linux-kernel - Re: [PATCH] bug: Fix no-return-statement warning with !CONFIG

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Message-Id: <e0cf6827-06c2-4212-848c-10d275c75546@app.fastmail.com>
Date: Mon, 15 Apr 2024 17:35:30 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Michael Ellerman" <mpe@...erman.id.au>,
 "Adrian Hunter" <adrian.hunter@...el.com>,
 "Christophe Leroy" <christophe.leroy@...roup.eu>
Cc: "Peter Zijlstra" <peterz@...radead.org>,
 "Dave Hansen" <dave.hansen@...ux.intel.com>,
 "John Stultz" <jstultz@...gle.com>, "H. Peter Anvin" <hpa@...or.com>,
 "Alexander Gordeev" <agordeev@...ux.ibm.com>,
 "Vincenzo Frascino" <vincenzo.frascino@....com>,
 "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
 "Naresh Kamboju" <naresh.kamboju@...aro.org>,
 "x86@...nel.org" <x86@...nel.org>,
 "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
 "Ingo Molnar" <mingo@...hat.com>,
 "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
 "Christian Borntraeger" <borntraeger@...ux.ibm.com>,
 "Vasily Gorbik" <gor@...ux.ibm.com>,
 "Heiko Carstens" <hca@...ux.ibm.com>,
 "Nicholas Piggin" <npiggin@...il.com>, "Borislav Petkov" <bp@...en8.de>,
 "Andy Lutomirski" <luto@...nel.org>,
 "Bjorn Helgaas" <bhelgaas@...gle.com>,
 "Thomas Gleixner" <tglx@...utronix.de>,
 "Anna-Maria Gleixner" <anna-maria@...utronix.de>,
 "Stephen Boyd" <sboyd@...nel.org>,
 "Randy Dunlap" <rdunlap@...radead.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "Sven Schnelle" <svens@...ux.ibm.com>,
 "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH] bug: Fix no-return-statement warning with !CONFIG_BUG

On Mon, Apr 15, 2024, at 04:19, Michael Ellerman wrote:
> "Arnd Bergmann" <arnd@...db.de> writes:
>> On Thu, Apr 11, 2024, at 11:27, Adrian Hunter wrote:
>>> On 11/04/24 11:22, Christophe Leroy wrote:
>>>
>>> That is fragile because it depends on defined(__OPTIMIZE__),
>>> so it should still be:
>>
>> If there is a function that is defined but that must never be
>> called, I think we are doing something wrong.
>
> It's a pretty inevitable result of using IS_ENABLED(), which the docs
> encourage people to use.

Using IS_ENABLED() is usually a good idea, as it helps avoid
adding extra #ifdef checks and just drops static functions as
dead code, or lets you call extern functions that are conditionally
defined in a different file.

The thing is that here it does not do either of those and
adds more complexity than it avoids.

> In this case it could easily be turned into a build error by just making
> it an extern rather than a static inline.
>
> But I think Christophe's solution is actually better, because it's more
> explicit, ie. this function should not be called and if it is that's a
> build time error.

I haven't seen a good solution here. Ideally we'd just define
the functions unconditionally and have IS_ENABLED() take care
of letting the compiler drop them silently, but that doesn't
build because of missing struct members.

I won't object to either an 'extern' declaration or the
'BUILD_BUG_ON()' if you and others prefer that, both are better
than BUG() here. I still think my suggestion would be a little
simpler.

     Arnd