void tcp_send_ack(struct sock *sk)

 {

     struct sk_buff *buff;

     /* If we have been reset, we may not send again. */

     /* 已经是CLOSE状态 */

     if (sk->sk_state == TCP_CLOSE)

         return;

     /* 拥塞控制处理 */

     tcp_ca_event(sk, CA_EVENT_NON_DELAYED_ACK);

     /* We are not putting this on the write queue, so

      * tcp_transmit_skb() will set the ownership to this

      * sock.

      */

     /* 分配skb，失败需要启用延迟ack定时器 */

     buff = alloc_skb(MAX_TCP_HEADER,

              sk_gfp_mask(sk, GFP_ATOMIC | __GFP_NOWARN));

     if (unlikely(!buff)) {

         inet_csk_schedule_ack(sk);

         inet_csk(sk)->icsk_ack.ato = TCP_ATO_MIN;

         inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,

                       TCP_DELACK_MAX, TCP_RTO_MAX);

         return;

     }

     /* Reserve space for headers and prepare control bits. */

     /* 预留头部空间，准备控制信息 */

     skb_reserve(buff, MAX_TCP_HEADER);

     tcp_init_nondata_skb(buff, tcp_acceptable_seq(sk), TCPHDR_ACK);

     /* We do not want pure acks influencing TCP Small Queues or fq/pacing

      * too much.

      * SKB_TRUESIZE(max(1 .. 66, MAX_TCP_HEADER)) is unfortunately ~784

      */

     /* 标记纯ack，做法是将skb->truesize设置为2 */

     skb_set_tcp_pure_ack(buff);

     /* Send it off, this clears delayed acks for us. */

     /* 记录时间戳，发送skb */

     skb_mstamp_get(&buff->skb_mstamp);

     tcp_transmit_skb(sk, buff, , (__force gfp_t));

 }