linux-kernel - Kconfig, DEFAULT_NETSCH, and shooting yourself in the foot..

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Message-ID: <16871.1609618487@turing-police>
Date:   Sat, 02 Jan 2021 15:14:47 -0500
From:   "Valdis Klētnieks" <valdis.kletnieks@...edu>
To:     Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        "David S. Miller" <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Kconfig, DEFAULT_NETSCH, and shooting yourself in the foot..

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.

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