Received-SPF: pass (google.com: domain of linux-kernel+bounces-139913-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.199.223 as permitted sender) client-ip=147.75.199.223;
Feedback-ID: i56a14606:Fastmail
User-Agent: Cyrus-JMAP/3.11.0-alpha0-379-gabd37849b7-fm-20240408.001-gabd37849
Precedence: bulk
MIME-Version: 1.0
Message-Id: <87be83da-6102-483d-b1dc-a77eecc9f780@app.fastmail.com>
In-Reply-To: <20240410153212.127477-1-adrian.hunter@intel.com>
References: <20240410153212.127477-1-adrian.hunter@intel.com>
Date: Thu, 11 Apr 2024 09:04:50 +0200
From: "Arnd Bergmann" <arnd@arndb.de>
To: "Adrian Hunter" <adrian.hunter@intel.com>,
 "Thomas Gleixner" <tglx@linutronix.de>
Cc: "Naresh Kamboju" <naresh.kamboju@linaro.org>,
 "Michael Ellerman" <mpe@ellerman.id.au>,
 "Nicholas Piggin" <npiggin@gmail.com>,
 "Christophe Leroy" <christophe.leroy@csgroup.eu>,
 "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>,
 "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>,
 "Heiko Carstens" <hca@linux.ibm.com>,
 "Vasily Gorbik" <gor@linux.ibm.com>,
 "Alexander Gordeev" <agordeev@linux.ibm.com>,
 "Christian Borntraeger" <borntraeger@linux.ibm.com>,
 "Sven Schnelle" <svens@linux.ibm.com>, "Ingo Molnar" <mingo@redhat.com>,
 "Borislav Petkov" <bp@alien8.de>,
 "Dave Hansen" <dave.hansen@linux.intel.com>, x86@kernel.org,
 "H. Peter Anvin" <hpa@zytor.com>, "Andy Lutomirski" <luto@kernel.org>,
 "Vincenzo Frascino" <vincenzo.frascino@arm.com>,
 "John Stultz" <jstultz@google.com>, "Stephen Boyd" <sboyd@kernel.org>,
 "Peter Zijlstra" <peterz@infradead.org>,
 "Randy Dunlap" <rdunlap@infradead.org>,
 "Bjorn Helgaas" <bhelgaas@google.com>,
 "Anna-Maria Gleixner" <anna-maria@linutronix.de>,
 linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
 linux-s390@vger.kernel.org
Subject: Re: [PATCH] bug: Fix no-return-statement warning with !CONFIG_BUG
Content-Type: text/plain;charset=utf-8
Content-Transfer-Encoding: quoted-printable

On Wed, Apr 10, 2024, at 17:32, Adrian Hunter wrote:
> BUG() does not return, and arch implementations of BUG() use unreachab=
le()
> or other non-returning code. However with !CONFIG_BUG, the default
> implementation is often used instead, and that does not do that. x86 a=
lways
> uses its own implementation, but powerpc with !CONFIG_BUG gives a build
> error:
>
>   kernel/time/timekeeping.c: In function =E2=80=98timekeeping_debug_ge=
t_ns=E2=80=99:
>   kernel/time/timekeeping.c:286:1: error: no return statement in funct=
ion
>   returning non-void [-Werror=3Dreturn-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