*fsync) (struct file *, struct dentry *, int datasync);
    int (*aio_fsync) (struct kiocb *, int datasync);
    int (*fasync) (int, struct file *, int);
    int (*lock) (struct file *, int, struct file_lock *);
    ssize_t (*readv) (struct file *, const struct iovec *, unsigned long, loff_t *);
    ssize_t (*writev) (struct file *, const struct iovec *, unsigned long, loff_t *);
    ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t, void *);
    ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
    unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
    int (*check_flags)(int);
    int (*dir_notify)(struct file *filp, unsigned long arg);
    int (*flock) (struct file *, int, struct file_lock *);
};

概念与关系

  • 文件描述符fd:进程打开的文件的数字代表形式,是文件指针的索引。
  • struct file:在内核中表示进程打开的文件。task.files[fd]=file
  • struct inode:静态的文件表示。

一. sys_epoll_create

代码如下:为了方便理解原理,无关紧要的代码逻辑和异常处理删掉了


asmlinkage long sys_epoll_create(int size)
{
    int error, fd;
    struct inode *inode;
    struct file *file;
    
    .....
      
    /*
     * 创建一个新的file,inode,获得file对应的fd。
     * 并且将file加入到当前进程打开文件列表。
     */
    error = ep_getfd(&fd, &inode, &file);
    /* 创建struct eventpoll,并挂在file的private_data指针上*/
    error = ep_file_init(file);
    .....
    return fd;
}

ep_getfd简单流程



static int ep_getfd(int *efd, struct inode **einode, struct file **efile)
{
    struct qstr this;
    char name[32];
    struct dentry *dentry;
    struct inode *inode;
    struct file *file;
    int error, fd;

    /* Get an ready to use file */
    error = -ENFILE;
    file = get_empty_filp();
    if (!file)
        goto eexit_1;

    /* Allocates an inode from the eventpoll file system */
    inode = ep_eventpoll_inode();
    error = PTR_ERR(inode);
    if (IS_ERR(inode))
        goto eexit_2;

    /* Allocates a free descriptor to plug the file onto */
    error = get_unused_fd();
    if (error < 0)
        goto eexit_3;
    fd = error;

    /*
     * Link the inode to a directory entry by creating a unique name
     * using the inode number.
     */
    error = -ENOMEM;
    sprintf(name, "[%lu]", inode->i_ino);
    this.name = name;
    this.len = strlen(name);
    this.hash = inode->i_ino;
    dentry = d_alloc(eventpoll_mnt->mnt_sb->s_root, &this);
    if (!dentry)
        goto eexit_4;
    dentry->d_op = &eventpollfs_dentry_operations;
    d_add(dentry, inode);
    file->f_vfsmnt = mntget(eventpoll_mnt);
    file->f_dentry = dentry;
    file->f_mapping = inode->i_mapping;

    file->f_pos = 0;
    file->f_flags = O_RDONLY;
    file->f_op = &eventpoll_fops;
    file->f_mode = FMODE_READ;
    file->f_version = 0;
    file->private_data = NULL;

    /* Install the new setup file into the allocated fd. */
    fd_install(fd, file);

    *efd = fd;
    *einode = inode;
    *efile = file;
    return 0;

eexit_4:
    put_unused_fd(fd);
eexit_3:
    iput(inode);
eexit_2:
    put_filp(file);
eexit_1:
    return error;
}
  1. 查找一个没有用的文件描述符。记为fd

  2. 创建一个空文件file结构体。记为epfile

  3. 在epoll的文件系统中创建一个inode

  4. epfile和inode做关联。

  5. epfile的f_ops成员(文件操作指针)和epoll的自定义函数组eventpoll_fops做关联。比较重要的一点是eventpoll_fops有一个自定义的poll函数,这个函数很重要,是实现epoll级联模型的关键。后面可以通过比较f_ops是否等于eventpoll_fops来判断file是不是epoll file。

   static struct file_operations eventpoll_fops = {
    .release    = ep_eventpoll_close,
    .poll       = ep_eventpoll_poll
   };

?

