linux-kernel - Re: [RFC PATCH 1/8] compiler/gcc: Emit build-time warning for GCC prior to version 4.8

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Date:   Tue, 14 Jan 2020 00:35:35 +0900
From:   Masahiro Yamada <masahiroy@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Joe Perches <joe@...ches.com>, Will Deacon <will@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Android Kernel Team <kernel-team@...roid.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>
Subject: Re: [RFC PATCH 1/8] compiler/gcc: Emit build-time warning for GCC
 prior to version 4.8

On Mon, Jan 13, 2020 at 11:40 PM Arnd Bergmann <arnd@...db.de> wrote:
>
> On Fri, Jan 10, 2020 at 6:54 PM Joe Perches <joe@...ches.com> wrote:
> >
> > On Fri, 2020-01-10 at 18:35 +0100, Arnd Bergmann wrote:
> > > On Fri, Jan 10, 2020 at 5:56 PM Will Deacon <will@...nel.org> wrote:
> > > > Prior to version 4.8, GCC may miscompile READ_ONCE() by erroneously
> > > > discarding the 'volatile' qualifier:
> > > >
> > > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145
> > > >
> > > > We've been working around this using some nasty hacks which make
> > > > READ_ONCE() both horribly complicated and also prevent us from enforcing
> > > > that it is only used on scalar types. Since GCC 4.8 is pretty old for
> > > > kernel builds now, emit a warning if we detect it during the build.
> > >
> > > No objection to recommending gcc-4.8, but I think this should either
> > > just warn once during the kernel build instead of for every file, or
> > > it should become a hard requirement.
> >
> > It might as well be a hard requirement as
> > gcc 4.8.0 is already nearly 7 years old.
> >
> > gcc 4.6.0 released 2011-03-25
> > gcc 4.8.0 released 2013-03-22
> >
> > Perhaps there are exceedingly few to zero new
> > instances using gcc compiler versions < 4.8
>
> The last time we had this discussion, the result was that gcc-4.8 is
> probably ok as a minimum version, but moving to 5.1+ (from 2015)
> was not an obvious choice:
>
> https://www.spinics.net/lists/linux-kbuild/msg23648.html
>
> If nobody complains about the move to 4.8, we can try moving to
> gcc-5.1 and GNU99/GNU11 next year  ;-)
>
>        Arnd


I agree.

"Your compiler is old and may miscompile the kernel due to
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145 - please upgrade it."
sounds like an error rather than a warning.



-- 
Best Regards
Masahiro Yamada