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Subject[PATCH net-next v2 09/10] ipv6: optimise dst refcounting on cork init
Date
udpv6_sendmsg() doesn't need dst after calling ip6_make_skb(), so
instead of taking an additional reference inside ip6_setup_cork()
and releasing the initial one afterwards, we can hand over a reference
into ip6_make_skb() saving two atomics. The only other user of
ip6_setup_cork() is ip6_append_data() and it requires an extra
dst_hold().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
net/ipv6/ip6_output.c | 13 +++++++++----
net/ipv6/udp.c | 3 ++-
2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 0cc490f2cfbf..0c6c971ce0a5 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -1356,6 +1356,11 @@ static int ip6_setup_cork(struct sock *sk, struct inet_cork_full *cork,
unsigned int mtu;
struct ipv6_txoptions *nopt, *opt = ipc6->opt;

+ /* callers pass dst together with a reference, set it first so
+ * ip6_cork_release() can put it down even in case of an error.
+ */
+ cork->base.dst = &rt->dst;
+
/*
* setup for corking
*/
@@ -1389,8 +1394,6 @@ 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 +1787,7 @@ 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)
@@ -1974,15 +1978,16 @@ 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);

cork->base.flags = 0;
cork->base.addr = 0;
cork->base.opt = NULL;
- cork->base.dst = NULL;
v6_cork.opt = NULL;
err = ip6_setup_cork(sk, cork, &v6_cork, ipc6, rt);
if (err) {
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index cfcf08c3df4d..c6872596b408 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
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