  1. 将epfile放到进程的打开文件列表中管理,用fd做索引。

  2. 初始化eventpoll结构,初始化等待队列和就绪队列等。

  3. 将epfile的private_data指向eventpoll结构。方便后面取eventpoll的数据。

  4. 返回给调用线程fd。

二、sys_epoll_ctl

sys_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;

    DNPRINTK(3, (KERN_INFO "[%p] eventpoll: sys_epoll_ctl(%d, %d, %d, %p)
",
             current, epfd, op, fd, event));

    error = -EFAULT;
    // 1. 从用户空间拷贝event数据。
    if (EP_OP_HASH_EVENT(op) &&
        copy_from_user(&epds, event, sizeof(struct epoll_event)))
        goto eexit_1;

    /* 2. 根据epollfile的文件描述符获得对应的file结构体,内核中fd和file是有一个映射关系的*/
    error = -EBADF;
    file = fget(epfd);
    if (!file)
        goto eexit_1;

    /* 3. 获得要操作的描述符的file指针,例如socket描述符 */
    tfile = fget(fd);
    if (!tfile)
        goto eexit_2;

    /* 4. 校验tfile是否支持poll操作,必须支持poll才能使用epoll */
    error = -EPERM;
    if (!tfile->f_op || !tfile->f_op->poll)
        goto eexit_3;

    /*
     * 5. 校验是否是epoll的file指针
     */
    error = -EINVAL;
    if (file == tfile || !IS_FILE_EPOLL(file))
        goto eexit_3;

    /*
     * 6. 取eventpoll,从创建时,我们知道epoll把自己的eventpoll结构体放在file->private_data了里面。
     */
    ep = file->private_data;
    
    down_write(&ep->sem);

    /* Try to lookup the file inside our hash table */
    epi = ep_find(ep, tfile, fd);
    // 7. 具体的逻辑操作
    error = -EINVAL;
    switch (op) {
    // 添加
    case EPOLL_CTL_ADD:
        if (!epi) {
            epds.events |= POLLERR | POLLHUP;

            error = ep_insert(ep, &epds, tfile, fd);
        } else
            error = -EEXIST;
        break;
    // 删除
    case EPOLL_CTL_DEL:
        if (epi)
            error = ep_remove(ep, epi);
        else
            error = -ENOENT;
        break;
    // 修改
    case EPOLL_CTL_MOD:
        if (epi) {
            epds.events |= POLLERR | POLLHUP;
            error = ep_modify(ep, epi, &epds);
        } else
            error = -ENOENT;
        break;
    }

    /*
     * The function ep_find() increments the usage count of the structure
     * so, if this is not NULL, we need to release it.
     */
    if (epi)
        ep_release_epitem(epi);

    up_write(&ep->sem);

eexit_3:
    fput(tfile);
eexit_2:
    fput(file);
eexit_1:
    DNPRINTK(3, (KERN_INFO "[%p] eventpoll: sys_epoll_ctl(%d, %d, %d, %p) = %d
",
             current, epfd, op, fd, event, error));

    return error;
}

上面的逻辑很简单

  1. 验证输入有效性

逻辑上,只需要了解添加即可。epoll的添加是理解整个流程的关键

epoll添加

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;

    error = -ENOMEM;
    if (!(epi = EPI_MEM_ALLOC()))
        goto eexit_1;

    /* Item initialization follow here ... */
    EP_RB_INITNODE(&epi->rbn);
    INIT_LIST_HEAD(&epi->rdllink);
    INIT_LIST_HEAD(&epi->fllink);
    INIT_LIST_HEAD(&epi->txlink);
    INIT_LIST_HEAD(&epi->pwqlist);
    epi->ep = ep;
    EP_SET_FFD(&epi->ffd, tfile, fd);
    epi->event = *event;
    atomic_set(&epi->usecnt, 1);
    epi->nwait = 0;

