bool tcp_prequeue(struct sock *sk, struct sk_buff *skb)

 {

     struct tcp_sock *tp = tcp_sk(sk);

     /* 启用了latency || 没有进程读取数据 */

     if (sysctl_tcp_low_latency || !tp->ucopy.task)

         return false;

     /* 长度<= tcp头部长度，无数据&& prequeue队列长度为0，空队列 */

     /* 队列为空，无数据skb不入队，

        有数据或者之前队列中有skb，则入队，防止乱序??

     */

     if (skb->len <= tcp_hdrlen(skb) &&

         skb_queue_len(&tp->ucopy.prequeue) == )

         return false;

     /* Before escaping RCU protected region, we need to take care of skb

      * dst. Prequeue is only enabled for established sockets.

      * For such sockets, we might need the skb dst only to set sk->sk_rx_dst

      * Instead of doing full sk_rx_dst validity here, let's perform

      * an optimistic check.

      */

     /* 释放skb路由缓存 */

     if (likely(sk->sk_rx_dst))

         skb_dst_drop(skb);

     else

         skb_dst_force_safe(skb);

     /* 加入队列尾部 */

     __skb_queue_tail(&tp->ucopy.prequeue, skb);

     /* 内存增加 */

     tp->ucopy.memory += skb->truesize;

     /* 队列长度>=32 || 内存消耗> rcvbuf */

     if (skb_queue_len(&tp->ucopy.prequeue) >=  ||

         tp->ucopy.memory + atomic_read(&sk->sk_rmem_alloc) > sk->sk_rcvbuf) {

         struct sk_buff *skb1;

         BUG_ON(sock_owned_by_user(sk));

         __NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPPREQUEUEDROPPED,

                 skb_queue_len(&tp->ucopy.prequeue));

         /* skb出队，调用tcp_v4_do_rcv处理 */

         while ((skb1 = __skb_dequeue(&tp->ucopy.prequeue)) != NULL)

             sk_backlog_rcv(sk, skb1);

         /* 重置消耗内存 */

         tp->ucopy.memory = ;

     }

     /* 空队列新增的第一个skb */

     else if (skb_queue_len(&tp->ucopy.prequeue) == ) {

         /* 唤醒进程可读 */

         wake_up_interruptible_sync_poll(sk_sleep(sk),

                        POLLIN | POLLRDNORM | POLLRDBAND);

         /* 没有ack要发送，则设置延迟ack定时器 */

         if (!inet_csk_ack_scheduled(sk))

             inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,

                           ( * tcp_rto_min(sk)) / ,

                           TCP_RTO_MAX);

     }

     return true;

 }

 int tcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, int nonblock,

         int flags, int *addr_len)

 {

        do {

             if (!sysctl_tcp_low_latency && tp->ucopy.task == user_recv) {

             /* Install new reader */

             if (!user_recv && !(flags & (MSG_TRUNC | MSG_PEEK))) {

                 user_recv = current;

                                 /* 设置task */

                 tp->ucopy.task = user_recv;

                 tp->ucopy.msg = msg;

             }

           } while (len > );

         /*...*/

     if (user_recv) {

         if (!skb_queue_empty(&tp->ucopy.prequeue)) {

             int chunk;

             tp->ucopy.len = copied >  ? len : ;

             tcp_prequeue_process(sk);

             if (copied >  && (chunk = len - tp->ucopy.len) != ) {

                 NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPDIRECTCOPYFROMPREQUEUE, chunk);

                 len -= chunk;

                 copied += chunk;

             }

         }

                 /* 清除task */

         tp->ucopy.task = NULL;

         tp->ucopy.len = ;

     }

 }