[PATCH net-next v8 08/11] net: tunnel: convert ip_tunnel_xmit to use a noref dst when possible

[Marek Mietus <mmietus97@yahoo.com>]

ip_tunnel_xmit unnecessarily references the dst_entry from the
dst_cache when interacting with the cache.

Reduce this overhead by avoiding the redundant refcount increments.

When using the cache, the found dst is either taken noref from the
cache or gets stolen into it. To reconcile both cache and the no cache
dst lookup flows to use a noref dst, we drop the found dst immediately
upon lookup in the no cache route lookup case.

This change is safe since ipv4 supports noref xmit under RCU which is
already the case for ip_tunnel_xmit.

Signed-off-by: Marek Mietus <mmietus97@yahoo.com>
---
 net/ipv4/ip_tunnel.c | 22 ++++++++++------------
 1 file changed, 10 insertions(+), 12 deletions(-)

diff --git a/net/ipv4/ip_tunnel.c b/net/ipv4/ip_tunnel.c
index d0e8fb7f040b..ec5d5bb74aae 100644
--- a/net/ipv4/ip_tunnel.c
+++ b/net/ipv4/ip_tunnel.c
@@ -769,11 +769,11 @@ void ip_tunnel_xmit(struct sk_buff *skb, struct net_device *dev,
 	if (connected && md) {
 		use_cache = ip_tunnel_dst_cache_usable(skb, tun_info);
 		if (use_cache)
-			rt = dst_cache_get_ip4(&tun_info->dst_cache,
-					       &fl4.saddr);
+			rt = dst_cache_get_ip4_rcu(&tun_info->dst_cache,
+						   &fl4.saddr);
 	} else {
-		rt = connected ? dst_cache_get_ip4(&tunnel->dst_cache,
-						&fl4.saddr) : NULL;
+		rt = connected ? dst_cache_get_ip4_rcu(&tunnel->dst_cache,
+						       &fl4.saddr) : NULL;
 	}
 
 	if (!rt) {
@@ -784,15 +784,16 @@ void ip_tunnel_xmit(struct sk_buff *skb, struct net_device *dev,
 			goto tx_error;
 		}
 		if (use_cache)
-			dst_cache_set_ip4(&tun_info->dst_cache, &rt->dst,
-					  fl4.saddr);
+			dst_cache_steal_ip4(&tun_info->dst_cache, &rt->dst,
+					    fl4.saddr);
 		else if (!md && connected)
-			dst_cache_set_ip4(&tunnel->dst_cache, &rt->dst,
-					  fl4.saddr);
+			dst_cache_steal_ip4(&tunnel->dst_cache, &rt->dst,
+					    fl4.saddr);
+		else
+			ip_rt_put(rt);
 	}
 
 	if (rt->dst.dev == dev) {
-		ip_rt_put(rt);
 		DEV_STATS_INC(dev, collisions);
 		goto tx_error;
 	}
@@ -802,7 +803,6 @@ void ip_tunnel_xmit(struct sk_buff *skb, struct net_device *dev,
 		df |= (inner_iph->frag_off & htons(IP_DF));
 
 	if (tnl_update_pmtu(dev, skb, rt, df, inner_iph, 0, 0, false)) {
-		ip_rt_put(rt);
 		goto tx_error;
 	}
 
@@ -833,7 +833,6 @@ void ip_tunnel_xmit(struct sk_buff *skb, struct net_device *dev,
 			+ rt->dst.header_len + ip_encap_hlen(&tunnel->encap);
 
 	if (skb_cow_head(skb, max_headroom)) {
-		ip_rt_put(rt);
 		DEV_STATS_INC(dev, tx_dropped);
 		kfree_skb(skb);
 		return;
@@ -843,7 +842,6 @@ void ip_tunnel_xmit(struct sk_buff *skb, struct net_device *dev,
 
 	iptunnel_xmit(NULL, rt, skb, fl4.saddr, fl4.daddr, protocol, tos, ttl,
 		      df, !net_eq(tunnel->net, dev_net(dev)), 0);
-	ip_rt_put(rt);
 	return;
 
 #if IS_ENABLED(CONFIG_IPV6)
-- 
2.51.0