linux-kernel - Re: [PATCH net-next] tcp: Fix tcp_syn_flood_action() if CONFIG

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Date:   Fri, 18 Nov 2022 11:45:42 +1000
From:   Jamie Bainbridge <jamie.bainbridge@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Geert Uytterhoeven <geert+renesas@...der.be>,
        Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Chris Down <chris@...isdown.name>,
        Stephen Hemminger <stephen@...workplumber.org>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] tcp: Fix tcp_syn_flood_action() if CONFIG_IPV6=n

On Thu, 17 Nov 2022 at 08:15, Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Thu, 17 Nov 2022 08:39:43 +1100 Jamie Bainbridge wrote:
> > >         if (v6) {
> > > #ifdef v6
> > >                 expensive_call6();
> > > #endif
> > >         } else {
> > >                 expensive_call6();
> > >         }
> >
> > These should work, but I expect they cause a comparison which can't be
> > optimised out at compile time. This is probably why the first style
> > exists.
> >
> > In this SYN flood codepath optimisation doesn't matter because we're
> > doing ratelimited logging anyway. But if we're breaking with existing
> > style, then wouldn't the others also have to change to this style? I
> > haven't reviewed all the other usage to tell if they're in an oft-used
> > fastpath where such a thing might matter.
>
> I think the word style already implies subjectivity.

You are right. Looking further, there are many other ways
IF_ENABLED(CONFIG_IPV6) is used, including similar to the ways you
have suggested.

I don't mind Geert's original patch, but if you want a different
style, I like your suggestion with v4 first:

        if (v4) {
                expensive_call4();
#ifdef v6
        } else {
                expensive_call6();
#endif
        }

Jamie