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

[Pavel Begunkov]

On 1/11/22 17:11, Paolo Abeni wrote:
> On Tue, 2022-01-11 at 01:21 +0000, Pavel Begunkov wrote:
> > 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.
>
> What about the corked path in udp6_sendmsg()? I mean:

It doesn't change it for callers, so the ref stays with udp6_sendmsg() when
corking. To compensate for ip6_setup_cork() there is an explicit dst_hold()
in ip6_append_data, should be fine.

@@ -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);


I don't care much about corking perf, but might be better to implement
this "handing away" for ip6_append_data() as well to be more consistent
with ip6_make_skb().


> udp6_sendmsg(MSG_MORE) -> ip6_append_data() -> ip6_setup_cork()
>
> what if ip6_setup_cork() errors out in that path?

--
Pavel Begunkov