概述

面试时候选人提到了半连贯，全连贯队列别离由内核中的常量，backlog参数决定。这个和之前的了解不符，于是看代码一探到底。

源码分析

源码版本：
https://github.com/torvalds/l...
d992fe5318d8d7af9510b879439a3c7f283da442

入口

搜寻"sys_listen", 能够看到，backlog的确是取了用户传入和sysctl_somaxconn的最小值。

int __sys_listen(int fd, int backlog){    struct socket *sock;    int err, fput_needed;    int somaxconn;    sock = sockfd_lookup_light(fd, &err, &fput_needed);    if (sock) {        somaxconn = sock_net(sock->sk)->core.sysctl_somaxconn;        if ((unsigned int)backlog > somaxconn)            backlog = somaxconn;        err = security_socket_listen(sock, backlog);        if (!err)            err = sock->ops->listen(sock, backlog);        fput_light(sock->file, fput_needed);    }    return err;}

listen的实现

能够看到sock->ops是一个函数指针构造体，搜寻"sock->ops = ",能够找到一系列proto_ops。挑base_sock_ops持续看。

static const struct proto_ops base_sock_ops = {    .family        = PF_ISDN,    .owner        = THIS_MODULE,    .release    = base_sock_release,    .ioctl        = base_sock_ioctl,    .bind        = base_sock_bind,    .getname    = sock_no_getname,    .sendmsg    = sock_no_sendmsg,    .recvmsg    = sock_no_recvmsg,    .listen        = sock_no_listen,    .shutdown    = sock_no_shutdown,    .connect    = sock_no_connect,    .socketpair    = sock_no_socketpair,    .accept        = sock_no_accept,    .mmap        = sock_no_mmap};

发现大部分的proto_ops都没有定义listen的实现。
搜寻".listen ="，能够找到有 svc_listen, dn_listen, unix_listen, vsock_listen, x25_listen等. 从命名和文件头的形容，猜想inet_listen是理论用的listen，即socket_addr中指定的协定。持续看inet_listen。
能够看到backlog赋值给了sk->sk_max_ack_backlog，搜寻发现其它listen函数也是如此。

int inet_listen(struct socket *sock, int backlog){    struct sock *sk = sock->sk;    unsigned char old_state;    int err, tcp_fastopen;    lock_sock(sk);    err = -EINVAL;    if (sock->state != SS_UNCONNECTED || sock->type != SOCK_STREAM)        goto out;    old_state = sk->sk_state;    if (!((1 << old_state) & (TCPF_CLOSE | TCPF_LISTEN)))        goto out;    WRITE_ONCE(sk->sk_max_ack_backlog, backlog);    /* Really, if the socket is already in listen state     * we can only allow the backlog to be adjusted.     */

从inet_connection_sock.c:795 能够看到，这个值决定的icsk_accept_queue队列大小，从代码上来看，并没有辨别所谓的半，全链接队列。并且做了一个新生代老年代的优化.
一个是reqsk_timer_handler中有解决syn_ack的重发。

    icsk = inet_csk(sk_listener);    net = sock_net(sk_listener);    max_syn_ack_retries = icsk->icsk_syn_retries ? : net->ipv4.sysctl_tcp_synack_retries;    /* Normally all the openreqs are young and become mature     * (i.e. converted to established socket) for first timeout.     * If synack was not acknowledged for 1 second, it means     * one of the following things: synack was lost, ack was lost,     * rtt is high or nobody planned to ack (i.e. synflood).     * When server is a bit loaded, queue is populated with old     * open requests, reducing effective size of queue.     * When server is well loaded, queue size reduces to zero     * after several minutes of work. It is not synflood,     * it is normal operation. The solution is pruning     * too old entries overriding normal timeout, when     * situation becomes dangerous.     *     * Essentially, we reserve half of room for young     * embrions; and abort old ones without pity, if old     * ones are about to clog our table.     */    queue = &icsk->icsk_accept_queue;    qlen = reqsk_queue_len(queue);    if ((qlen << 1) > max(8U, READ_ONCE(sk_listener->sk_max_ack_backlog))) {        int young = reqsk_queue_len_young(queue) << 1;        while (max_syn_ack_retries > 2) {            if (qlen < young)                break;            max_syn_ack_retries--;            young <<= 1;        }    }

一个是accept中有去find already established connection。

/* * 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)        goto out_err;    /* Find already established connection */    if (reqsk_queue_empty(queue)) {        long timeo = sock_rcvtimeo(sk, flags & O_NONBLOCK);        /* If this is a non blocking socket don't sleep */        error = -EAGAIN;        if (!timeo)            goto out_err;        error = inet_csk_wait_for_connect(sk, timeo);        if (error)            goto out_err;    }    req = reqsk_queue_remove(queue, sk);    newsk = req->sk;

论断

backlog决定icsk_accept_queue队列大小。该队列中存在SYNC_RECV/ESTABLISHTED状态的链接。