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

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Message-ID: <CAMuHMdVQdax10pAgNBbAVDXgVVTAQC93GR1f_4DuKfdAXngNMA@mail.gmail.com>
Date:   Fri, 18 Nov 2022 09:29:13 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Jamie Bainbridge <jamie.bainbridge@...il.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        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

Hi Jamie,

On Fri, Nov 18, 2022 at 2:50 AM Jamie Bainbridge
<jamie.bainbridge@...il.com> wrote:
> 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
>         }

IMHO this is worse, as the #ifdef/#endif is spread across the two branches
of an if-conditional.

Hence this is usually written as:

            if (cond1) {
                    expensive_call1();
            }
    #ifdef cond2_enabled
           else {
                    expensive_call1();
            }
    #endif

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds