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

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Date:   Wed, 12 Jan 2022 12:15:01 +0100
From:   Paolo Abeni <pabeni@...hat.com>
To:     Pavel Begunkov <asml.silence@...il.com>, netdev@...r.kernel.org,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 09/14] ipv6: hand dst refs to cork setup

On Tue, 2022-01-11 at 20:39 +0000, Pavel Begunkov wrote:
> 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.

Whoops, I underlooked that chunk, thanks for pointing it out!

Yes, it looks 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().

I'm personally fine with the the added dst_hold() in ip6_append_data()

Thanks!

Paolo