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

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Message-ID: <CAK8P3a3ueJ_rQc-1JTg=3N0JSuY9BduJ6FrrPFG1K2FWVzJdfA@mail.gmail.com>
Date:   Fri, 10 Jan 2020 18:35:02 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Will Deacon <will@...nel.org>
Cc:     "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 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.

       Arnd