Re: [PATCH 1/1] bug: mark generic BUG() as unreachable

[Tom Rix]

On 6/18/21 1:20 AM, Arnd Bergmann wrote:
> On Thu, Jun 17, 2021 at 11:44 PM <trix@xxxxxxxxxx> wrote:
> > From: Tom Rix <trix@xxxxxxxxxx>
> >
> > This spurious error is reported for powerpc64, CONFIG_BUG=n
> >
> > diff --git a/include/asm-generic/bug.h b/include/asm-generic/bug.h
> > index f152b9bb916fc..b250e06d7de26 100644
> > --- a/include/asm-generic/bug.h
> > +++ b/include/asm-generic/bug.h
> > @@ -177,7 +177,10 @@ void __warn(const char *file, int line, void *caller, unsigned taint,
> >
> >   #else /* !CONFIG_BUG */
> >   #ifndef HAVE_ARCH_BUG
> > -#define BUG() do {} while (1)
> > +#define BUG() do {                                             \
> > +               do {} while (1);                                \
> > +               unreachable();                                  \
> > +       } while (0)
> >   #endif
> Please let's not go back to this version, we had good reasons to use
> the infinite loop,
> mostly to avoid undefined behavior that would lead to the compiler producing
> completely random output in code paths that lead to a BUG() statement. Those
> do cause other kinds of warnings from objtool and from other compilers.
>
> The obvious workaround here would be to add a return statement locally, but
> it may also help to figure out what exactly triggers the warning, as I don't see
> it in my randconfig builds and it may be that there is a bug elsewhere.
>
> I've tried a simple reproducer on https://godbolt.org/z/341P949bG that did not
> show this warning in any of the compilers I tried. Can you try to narrow down
> the exact compiler versions and commmand line options that produce the
> warning? https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/ has
> most of the supported gcc versions in case you need those.

Please follow the link in the cover letter to the original issue 
reported for fs/afs/dir + gcc ppc64 9.x / 10.3.1

Adding the return was the first, rejected solution.

Tom

>        Arnd