[PATCH 09/14] ipv6: hand dst refs to cork setup

From: Pavel Begunkov
Date: Mon Jan 10 2022 - 20:25:44 EST


During cork->dst setup, ip6_make_skb() gets an additional reference to
a passed in dst. However, udpv6_sendmsg() doesn't need dst after calling
ip6_make_skb(), and so we can save two additional atomics by passing
dst references to ip6_make_skb(). udpv6_sendmsg() is the only caller, so
it's enough to make sure it doesn't use dst afterwards.

Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
---
net/ipv6/ip6_output.c | 9 ++++++---
net/ipv6/udp.c | 3 ++-
2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 0cc490f2cfbf..6a7bba4dd04d 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1356,6 +1356,8 @@ static int ip6_setup_cork(struct sock *sk, struct inet_cork_full *cork,
unsigned int mtu;
struct ipv6_txoptions *nopt, *opt = ipc6->opt;

+ cork->base.dst = &rt->dst;
+
/*
* setup for corking
*/
@@ -1389,8 +1391,6 @@ static int ip6_setup_cork(struct sock *sk, struct inet_cork_full *cork,

/* need source address above miyazawa*/
}
- dst_hold(&rt->dst);
- cork->base.dst = &rt->dst;
v6_cork->hop_limit = ipc6->hlimit;
v6_cork->tclass = ipc6->tclass;
if (rt->dst.flags & DST_XFRM_TUNNEL)
@@ -1784,6 +1784,7 @@ int ip6_append_data(struct sock *sk,
/*
* setup for corking
*/
+ dst_hold(&rt->dst);
err = ip6_setup_cork(sk, &inet->cork, &np->cork,
ipc6, rt);
if (err)
@@ -1974,8 +1975,10 @@ struct sk_buff *ip6_make_skb(struct sock *sk,
int exthdrlen = (ipc6->opt ? ipc6->opt->opt_flen : 0);
int err;

- if (flags & MSG_PROBE)
+ if (flags & MSG_PROBE) {
+ dst_release(&rt->dst);
return NULL;
+ }

__skb_queue_head_init(&queue);

diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index eec83e34ae27..3039dff7fe64 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -1541,7 +1541,8 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
err = PTR_ERR(skb);
if (!IS_ERR_OR_NULL(skb))
err = udp_v6_send_skb(skb, fl6, &cork.base);
- goto out;
+ /* ip6_make_skb steals dst reference */
+ goto out_no_dst;
}

lock_sock(sk);
--
2.34.1