Does listen's backlog number include SYN-received connections count in case of TCP in Linux?

Question

I read some posts and checked Linux kernel code like inet_listen()->inet_csk_listen_start() and it seems that backlog argument of listen(

Answer 1

In case of 4.3 kernel you specified it's something like:

tcp_v4_do_rcv()->tcp_rcv_state_process()->tcp_v4_conn_request()->tcp_conn_request()->inet_csk_reqsk_queue_is_full()

Here we can see the most important details about queues:

/* TW buckets are converted to open requests without
 * limitations, they conserve resources and peer is
 * evidently real one.
 */
if ((sysctl_tcp_syncookies == 2 ||
     inet_csk_reqsk_queue_is_full(sk)) && !isn) {
    want_cookie = tcp_syn_flood_action(sk, skb, rsk_ops->slab_name);
    if (!want_cookie)
        goto drop;
}

/* Accept backlog is full. If we have already queued enough
 * of warm entries in syn queue, drop request. It is better than
 * clogging syn queue with openreqs with exponentially increasing
 * timeout.
 */
if (sk_acceptq_is_full(sk) && inet_csk_reqsk_queue_young(sk) > 1) {
    NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_LISTENOVERFLOWS);
    goto drop;
}

Pay your attention to inet_csk_reqsk_queue_is_full():

static inline int inet_csk_reqsk_queue_is_full(const struct sock *sk)
{
    return inet_csk_reqsk_queue_len(sk) >= sk->sk_max_ack_backlog;
}

Finally it compares current queue icsk_accept_queue with sk_max_ack_backlog size which was previously set by inet_csk_listen_start(). So yep, backlog affects incoming queue in current case.

You can see that both sk_acceptq_is_full() and inet_csk_reqsk_queue_is_full() make comparison with the same socket's sk_max_ack_backlog which is set through the listen():

static inline bool sk_acceptq_is_full(const struct sock *sk)
{
    return sk->sk_ack_backlog > sk->sk_max_ack_backlog;
}

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