    /* 初始化polltable,当调用poll的时候,会调用ep_ptable_queue_proc函数将自身加入等待队列中 */
    epq.epi = epi;
    init_poll_funcptr(&epq.pt, ep_ptable_queue_proc);

    /*
     * 将epq.pt的结构体传入tfile进行poll,poll最终调用ep_ptable_queue_proc函数。
     */
    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.
     */
    if (epi->nwait < 0)
        goto eexit_2;

    /* 操作tfile,把当前项加入到epoll列表中。
     */
    spin_lock(&tfile->f_ep_lock);
    list_add_tail(&epi->fllink, &tfile->f_ep_links);
    spin_unlock(&tfile->f_ep_lock);

    /* We have to drop the new item inside our item list to keep track of it */
    write_lock_irqsave(&ep->lock, flags);

    /* Add the current item to the rb-tree */
    ep_rbtree_insert(ep, epi);

    /* 如果已经有就绪的,就唤醒epollwait等待队列和poll等待队列 */
    if ((revents & event->events) && !EP_IS_LINKED(&epi->rdllink)) {
        list_add_tail(&epi->rdllink, &ep->rdllist);

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

    write_unlock_irqrestore(&ep->lock, flags);

    /* We have to call this outside the lock */
    if (pwake)
        ep_poll_safewake(&psw, &ep->poll_wait);

    DNPRINTK(3, (KERN_INFO "[%p] eventpoll: ep_insert(%p, %p, %d)
",
             current, ep, tfile, fd));

    return 0;

eexit_2:
    ep_unregister_pollwait(ep, epi);

    /*
     * We need to do this because an event could have been arrived on some
     * allocated wait queue.
     */
    write_lock_irqsave(&ep->lock, flags);
    if (EP_IS_LINKED(&epi->rdllink))
        EP_LIST_DEL(&epi->rdllink);
    write_unlock_irqrestore(&ep->lock, flags);

    EPI_MEM_FREE(epi);
eexit_1:
    return error;
}

整理一下,向epoll添加一个描述符主要步骤如下:

  1. 构建epitem,epitem之后会加入到eventpoll.rbr中。

  2. 调用init_poll_funcptr,将ep_ptable_queue_proc函数指针赋值给poll_table的qproc,poll_table记为epq.pt,在file的poll函数中,可以传入poll_table作为参数,poll函数会主动调用poll_table的qproc函数。

poll_table的结构体如下:

poll_table de jie gou ti ru xia:

```C
/**
*@param f:poll的file指针
*@param whead f的等待队列
*@param pt
/
typedef void (
poll_queue_proc)(struct file f, wait_queue_head_t whead, struct poll_table_struct *pt);

typedef struct poll_table_struct {
poll_queue_proc qproc;
} poll_table;

```
poll函数原型

C // 当上层传入pt结构体时,驱动函数当调用poll_table_struct.qproc来实现阻塞队列的添加工作。 unsigned int (*poll) (struct file *f, struct poll_table_struct *pt);

?

  1. 调用待监控的文件的poll函数,按第2步所说,poll函数规范的实现应该最终会调用到ep_ptable_queue_proc函数,ep_ptable_queue_proc主要是初始化一个等待队列项(以ep_ptable_queue_proc为回调函数),然后将等待队列项塞到驱动的等待队列中。ep_ptable_queue_proc注释如下:
   struct __wait_queue {
       
    unsigned int flags;
   #define WQ_FLAG_EXCLUSIVE    0x01
       // 线程指针,如果func为默认的执行函数,这个需要赋值。
    struct task_struct * task;
       // 等待队列唤醒执行的函数
    wait_queue_func_t func;
       
    struct list_head task_list;
   };
   typedef struct __wait_queue wait_queue_t;

   static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead,
                 poll_table *pt)
   {
       // 这是一个特殊的宏操作,因为pt和epitem是包含在ep_queue结构体里面的,所以可以根据偏移取同级别的epitem。
    struct epitem *epi = EP_ITEM_FROM_EPQUEUE(pt);
    struct eppoll_entry *pwq;
    
