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
Date: Thu, 11 Apr 2024 09:56:02 +0200 [thread overview]
Message-ID: <cac3f357-0dc2-46ba-9ea0-7b1f4278e8ff@app.fastmail.com> (raw)
In-Reply-To: <c9f382b2-cd96-4ee3-ad68-95381d9e09c0@intel.com>
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 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?
>
> 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
next prev parent reply other threads:[~2024-04-11 7:56 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-10 15:32 [PATCH] bug: Fix no-return-statement warning with !CONFIG_BUG Adrian Hunter
2024-04-10 15:32 ` Adrian Hunter
2024-04-10 17:02 ` Naresh Kamboju
2024-04-10 17:02 ` Naresh Kamboju
2024-04-10 20:05 ` [tip: timers/urgent] " tip-bot2 for Adrian Hunter
2024-04-11 7:04 ` [PATCH] " Arnd Bergmann
2024-04-11 7:04 ` Arnd Bergmann
2024-04-11 7:16 ` Adrian Hunter
2024-04-11 7:16 ` Adrian Hunter
2024-04-11 7:56 ` Arnd Bergmann [this message]
2024-04-11 9:03 ` Adrian Hunter
2024-04-11 10:27 ` David Laight
2024-04-11 8:13 ` Christophe Leroy
2024-04-11 8:13 ` Christophe Leroy
2024-04-11 8:22 ` Christophe Leroy
2024-04-11 8:22 ` Christophe Leroy
2024-04-11 9:27 ` Adrian Hunter
2024-04-11 9:27 ` Adrian Hunter
2024-04-11 11:26 ` Arnd Bergmann
2024-04-11 11:26 ` Arnd Bergmann
2024-04-15 2:19 ` Michael Ellerman
2024-04-15 15:35 ` Arnd Bergmann
2024-04-15 15:35 ` Arnd Bergmann
2024-04-15 17:07 ` Christophe Leroy
2024-04-15 17:32 ` Arnd Bergmann
2024-04-15 17:32 ` Arnd Bergmann
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