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Date: Thu, 11 Apr 2024 09:56:02 +0200
From: "Arnd Bergmann" <arnd@arndb.de>
To: "Adrian Hunter" <adrian.hunter@intel.com>
Cc: "Peter Zijlstra" <peterz@infradead.org>,
 "Dave Hansen" <dave.hansen@linux.intel.com>,
 "John Stultz" <jstultz@google.com>, "H. Peter Anvin" <hpa@zytor.com>,
 "Alexander Gordeev" <agordeev@linux.ibm.com>,
 "Vincenzo Frascino" <vincenzo.frascino@arm.com>,
 linux-s390@vger.kernel.org, "Naresh Kamboju" <naresh.kamboju@linaro.org>,
 x86@kernel.org, "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>,
 "Ingo Molnar" <mingo@redhat.com>,
 "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>,
 "Christian Borntraeger" <borntraeger@linux.ibm.com>,
 "Vasily Gorbik" <gor@linux.ibm.com>,
 "Heiko Carstens" <hca@linux.ibm.com>,
 "Nicholas Piggin" <npiggin@gmail.com>, "Borislav Petkov" <bp@alien8.de>,
 "Andy Lutomirski" <luto@kernel.org>,
 "Bjorn Helgaas" <bhelgaas@google.com>,
 "Thomas Gleixner" <tglx@linutronix.de>,
 "Anna-Maria Gleixner" <anna-maria@linutronix.de>,
 "Stephen Boyd" <sboyd@kernel.org>,
 "Randy Dunlap" <rdunlap@infradead.org>, linux-kernel@vger.kernel.org,
 "Sven Schnelle" <svens@linux.ibm.com>, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH] bug: Fix no-return-statement warning with !CONFIG_BUG
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On Thu, Apr 11, 2024, at 09:16, Adrian Hunter wrote:
> On 11/04/24 10:04, Arnd Bergmann wrote:
>> On Wed, Apr 10, 2024, at 17:32, Adrian Hunter wrote:
>>> BUG() does not return, and arch implementations of BUG() use unreach=
able()
>>> 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 bu=
ild
>>> error:
>>>
>>>   kernel/time/timekeeping.c: In function =E2=80=98timekeeping_debug_=
get_ns=E2=80=99:
>>>   kernel/time/timekeeping.c:286:1: error: no return statement in fun=
ction
>>>   returning non-void [-Werror=3Dreturn-type]
>>>
>>> Add unreachable() to default !CONFIG_BUG BUG() implementation.
>>=20
>> 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.
>>=20
>> 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.
>>=20
>> Do you know which compiler version show the warning above?
>
> Original report has a list
>

It looks like it's all versions of gcc, though no versions
of clang show the warnings. I did a few more tests and could
not find any differences on actual code generation, but
I'd still feel more comfortable changing the caller than
the BUG() macro. It's trivial to add a 'return 0' there.

Another interesting observation is that clang-11 and earlier
versions end up skipping the endless loop, both with and
without the __builtin_unreachable, see
https://godbolt.org/z/aqa9zqz8x

clang-12 and above do work like gcc, so I guess that is good.

     Arnd