    if (epi->nwait >= 0 && (pwq = PWQ_MEM_ALLOC())) {
           // 初始化一个等待队列项,并且设置当等待队列唤醒时的执行函数为ep_poll_callback
           // 这个很关键。等下我们分析这个ep_poll_call
        init_waitqueue_func_entry(&pwq->wait, ep_poll_callback);
        pwq->whead = whead;
        pwq->base = epi;
           // 把刚创建的等待队列项加入到等待队列中。
        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;
    }
   }

   static inline void init_waitqueue_func_entry(wait_queue_t *q,
                    wait_queue_func_t func)
   {
    q->flags = 0;
    q->task = NULL;
    q->func = func;
   }
  1. 至此,添加一个文件描述符到epoll监控内的流程完成了,总的来讲,就是在对应的file中设置等待队列。等待回调ep_poll_callback,。至于对应的file用什么机制来确保文件异步就绪,epoll不管。不过一般是通过中断来实现的。

epoll模型的poll函数实现:

* 
 * structures and helpers for f_op->poll implementations
 */
typedef void (*poll_queue_proc)(struct file *, wait_queue_head_t *, struct poll_table_struct *);

typedef struct poll_table_struct {
   poll_queue_proc qproc;
} poll_table;
//poll_wait函数实现,其实内部调用了poll_table.qproc成员,poll_table.qproc在epoll中对应了上面的ep_ptable_queue_proc函数
static inline void poll_wait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p)
{
   if (p && wait_address)
       p->qproc(filp, wait_address, p);
}
// epollevent的poll函数实现,驱动的逻辑都差不多,有参考意义
static unsigned int ep_eventpoll_poll(struct file *file, poll_table *wait)
{
   unsigned int pollflags = 0;
   unsigned long flags;
   struct eventpoll *ep = file->private_data;

   /* 1. 加入等待队列中*/
   poll_wait(file, &ep->poll_wait, wait);

   /* Check our condition */
   read_lock_irqsave(&ep->lock, flags);
   if (!list_empty(&ep->rdllist))
       pollflags = POLLIN | POLLRDNORM;
   read_unlock_irqrestore(&ep->lock, flags);

   return pollflags;
}

?

sys_epoll_wait

了解了ep_insert的话,这个其实就很容易理解了:

static struct file_operations eventpoll_fops = {
    .release    = ep_eventpoll_close,
    .poll       = ep_eventpoll_poll
};

/*
 * sys_epoll_wait实现
 */
asmlinkage long sys_epoll_wait(int epfd, struct epoll_event __user *events,
                   int maxevents, int timeout)
{
    int error;
    struct file *file;
    struct eventpoll *ep;

    DNPRINTK(3, (KERN_INFO "[%p] eventpoll: sys_epoll_wait(%d, %p, %d, %d)
",
             current, epfd, events, maxevents, timeout));

    /**
     * 验证输入的代码忽略
     */
    error = -EBADF;
    // 1. 根据epfd获得对应的file
    file = fget(epfd);
    if (!file)
        goto eexit_1;

    // 2. 验证是否是epoll的file,就是验证f_op是否等于eventpoll_fops
    error = -EINVAL;
    if (!IS_FILE_EPOLL(file))
        goto eexit_2;

    /*
     * 3. 取eventpoll结构体
     */
    ep = file->private_data;

    /* 4. 调用ep_poll实现具体逻辑。不要被ep_poll名字忽悠了,这个不是poll实现 */
    error = ep_poll(ep, events, maxevents, timeout);

eexit_2:
    fput(file);
eexit_1:
    DNPRINTK(3, (KERN_INFO "[%p] eventpoll: sys_epoll_wait(%d, %p, %d, %d) = %d
",
             current, epfd, events, maxevents, timeout, error));

    return error;
}

epoll_wait最终调用ep_poll来实现核心功能。

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;

