Linux的poll与epoll实现（2）---epoll_create

    昨晚分析了poll，通过代码的阅读可以发现，poll操作有很多可以优化的地方。epoll是eventpoll的简称，他的效率是非常高的，我们今天来看看他的实现。他的实现在FS/Eventpoll.c，代码有1500多行，呵呵，怕了吧。

    大家都知道，epoll有三个系统调用，C库封装成以下三个：

1. int epoll_create(int size);
2. int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event);
3. int epoll_wait(int epfd, struct epoll_event *events,int maxevents, int timeout);

epoll的源码这么多，我们就干脆跟着他们三个走着瞧。今天先搞定第一个---epoll_create

 

第一个是

/*
 * It opens an eventpoll file descriptor by suggesting a storage of "size"
 * file descriptors. The size parameter is just an hint about how to size
 * data structures. It won't prevent the user to store more than "size"
 * file descriptors inside the epoll interface. It is the kernel part of
 * the userspace epoll_create(2).
 */
asmlinkage long sys_epoll_create(int size)
{
    int error, fd;
    struct inode *inode;
    struct file *file;

    DNPRINTK(3, (KERN_INFO "[%p] eventpoll: sys_epoll_create(%d)\n",
         current, size));

    /* Sanity check on the size parameter */
    error = -EINVAL;
    if (size <= 0)
        goto eexit_1;

    /*
     * Creates all the items needed to setup an eventpoll file. That is,
     * a file structure, and inode and a free file descriptor.
     */
    error = ep_getfd(&fd, &inode, &file);                  //(1)
    if (error)
        goto eexit_1;

    /* Setup the file internal data structure ( "struct eventpoll" ) */
    error = ep_file_init(file);                            //(2)
    if (error)
        goto eexit_2;


    DNPRINTK(3, (KERN_INFO "[%p] eventpoll: sys_epoll_create(%d) = %d\n",
         current, size, fd));

    return fd;

eexit_2:
    sys_close(fd);
eexit_1:
    DNPRINTK(3, (KERN_INFO "[%p] eventpoll: sys_epoll_create(%d) = %d\n",
         current, size, error));
    return error;
}

(1)这里用到了一个ep_getfd函数，从注释我们知道，这个函数建立eventpoll相关的file，当然，一个file要包括文件描述符、inode、还有文件对象，这也是我们传的三个参数。废话不说，看源码：

/*
 * Creates the file descriptor to be used by the epoll interface.
 */
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;
}

这个函数的注释都比较全，这里简单提一下，况且因为涉及到的函数太多，要深究起来涉及的知识太多，也不可能逐一去列代码。不过这个函数个人觉得比较经典，这函数就是创建一个文件的流程。

首先，我们得拿到一个file结构体，通过内核分配给我们；然后我们要拿到inode，调用这个ep_eventpoll_inode()就可以了；接着是get_unused_fd()拿到文件描述符；接着d_alloc()函数为我们拿到一个dentry；d_add(dentry, inode)函数把dentry建立hash里面并且绑定inode；后面是继续填充文件对象file；fd_install(fd, file)向进程注册文件，并通过这样的方式把文件描述符和文件对象关联起来。

(2)在跟踪ep_file_init函数之前，我们先来看一下eventpoll结构体：

/*
 * This structure is stored inside the "private_data" member of the file
 * structure and rapresent the main data sructure for the eventpoll
 * interface.
 */
struct eventpoll {
    /* Protect the this structure access */
    rwlock_t lock;

    /*
     * This semaphore is used to ensure that files are not removed
     * while epoll is using them. This is read-held during the event
     * collection loop and it is write-held during the file cleanup
     * path, the epoll file exit code and the ctl operations.
     */
    struct rw_semaphore sem;

    /* 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;
};

注释也是相当清楚。这个eventpoll可以看得出来，是epoll的核心，它将会存储你想要监听的文件描述符，这也是为什么epoll高效之所在。

好，我们回到sys_epoll_create函数，开始跟踪ep_file_init函数：

static int ep_file_init(struct file *file)
{
    struct eventpoll *ep;

    if (!(ep = kmalloc(sizeof(struct eventpoll), GFP_KERNEL)))
        return -ENOMEM;

    memset(ep, 0, sizeof(*ep));
    rwlock_init(&ep->lock);
    init_rwsem(&ep->sem);
    init_waitqueue_head(&ep->wq);
    init_waitqueue_head(&ep->poll_wait);
    INIT_LIST_HEAD(&ep->rdllist);
    ep->rbr = RB_ROOT;

    file->private_data = ep;

    DNPRINTK(3, (KERN_INFO "[%p] eventpoll: ep_file_init() ep=%p\n",
         current, ep));
    return 0;
}

其实也就是eventpoll结构体的初始化。

sys_epoll_create函数大概就这样了，明天接着看sys_epoll_ctl。