TCP连接下载文件失败，也许该看看这篇文章

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unsigned int tcp_poll(struct file *file, struct socket *sock, poll_table *wait)
{
    unsigned int mask;
    struct sock *sk = sock->sk;
    struct tcp_sock *tp = tcp_sk(sk);

    sock_poll_wait(file, sk->sk_sleep, wait);
    if (sk->sk_state == TCP_LISTEN) // 侦听状态，进入listen poll，即检查侦听socket的accpet队列是否为空
        return inet_csk_listen_poll(sk);

    /* Socket is not locked. We are protected from async events
     * by poll logic and correct handling of state changes
     * made by other threads is impossible in any case.
     */

    mask = 0;

    /*
     * POLLHUP is certainly not done right. But poll() doesn't
     * have a notion of HUP in just one direction, and for a
     * socket the read side is more interesting.
     *
     * Some poll() documentation says that POLLHUP is incompatible
     * with the POLLOUT/POLLWR flags, so somebody should check this
     * all. But careful, it tends to be safer to return too many
     * bits than too few, and you can easily break real applications
     * if you don't tell them that something has hung up!
     *
     * Check-me.
     *
     * Check number 1. POLLHUP is _UNMASKABLE_ event (see UNIX98 and
     * our fs/select.c). It means that after we received EOF,
     * poll always returns immediately, making impossible poll() on write()
     * in state CLOSE_WAIT. One solution is evident --- to set POLLHUP
     * if and only if shutdown has been made in both directions.
     * Actually, it is interesting to look how Solaris and DUX
     * solve this dilemma. I would prefer, if POLLHUP were maskable,
     * then we could set it on SND_SHUTDOWN. BTW examples given
     * in Stevens' books assume exactly this behaviour, it explains
     * why POLLHUP is incompatible with POLLOUT.    --ANK
     *
     * NOTE. Check for TCP_CLOSE is added. The goal is to prevent
     * blocking on fresh not-connected or disconnected socket. --ANK
     */
     /* socket 与tcp 状态转化 poll事件 */
    if (sk->sk_shutdown == SHUTDOWN_MASK || sk->sk_state == TCP_CLOSE)
        mask |= POLLHUP;
    if (sk->sk_shutdown & RCV_SHUTDOWN)
        mask |= POLLIN | POLLRDNORM | POLLRDHUP;

    /* Connected? */
    if ((1 << sk->sk_state) & ~(TCPF_SYN_SENT | TCPF_SYN_RECV)) {
        int target = sock_rcvlowat(sk, 0, INT_MAX);

        if (tp->urg_seq == tp->copied_seq &&
            !sock_flag(sk, SOCK_URGINLINE) &&
            tp->urg_data)
            target--;

        /* Potential race condition. If read of tp below will
         * escape above sk->sk_state, we can be illegally awaken
         * in SYN_* states. */
         /*
         未处理接收报文字节数超过了最小阈值，满足可读条件
         */
        if (tp->rcv_nxt - tp->copied_seq >= target)
            mask |= POLLIN | POLLRDNORM;

        if (!(sk->sk_shutdown & SEND_SHUTDOWN)) {
            /*  
            sk->sk_sndbuf - sk->sk_wmem_queued >=   sk->sk_wmem_queued>>1 (简单理解为最大值 0.5* sk->sk_wmem_queued)
            如果未发送报文超过了66%，那么不会继续上报POLLOUT事件
            */
            if (sk_stream_wspace(sk) >= sk_stream_min_wspace(sk)) {
                mask |= POLLOUT | POLLWRNORM;
            } else { 
                /* send SIGIO later */
                /* 发送SIGIO  */
                set_bit(SOCK_ASYNC_NOSPACE,
                    &sk->sk_socket->flags);
                set_bit(SOCK_NOSPACE, &sk->sk_socket->flags);

                /* Race breaker. If space is freed after
                 * wspace test but before the flags are set,
                 * IO signal will be lost.
                 */
                if (sk_stream_wspace(sk) >= sk_stream_min_wspace(sk))
                    mask |= POLLOUT | POLLWRNORM;
            }
        } else
            mask |= POLLOUT | POLLWRNORM;

        if (tp->urg_data & TCP_URG_VALID)
            mask |= POLLPRI;
    }
    /* This barrier is coupled with smp_wmb() in tcp_reset() */
    smp_rmb();
    if (sk->sk_err)
        mask |= POLLERR;

    return mask;
}