tcp: tcp_make_synack() consumes dst parameter

tcp_make_synack() clones the dst, and callers release it.

We can avoid two atomic operations per SYNACK if tcp_make_synack()
consumes dst instead of cloning it.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_ipv4.c

@@ -848,7 +848,6 @@ static int tcp_v4_send_synack(struct sock *sk, struct dst_entry *dst,
 		err = net_xmit_eval(err);
 	}
 
-	dst_release(dst);
 	return err;
 }
 

net/ipv4/tcp_output.c

@@ -2442,7 +2442,16 @@ int tcp_send_synack(struct sock *sk)
 	return tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
 }
 
-/* Prepare a SYN-ACK. */
+/**
+ * tcp_make_synack - Prepare a SYN-ACK.
+ * sk: listener socket
+ * dst: dst entry attached to the SYNACK
+ * req: request_sock pointer
+ * rvp: request_values pointer
+ *
+ * Allocate one skb and build a SYNACK packet.
+ * @dst is consumed : Caller should not use it again.
+ */
 struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
 				struct request_sock *req,
 				struct request_values *rvp)
@@ -2462,13 +2471,14 @@ struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst,
 	if (cvp != NULL && cvp->s_data_constant && cvp->s_data_desired)
 		s_data_desired = cvp->s_data_desired;
 	skb = alloc_skb(MAX_TCP_HEADER + 15 + s_data_desired, GFP_ATOMIC);
-	if (skb == NULL)
+	if (unlikely(!skb)) {
+		dst_release(dst);
 		return NULL;
-
+	}
 	/* Reserve space for headers. */
 	skb_reserve(skb, MAX_TCP_HEADER);
 
-	skb_dst_set(skb, dst_clone(dst));
+	skb_dst_set(skb, dst);
 
 	mss = dst_metric_advmss(dst);
 	if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < mss)

net/ipv6/tcp_ipv6.c

@@ -522,7 +522,6 @@ static int tcp_v6_send_synack(struct sock *sk, struct request_sock *req,
 done:
 	if (opt && opt != np->opt)
 		sock_kfree_s(sk, opt, opt->tot_len);
-	dst_release(dst);
 	return err;
 }