linux-kernel - Re: [PATCH v1] driver core: Silence 'unused-but-set variable' warning

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Message-ID: <Y6Xl3M7Tx+c2SCa9@dev-arch.thelio-3990X>
Date:   Fri, 23 Dec 2022 10:31:08 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Ammar Faizi <ammarfaizi2@...weeb.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>,
        LLVM Mailing List <llvm@...ts.linux.dev>
Subject: Re: [PATCH v1] driver core: Silence 'unused-but-set variable' warning

On Fri, Dec 23, 2022 at 10:31:57PM +0700, Ammar Faizi wrote:
> On 12/23/22 10:02 PM, Greg Kroah-Hartman wrote:
> > Ick, no, that's horrid and is NOT ok for kernel code, sorry.
> > 
> > Please fix the compiler, this is not a "fix" in any sense of the word
> > and is not going to work at all for kernel code.

It is possible that the compiler should not emit
-Wunused-but-set-variable when the variable is assigned the return
value of a function marked with __attribute__((warn_unused_result)) but
neither compiler does that today, you'll see the exact same warning from
GCC 12.2.0:

  drivers/base/module.c: In function ‘module_add_driver’:
  drivers/base/module.c:36:13: error: variable ‘no_warn’ set but not used [-Werror=unused-but-set-variable]
     36 |         int no_warn;
        |             ^~~~~~~
  cc1: all warnings being treated as errors

This has come up before too:

https://lore.kernel.org/20210726201924.3202278-2-morbo@google.com/

> Agree.
> 
> Sorry for the noise. It turned out I messed up my clang compiler flags.
> I forgot to do a "git reset --hard" before recompiling.
> 
> This has nothing todo with the upstream kernel.

This warning is in W=1, which are warnings that we want to turn on at
some point but the current instances need to be fixed first, so I would
say this is still relevant to the upstream kernel.

Cheers,
Nathan