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From:   Pavel Begunkov <asml.silence@gmail.com>
To:     netdev@vger.kernel.org, "David S . Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>
Cc:     Eric Dumazet <edumazet@google.com>,
        Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
        linux-kernel@vger.kernel.org, Paolo Abeni <pabeni@redhat.com>,
        Pavel Begunkov <asml.silence@gmail.com>
Subject: [PATCH net-next v2 01/10] ipv6: optimise dst refcounting on skb init
Date:   Thu, 27 Jan 2022 00:36:22 +0000
Message-Id: <647e136457fb586846933244448652547dd306f9.1643243772.git.asml.silence@gmail.com>
In-Reply-To: <cover.1643243772.git.asml.silence@gmail.com>
References: <cover.1643243772.git.asml.silence@gmail.com>
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Precedence: bulk

__ip6_make_skb() gets a cork->dst ref, hands it over to skb and shortly
after puts cork->dst. Save two atomics by stealing it without extra
referencing, ip6_cork_release() handles NULL cork->dst.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 net/ipv6/ip6_output.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 2995f8d89e7e..14d607ccfeea 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1807,6 +1807,15 @@ int ip6_append_data(struct sock *sk,
 }
 EXPORT_SYMBOL_GPL(ip6_append_data);
 
+static void ip6_cork_steal_dst(struct sk_buff *skb, struct inet_cork_full *cork)
+{
+	struct dst_entry *dst = cork->base.dst;
+
+	cork->base.dst = NULL;
+	cork->base.flags &= ~IPCORK_ALLFRAG;
+	skb_dst_set(skb, dst);
+}
+
 static void ip6_cork_release(struct inet_cork_full *cork,
 			     struct inet6_cork *v6_cork)
 {
@@ -1889,7 +1898,7 @@ struct sk_buff *__ip6_make_skb(struct sock *sk,
 
 	skb->tstamp = cork->base.transmit_time;
 
-	skb_dst_set(skb, dst_clone(&rt->dst));
+	ip6_cork_steal_dst(skb, cork);
 	IP6_UPD_PO_STATS(net, rt->rt6i_idev, IPSTATS_MIB_OUT, skb->len);
 	if (proto == IPPROTO_ICMPV6) {
 		struct inet6_dev *idev = ip6_dst_idev(skb_dst(skb));
-- 
2.34.1