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SubjectRe: Kconfig, DEFAULT_NETSCH, and shooting yourself in the foot..
On Sun, 3 Jan 2021 15:30:30 +0900
Masahiro Yamada <masahiroy@kernel.org> wrote:

> On Sun, Jan 3, 2021 at 5:14 AM Valdis Klētnieks <valdis.kletnieks@vt.edu> wrote:
> >
> > Consider the following own goal I just discovered I scored:
> >
> > [~] zgrep -i fq_codel /proc/config.gz
> > CONFIG_NET_SCH_FQ_CODEL=m
> > CONFIG_DEFAULT_FQ_CODEL=y
> > CONFIG_DEFAULT_NET_SCH="fq_codel"
> >
> > Obviously, fq_codel didn't get set as the default, because that happens
> > before the module gets loaded (which may never happen if the sysadmin
> > thinks the DEFAULT_NET_SCH already made it happen)
> >
> > Whoops. My bad, probably - but....
> >
> > The deeper question, part 1:
> >
> > There's this chunk in net/sched/Kconfig:
> >
> > config DEFAULT_NET_SCH
> > string
> > default "pfifo_fast" if DEFAULT_PFIFO_FAST
> > default "fq" if DEFAULT_FQ
> > default "fq_codel" if DEFAULT_FQ_CODEL
> > default "fq_pie" if DEFAULT_FQ_PIE
> > default "sfq" if DEFAULT_SFQ
> > default "pfifo_fast"
> > endif
> >
> > (And a similar chunk right above it with a similar issue)
> >
> > Should those be "if (foo=y)" so =m can't be chosen? (I'll be
> > happy to write the patch if that's what we want)
> >
> > Deeper question, part 2:
> >
> > Should there be a way in the Kconfig language to ensure that
> > these two chunks can't accidentally get out of sync? There's other
> > places in the kernel where similar issues arise - a few days ago I was
> > chasing a CPU governor issue where it looked like it was possible
> > to set a default that was a module and thus possibly not actually loaded.
> >
>
>
> If there is a restriction where a modular discipline cannot be the default,
> I think you can add 'depends on FOO = y'.
>
>
>
> For example,
>
>
> choice
> prompt "Default"
>
> config DEFAULT_FOO
> bool "Use foo for default"
> depends on FOO = y
>
> config DEFAULT_BAR
> bool "Use bar for default"
> depends on BAR = y
>
> config DEFAULT_FALLBACK
> bool "fallback when nothing else is builtin"
>
> endchoice
>
>
>
>
>

You can use a qdisc that is a module, it just has to be available when device
is loaded. Typically that means putting it in initramfs.

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