Socket accept 简要分析

accept 用于从指定套接字的连接队列中取出第一个连接，并返回一个新的套接字用于与客户端进行通信，示例代码如下

#include <sys/types.h> /* See NOTES */
#include <sys/socket.h>
int accept(int sockfd, struct sockaddr *addr,  socklen_t *addrlen);
int accept4(int sockfd, struct sockaddr *addr, socklen_t *addrlen, int flags);

其中的参数解释如下：
·int sockfd ：处于监听状态的套接字。
·struct sockaddr*addr ：用于保存对端的地址信息。
·socklen_t*addrlen ：是一个输入输出值。调用者将其初始化为 addr 缓存的大小， accept 返回时，会
将其设置为 addr 的大小。
·int flags ：是新引入的系统调用 accept4 的标志位；目前支持 SOCK_NONBLOCK 和
SOCK_CLOEXEC

*
 *	For accept, we attempt to create a new socket, set up the link
 *	with the client, wake up the client, then return the new
 *	connected fd. We collect the address of the connector in kernel
 *	space and move it to user at the very end. This is unclean because
 *	we open the socket then return an error.
 *
 *	1003.1g adds the ability to recvmsg() to query connection pending
 *	status to recvmsg. We need to add that support in a way thats
 *	clean when we restucture accept also.
 */

SYSCALL_DEFINE4(accept4, int, fd, struct sockaddr __user *, upeer_sockaddr,
		int __user *, upeer_addrlen, int, flags)
{
	struct socket *sock, *newsock;
	struct file *newfile;
	int err, len, newfd, fput_needed;
	struct sockaddr_storage address;
 /* 只允许使用这两个标志 */
	if (flags & ~(SOCK_CLOEXEC | SOCK_NONBLOCK))
		return -EINVAL;

	if (SOCK_NONBLOCK != O_NONBLOCK && (flags & SOCK_NONBLOCK))
		flags = (flags & ~SOCK_NONBLOCK) | O_NONBLOCK;
 /* 通过文件描述符fd，找到对应的socket。
     * 以fd为索引从当前进程的文件描述符表files_struct中找到对应的file实例，
     * 然后从file实例的private_data成员中获取socket实例。
     */
	sock = sockfd_lookup_light(fd, &err, &fput_needed);
	if (!sock)
		goto out;

	err = -ENFILE;
	newsock = sock_alloc();
	if (!newsock)
		goto out_put;
 /* 新socket的类型 新socket的socket层操作 */
	newsock->type = sock->type;
	newsock->ops = sock->ops;

	/*
	 * We don't need try_module_get here, as the listening socket (sock)
	 * has the protocol module (sock->ops->owner) held.
Socekt层协议，对SOCK_STREAM来说是inet_stream_ops，它的引用计数加一
	 */
	__module_get(newsock->ops->owner);
 /*申请一个新的文件描述符*/
	newfd = get_unused_fd_flags(flags);
	if (unlikely(newfd < 0)) {
		err = newfd;
		sock_release(newsock);
		goto out_put;
	}
/* 为socket创建一个对应的file结构sock->file，返回fd */
	newfile = sock_alloc_file(newsock, flags, sock->sk->sk_prot_creator->name);
	if (IS_ERR(newfile)) {
		err = PTR_ERR(newfile);
		put_unused_fd(newfd);
		sock_release(newsock);
		goto out_put;
	}

	err = security_socket_accept(sock, newsock);
	if (err)
		goto out_fd;
/* SOCKET层的操作函数，如果是SOCK_STREAM，proto_ops为inet_stream_ops，
     * 接下来调用inet_accept()。
     */
	err = sock->ops->accept(sock, newsock, sock->file->f_flags, false);
	if (err < 0)
		goto out_fd;

	if (upeer_sockaddr) {/* 如果要保存对端地址 */
		if (newsock->ops->getname(newsock, (struct sockaddr *)&address,
					  &len, 2) < 0) {
			err = -ECONNABORTED;
			goto out_fd;
		}/* 把内核空间的socket地址复制到用户空间 */
		err = move_addr_to_user(&address,
					len, upeer_sockaddr, upeer_addrlen);
		if (err < 0)
			goto out_fd;
	}

	/* File flags are not inherited via accept() unlike another OSes.
* 以newfd为索引，把newfile加入当前进程的文件描述符表files_struct中
    */

	fd_install(newfd, newfile);
	err = newfd;

out_put:
	fput_light(sock->file, fput_needed);
out:
	return err;
out_fd:
	fput(newfile);
	put_unused_fd(newfd);
	goto out_put;
}

static inline void sock_graft(struct sock *sk, struct socket *parent)
{
    write_lock_bh(&sk->sk_callback_lock);
    sk->sk_wq = parent->wq;/指向parent 等待队列
    parent->sk = sk;
    sk_set_socket(sk, parent);
    sk->sk_uid = SOCK_INODE(parent)->i_uid;
    security_sock_graft(sk, parent);
    write_unlock_bh(&sk->sk_callback_lock);
}

SOCK_STREAM套接口的TCP层操作函数集实例为tcp_prot，其中连接接收函数为inet_csk_accept()

inet_csk_accept()用于从backlog队列（全连接队列）中取出一个ESTABLISHED状态的连接请求块，返回它所对应的连接sock。

1. 非阻塞的，且当前没有已建立的连接，则直接退出，返回-EAGAIN。

2. 阻塞的，且当前没有已建立的连接：

    -.1 用户没有设置超时时间，则无限期阻塞。

    -.2 用户设置了超时时间，超时后会退出。

1、当前全队列中有socket，则accept()直接返回对应的fd。
2、如果当前全队列中没有socket，则如果当前socket是阻塞的，直接睡眠。
3、如果当前全队列中没有socket，如果非阻塞，就直接返回-EAGAIN。
4、如果是阻塞的listenfd，需要将当前进程挂在listenfd对应socket的等待队列里面，当前进程让出cpu，并且等待唤醒

/*
 * This will accept the next outstanding connection.
 */
struct sock *inet_csk_accept(struct sock *sk, int flags, int *err, bool kern)
{
    struct inet_connection_sock *icsk = inet_csk(sk);
    struct request_sock_queue *queue = &icsk->icsk_accept_queue;
    struct request_sock *req;
    struct sock *newsk;
    int error;

    lock_sock(sk);

    /* We need to make sure that this socket is listening,
     * and that it has something pending.
     */
    error = -EINVAL;
    if (sk->sk_state != TCP_LISTEN)/* socket必须处于监听状态 */
        goto out_err;

    /* Find already established connection */
    if (reqsk_queue_empty(queue)) {//发没有现ESTABLISHED状态的连接请求块
     /* 等待超时时间，如果是非阻塞则为0 */
        long timeo = sock_rcvtimeo(sk, flags & O_NONBLOCK);

        /* If this is a non blocking socket don't sleep */
        error = -EAGAIN;
        if (!timeo)/* 如果是非阻塞的，则直接退出 */
            goto out_err;
        /* 阻塞等待，直到有全连接。如果用户有设置等待超时时间，超时后会退出 */
        error = inet_csk_wait_for_connect(sk, timeo);
        if (error)
            goto out_err;
    }
    //走到这里，说明全队列中有socket，直接取出来
    req = reqsk_queue_remove(queue, sk);
    newsk = req->sk;

    if (sk->sk_protocol == IPPROTO_TCP &&
        tcp_rsk(req)->tfo_listener) {
        spin_lock_bh(&queue->fastopenq.lock);
        if (tcp_rsk(req)->tfo_listener) {
            /* We are still waiting for the final ACK from 3WHS
             * so can't free req now. Instead, we set req->sk to
             * NULL to signify that the child socket is taken
             * so reqsk_fastopen_remove() will free the req
             * when 3WHS finishes (or is aborted).
             */
            req->sk = NULL;
            req = NULL;
        }
        spin_unlock_bh(&queue->fastopenq.lock);
    }
out:
    release_sock(sk);
    if (req)
        reqsk_put(req);
    return newsk;
out_err:
    newsk = NULL;
    req = NULL;
    *err = error;
    goto out;
}

/*
 * Wait for an incoming connection, avoid race conditions. This must be called
 * with the socket locked.
 */
static int inet_csk_wait_for_connect(struct sock *sk, long timeo)
{
    struct inet_connection_sock *icsk = inet_csk(sk);
    DEFINE_WAIT(wait);
    int err;

