tcp/dccp: do not touch listener sk_refcnt under synflood

When a SYNFLOOD targets a non SO_REUSEPORT listener, multiple
cpus contend on sk->sk_refcnt and sk->sk_wmem_alloc changes.

By letting listeners use SOCK_RCU_FREE infrastructure,
we can relax TCP_LISTEN lookup rules and avoid touching sk_refcnt

Note that we still use SLAB_DESTROY_BY_RCU rules for other sockets,
only listeners are impacted by this change.

Peak performance under SYNFLOOD is increased by ~33% :

On my test machine, I could process 3.2 Mpps instead of 2.4 Mpps

Most consuming functions are now skb_set_owner_w() and sock_wfree()
contending on sk->sk_wmem_alloc when cooking SYNACK and freeing them.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dccp/ipv6.c

@@ -642,6 +642,7 @@ discard:
 static int dccp_v6_rcv(struct sk_buff *skb)
 {
 	const struct dccp_hdr *dh;
+	bool refcounted;
 	struct sock *sk;
 	int min_cov;
 
@@ -670,7 +671,7 @@ static int dccp_v6_rcv(struct sk_buff *skb)
 lookup:
 	sk = __inet6_lookup_skb(&dccp_hashinfo, skb, __dccp_hdr_len(dh),
 			        dh->dccph_sport, dh->dccph_dport,
-				inet6_iif(skb));
+				inet6_iif(skb), &refcounted);
 	if (!sk) {
 		dccp_pr_debug("failed to look up flow ID in table and "
 			      "get corresponding socket\n");
@@ -699,6 +700,7 @@ lookup:
 			goto lookup;
 		}
 		sock_hold(sk);
+		refcounted = true;
 		nsk = dccp_check_req(sk, skb, req);
 		if (!nsk) {
 			reqsk_put(req);
@@ -752,7 +754,8 @@ discard_it:
 	return 0;
 
 discard_and_relse:
-	sock_put(sk);
+	if (refcounted)
+		sock_put(sk);
 	goto discard_it;
 }