net: No dst refcounting in ip_queue_xmit()

TCP outgoing packets can avoid two atomic ops, and dirtying
of previously higly contended cache line using new refdst
infrastructure.

Note 1: loopback device excluded because of !IFF_XMIT_DST_RELEASE
Note 2: UDP packets dsts are built before ip_queue_xmit().
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_output.c

@@ -318,10 +318,12 @@ int ip_queue_xmit(struct sk_buff *skb)
 	struct ip_options *opt = inet->opt;
 	struct rtable *rt;
 	struct iphdr *iph;
+	int res;
 
 	/* Skip all of this if the packet is already routed,
 	 * f.e. by something like SCTP.
 	 */
+	rcu_read_lock();
 	rt = skb_rtable(skb);
 	if (rt != NULL)
 		goto packet_routed;
@@ -359,7 +361,7 @@ int ip_queue_xmit(struct sk_buff *skb)
 		}
 		sk_setup_caps(sk, &rt->u.dst);
 	}
-	skb_dst_set(skb, dst_clone(&rt->u.dst));
+	skb_dst_set_noref(skb, &rt->u.dst);
 
 packet_routed:
 	if (opt && opt->is_strictroute && rt->rt_dst != rt->rt_gateway)
@@ -391,9 +393,12 @@ int ip_queue_xmit(struct sk_buff *skb)
 	skb->priority = sk->sk_priority;
 	skb->mark = sk->sk_mark;
 
-	return ip_local_out(skb);
+	res = ip_local_out(skb);
+	rcu_read_unlock();
+	return res;
 
 no_route:
+	rcu_read_unlock();
 	IP_INC_STATS(sock_net(sk), IPSTATS_MIB_OUTNOROUTES);
 	kfree_skb(skb);
 	return -EHOSTUNREACH;