    /*
     * 1. 内核中是是用滴答数作为时间计时的,所以下面代码是转换时间为滴答数。
     */
    jtimeout = timeout == -1 || timeout > (MAX_SCHEDULE_TIMEOUT - 1000) / HZ ?
        MAX_SCHEDULE_TIMEOUT: (timeout * HZ + 999) / 1000;

retry:
    write_lock_irqsave(&ep->lock, flags);

    res = 0;
    // 1. 如果就绪队列是空的,就进行等待
    if (list_empty(&ep->rdllist)) {
        /*
         * 2. 把当前调用epoll_wait的线程加入到wq等待队列中,当ep_poll_callback()会唤醒这个线程。
         * current是当前线程的代表,最终是从cpu中取得的。
         */
        init_waitqueue_entry(&wait, current);
        add_wait_queue(&ep->wq, &wait);
        //死循环处理。
        for (;;) {
            /*
             * 3. 设置为可打断,方便处理信号。
             */
            set_current_state(TASK_INTERRUPTIBLE);
            if (!list_empty(&ep->rdllist) || !jtimeout)
                break;
             // 4. 处理未处理信号
            if (signal_pending(current)) {
                res = -EINTR;
                break;
            }

            write_unlock_irqrestore(&ep->lock, flags);
             // 类似于睡眠。其返回值为剩余时间。该函数会将该cpu的任务切换掉。所以下一行代码在重新调度前不会执行。
            jtimeout = schedule_timeout(jtimeout);
            write_lock_irqsave(&ep->lock, flags);
        }
        //把调用线程从等待队列删除。
        remove_wait_queue(&ep->wq, &wait);

        set_current_state(TASK_RUNNING);
    }

    eavail = !list_empty(&ep->rdllist);

    write_unlock_irqrestore(&ep->lock, flags);

    /*
     * 将events数据传回用户空间
     */
    if (!res && eavail &&
        !(res = ep_events_transfer(ep, events, maxevents)) && jtimeout)
        goto retry;

    return res;
}

ep_poll的步骤如下:

  1. 转换超时时间为cpu滴答计数。

  2. 查询就绪队列是否就绪,如果有就绪的,就直接返回给上层。

  3. 如果没有就绪的,就等待。

a. 把调用线程添加到eventpoll.wq队列中。

b. 设置自身为可打断状态

c. 检查现在是否有就绪,有的话就直接返给上层。

d. 处理信号。

c. 发起调度,将自身切换为阻塞状态。等待被唤醒。唤醒的方式有:ep_poll_callback唤醒eventpoll.wq队列或者其他中断唤醒。ep_poll_callback是sys_epoll_ctl添加epoll监听的时候设置的等待队列回调。其实现为:


static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *key)
{
    int pwake = 0;
    unsigned long flags;
     // 1. 这是一个特殊的宏操作,因为wait和epitem是包含在ep_queue结构体里面的,所以可以根据偏移取同级别的epitem。
    struct epitem *epi = EP_ITEM_FROM_WAIT(wait);
    // 2. 获得对应的eventpoll
    struct eventpoll *ep = epi->ep;

    DNPRINTK(3, (KERN_INFO "[%p] eventpoll: poll_callback(%p) epi=%p ep=%p
",
             current, epi->file, epi, ep));

    write_lock_irqsave(&ep->lock, flags);
    ....
    // 3. 将就绪item加入到就绪
    list_add_tail(&epi->rdllink, &ep->rdllist);

is_linked:
    /*
     * 4. 唤醒wq等待队列(就是唤醒等待epoll_wait的线程)
     */
    if (waitqueue_active(&ep->wq))
        wake_up(&ep->wq);
    if (waitqueue_active(&ep->poll_wait))
        pwake++;

is_disabled:
    write_unlock_irqrestore(&ep->lock, flags);

    /* We have to call this outside the lock */
    if (pwake)
        ep_poll_safewake(&psw, &ep->poll_wait);

    return 1;
}

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