Received-SPF: pass (google.com: domain of linux-kernel+bounces-145473-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:4601:e00::3 as permitted sender) client-ip=2604:1380:4601:e00::3;
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: <e0cf6827-06c2-4212-848c-10d275c75546@app.fastmail.com>
In-Reply-To: <875xwjcqpl.fsf@mail.lhotse>
References: <20240410153212.127477-1-adrian.hunter@intel.com>
 <87be83da-6102-483d-b1dc-a77eecc9f780@app.fastmail.com>
 <c9f382b2-cd96-4ee3-ad68-95381d9e09c0@intel.com>
 <a434248a-1e9f-4f4f-8f90-d36d8e979f53@csgroup.eu>
 <ff9d7032-a3b6-4ecd-ac26-d7d4a06a5c7f@csgroup.eu>
 <4d429a10-eb45-4262-8e74-69af810ef1ac@intel.com>
 <dd6653b2-3a88-4b95-af13-c6fda5b27b39@app.fastmail.com>
 <875xwjcqpl.fsf@mail.lhotse>
Date: Mon, 15 Apr 2024 17:35:30 +0200
From: "Arnd Bergmann" <arnd@arndb.de>
To: "Michael Ellerman" <mpe@ellerman.id.au>,
 "Adrian Hunter" <adrian.hunter@intel.com>,
 "Christophe Leroy" <christophe.leroy@csgroup.eu>
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" <linux-s390@vger.kernel.org>,
 "Naresh Kamboju" <naresh.kamboju@linaro.org>,
 "x86@kernel.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" <linux-kernel@vger.kernel.org>,
 "Sven Schnelle" <svens@linux.ibm.com>,
 "linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH] bug: Fix no-return-statement warning with !CONFIG_BUG
Content-Type: text/plain

On Mon, Apr 15, 2024, at 04:19, Michael Ellerman wrote:
> "Arnd Bergmann" <arnd@arndb.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