From: Arnd Bergmann <arnd@arndb.de>
To: Tom Rix <trix@redhat.com>
Cc: linux-arch <linux-arch@vger.kernel.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/1] bug: mark generic BUG() as unreachable
Date: Fri, 18 Jun 2021 16:35:09 +0200 [thread overview]
Message-ID: <CAK8P3a29bdyw=K8fNm1_pkbrsLG3f5FeTZhNKUYhdP3cs_fO=A@mail.gmail.com> (raw)
In-Reply-To: <312e5b85-bfa5-e7f1-c1f7-a13a5d2583b8@redhat.com>
On Fri, Jun 18, 2021 at 3:43 PM Tom Rix <trix@redhat.com> wrote:
> On 6/18/21 1:20 AM, Arnd Bergmann wrote:
> > On Thu, Jun 17, 2021 at 11:44 PM <trix@redhat.com> wrote:
> >> From: Tom Rix <trix@redhat.com>
> >>
> >> 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.
Ok, I was able to reproduce it now and have a better idea of what is going on.
I also misread your patch (sorry about that), and missed that you keep the
"do {} while (1)" loop ahead of the unreachable(), and that should address
all the concerns I had. I would still try to find the old email thread about
the change, just to make sure we don't already know about other problems
with your version.
Arnd
prev parent reply other threads:[~2021-06-18 14:37 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-17 21:43 [PATCH 0/1] bug: mark generic BUG() as unreachable trix
2021-06-17 21:43 ` [PATCH 1/1] " trix
2021-06-18 8:20 ` Arnd Bergmann
2021-06-18 13:43 ` Tom Rix
2021-06-18 14:35 ` Arnd Bergmann [this message]
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