    /*
     * True wake-one mechanism for incoming connections: only
     * one process gets woken up, not the 'whole herd'.
     * Since we do not 'race & poll' for established sockets
     * anymore, the common case will execute the loop only once.
     *
     * Subtle issue: "add_wait_queue_exclusive()" will be added
     * after any current non-exclusive waiters, and we know that
     * it will always _stay_ after any new non-exclusive waiters
     * because all non-exclusive waiters are added at the
     * beginning of the wait-queue. As such, it's ok to "drop"
     * our exclusiveness temporarily when we get woken up without
     * having to remove and re-insert us on the wait queue.
     */
    for (;;) {
        //prepare_to_wait_exclusive很重要，把 wait 挂到当前sk的等待队列里面。
        prepare_to_wait_exclusive(sk_sleep(sk), &wait,
                      TASK_INTERRUPTIBLE);
        release_sock(sk);
        //icsk_accept_queue是全队列
        if (reqsk_queue_empty(&icsk->icsk_accept_queue))
            timeo = schedule_timeout(timeo);//阻塞情况下，只有主动唤醒当前进程，才会继续执行。
        lock_sock(sk);
        err = 0;

        //如果阻塞且非超时的情况从schedule_timeout返回，那么必然是全队列有值了。
        if (!reqsk_queue_empty(&icsk->icsk_accept_queue))
            break;//这个break是所有程序必经之路
        err = -EINVAL;
        if (sk->sk_state != TCP_LISTEN)
            break;
        err = sock_intr_errno(timeo);

        //有信号或者睡眠时间满了，则退出循环，否则接着睡。
        if (signal_pending(current))
            break;
        err = -EAGAIN;
        if (!timeo)
            break;
    }
    finish_wait(sk_sleep(sk), &wait);
    return err;
}

prepare_to_wait_exclusive函数很重要，把当前上下文加到listenfd对应的socket等待队列里面，如果是多进程，那么listenfd对应的socket等待队列里面会有多个进程的上下文

多进程accept，不考虑resuseport，那么多进程accept只会出现在父子进程同时accept的情况，那么上文也说过，prepare_to_wait_exclusive函数会被当前进程上下文加入到listenfd等待队列里面，所以父子进程的上下文都会加入到socket的等待队列里面。核心问题就是这么唤醒，我们可以相当，所谓的惊群，就是把等待队里里面的所有进程都唤醒。

int tcp_v4_do_rcv(struct sock *sk, struct sk_buff *skb)
{
    struct sock *rsk;

    ......
    if (sk->sk_state == TCP_LISTEN) {
        struct sock *nsk = tcp_v4_hnd_req(sk, skb);
        if (!nsk)
            goto discard;

        if (nsk != sk) {
            sock_rps_save_rxhash(nsk, skb);
            //当三次握手客户端的ack到来时，会走tcp_child_process这里
            if (tcp_child_process(sk, nsk, skb)) {
                rsk = nsk;
                goto reset;
            }
            return 0;
        }
    }
    ......
}

int tcp_child_process(struct sock *parent, struct sock *child,
              struct sk_buff *skb)
{
    int ret = 0;
    int state = child->sk_state;

    if (!sock_owned_by_user(child)) {
        ret = tcp_rcv_state_process(child, skb, tcp_hdr(skb),
                        skb->len);
        /* Wakeup parent, send SIGIO */
        if (state == TCP_SYN_RECV && child->sk_state != state)
            parent->sk_data_ready(parent, 0);//唤醒 在accept的进程，调用 sock_def_readable
    } else {
        /* Alas, it is possible again, because we do lookup
         * in main socket hash table and lock on listening
         * socket does not protect us more.
         */
        __sk_add_backlog(child, skb);
    }

    bh_unlock_sock(child);
    sock_put(child);
    return ret;
}

static void sock_def_readable(struct sock *sk, int len)
{
    struct socket_wq *wq;

    rcu_read_lock();
    wq = rcu_dereference(sk->sk_wq);
    //显然，我们在accept的时候调用了`prepare_to_wait_exclusive`加入了队列，故唤醒靠 wake_up_interruptible_sync_poll
    if (wq_has_sleeper(wq))
        wake_up_interruptible_sync_poll(&wq->wait, POLLIN | POLLPRI |
                        POLLRDNORM | POLLRDBAND);
    sk_wake_async(sk, SOCK_WAKE_WAITD, POLL_IN);
    rcu_read_unlock();
}

 多个进程accept的时候，内核只会唤醒1个等待的进程，且唤醒的逻辑是FIFO

其代码逻辑如下

static void __wake_up_common(wait_queue_head_t *q, unsigned int mode,
            int nr_exclusive, int wake_flags, void *key)
{
    wait_queue_t *curr, *next;

    list_for_each_entry_safe(curr, next, &q->task_list, task_list) {
        unsigned flags = curr->flags;

        //prepare_to_wait_exclusive时候，flags是WQ_FLAG_EXCLUSIVE，入参nr_exclusive是1，所以只执行一次就break了。
        if (curr->func(curr, mode, wake_flags, key) &&
                (flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
            break;
    }
}