doubt about the 3-way handshake

["R.wen" <rwen2012@xxxxxxxxx>]

HI, guys :

The implement of the 3 way handshaking confuse me:

      This is what the tcp_v4_do_rcv()
 mainly do:

1.     if (sk->sk_state == TCP_LISTEN) {
2.         struct sock *nsk = tcp_v4_hnd_req(sk, skb);
3.           ...
4.         if (nsk != sk) {
5.             if (tcp_child_process(sk, nsk, skb)) {
6.               ...
7.             }
8.             return 0;
9.         }
10.     }
11.    ...
12.     if (tcp_rcv_state_process(sk, skb, skb-> h.th, skb->len)) {
13.         ...
14.     }
    

It seems that, if the server is in the LISTEN state, it execs the line 2, which is tcp_v4_hnd_req(sk, skb); 


, That is the function really confuse me:

1. static struct sock *tcp_v4_hnd_req(struct sock *sk, struct sk_buff *skb)
2. {
3.     struct tcphdr *th = skb-> h.th;
4.     struct iphdr *iph = skb->nh.iph;
5.     struct sock *nsk;
6.     struct request_sock **prev;
7.     /* Find possible connection requests. */
8.     struct request_sock *req = inet_csk_search_req(sk, &prev, th->source,
9.                                iph->saddr, iph->daddr);
10.     if (req)
11.         return tcp_check_req(sk, skb, req, prev);
12.     nsk = inet_lookup_established(&tcp_hashinfo, skb->nh.iph->saddr,
13.                       th->source, skb-> nh.iph->daddr,
14.                       th->dest, inet_iif(skb));
15.     if (nsk) {
16.         if (nsk->sk_state != TCP_TIME_WAIT) {
17.             bh_lock_sock(nsk);
18.             return nsk;
19.         }
20.         inet_twsk_put(inet_twsk(nsk));
21.         return NULL;
22.     }
23. #ifdef CONFIG_SYN_COOKIES
24.     if (!th->rst && !th->syn && th->ack)
25.         sk = cookie_v4_check(sk, skb, &(IPCB(skb)->opt));
26. #endif
27.     return sk;
28. }

Because
the server  is running just now and at the LISTEN state, so  there is
nothing in the listen queue's request_sock array. So the return value
of the Line 8 -- inet_csk_search_req() should be NULL,  and then  the "
return tcp_check_req(sk, skb, req, prev);" at Line 11 should NOT be
executed.  
IS that Right?  Could you please explain this function for me? I am very grateful.

     R.wen