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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Mon, 13 Jan 2020 15:39:45 +0100
Message-ID: <CAK8P3a1t9757M5CRKNX_X+T9VuLX+5=z5_845rLJGTG50RogXA@mail.gmail.com>
Subject: Re: [RFC PATCH 1/8] compiler/gcc: Emit build-time warning for GCC
 prior to version 4.8
To:     Joe Perches <joe@perches.com>
Cc:     Will Deacon <will@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Android Kernel Team <kernel-team@android.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Peter Zijlstra <peterz@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Segher Boessenkool <segher@kernel.crashing.org>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
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On Fri, Jan 10, 2020 at 6:54 PM Joe Perches <joe@perches.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@kernel.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