linux-kernel - Re: [PATCH] asm-generic: bug.h: add unreachable() in BUG() for CONFIG

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Date:   Sun, 17 Oct 2021 12:17:32 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>
Subject: Re: [PATCH] asm-generic: bug.h: add unreachable() in BUG() for
 CONFIG_BUG not set

On 10/17/21 12:09 PM, Arnd Bergmann wrote:
> On Sun, Oct 17, 2021 at 7:49 PM Randy Dunlap <rdunlap@...radead.org> wrote:
>>
>> When CONFIG_BUG is not set/enabled, there is a warning
>> on ARCH=m68k, gcc version 11.1.0-nolibc from Arnd's crosstools:
>>
>> ../fs/afs/dir.c: In function 'afs_dir_set_page_dirty':
>> ../fs/afs/dir.c:51:1: error: no return statement in function returning non-void [-Werror=return-type]
>>
>> Adding "unreachable()" in the BUG() macro silences the warning.
> 
> No, I don't think this is the right solution:
> 
>> -#define BUG() do {} while (1)
>> +#define BUG() do {unreachable();} while (1)
> 
> Marking this code unreachable() means the compiler is free
> to assume any code path leading here will never be entered,
> which leads to additional undefined behavior and other warnings
> rather than just hanging reproducibly.
> 
> The endless loop here should normally be sufficient to tell the
> compiler that the function never returns, so it sounds like a
> problem in gcc for m68k.

Sounds likely.

> Did you see any other issues like this one on m68k, or the
> same one on another architecture?

No and no.

thanks.
-- 
~Randy