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

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Date: Thu, 11 Apr 2024 09:04:50 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Adrian Hunter" <adrian.hunter@...el.com>,
 "Thomas Gleixner" <tglx@...utronix.de>
Cc: "Naresh Kamboju" <naresh.kamboju@...aro.org>,
 "Michael Ellerman" <mpe@...erman.id.au>,
 "Nicholas Piggin" <npiggin@...il.com>,
 "Christophe Leroy" <christophe.leroy@...roup.eu>,
 "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
 "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
 "Heiko Carstens" <hca@...ux.ibm.com>,
 "Vasily Gorbik" <gor@...ux.ibm.com>,
 "Alexander Gordeev" <agordeev@...ux.ibm.com>,
 "Christian Borntraeger" <borntraeger@...ux.ibm.com>,
 "Sven Schnelle" <svens@...ux.ibm.com>, "Ingo Molnar" <mingo@...hat.com>,
 "Borislav Petkov" <bp@...en8.de>,
 "Dave Hansen" <dave.hansen@...ux.intel.com>, x86@...nel.org,
 "H. Peter Anvin" <hpa@...or.com>, "Andy Lutomirski" <luto@...nel.org>,
 "Vincenzo Frascino" <vincenzo.frascino@....com>,
 "John Stultz" <jstultz@...gle.com>, "Stephen Boyd" <sboyd@...nel.org>,
 "Peter Zijlstra" <peterz@...radead.org>,
 "Randy Dunlap" <rdunlap@...radead.org>,
 "Bjorn Helgaas" <bhelgaas@...gle.com>,
 "Anna-Maria Gleixner" <anna-maria@...utronix.de>,
 linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
 linux-s390@...r.kernel.org
Subject: Re: [PATCH] bug: Fix no-return-statement warning with !CONFIG_BUG

On Wed, Apr 10, 2024, at 17:32, Adrian Hunter wrote:
> BUG() does not return, and arch implementations of BUG() use unreachable()
> or other non-returning code. However with !CONFIG_BUG, the default
> implementation is often used instead, and that does not do that. x86 always
> uses its own implementation, but powerpc with !CONFIG_BUG gives a build
> error:
>
>   kernel/time/timekeeping.c: In function ‘timekeeping_debug_get_ns’:
>   kernel/time/timekeeping.c:286:1: error: no return statement in function
>   returning non-void [-Werror=return-type]
>
> Add unreachable() to default !CONFIG_BUG BUG() implementation.

I'm a bit worried about this patch, since we have had problems
with unreachable() inside of BUG() in the past, and as far as I
can remember, the current version was the only one that
actually did the right thing on all compilers.

One problem with an unreachable() annotation here is that if
a compiler misanalyses the endless loop, it can decide to
throw out the entire code path leading up to it and just
run into undefined behavior instead of printing a BUG()
message.

Do you know which compiler version show the warning above?

     Arnd