linux-kernel - Re: [PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when CLANG

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Message-ID: <CAK8P3a1MWBxHihcUaduqcpS2U61cKPujQG6mAfn3-pCokzLwUw@mail.gmail.com>
Date:   Wed, 1 Jul 2020 12:59:36 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Will Deacon <will@...nel.org>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Marco Elver <elver@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Matt Turner <mattst88@...il.com>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Richard Henderson <rth@...ddle.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Alan Stern <stern@...land.harvard.edu>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        alpha <linux-alpha@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH 18/18] arm64: lto: Strengthen READ_ONCE() to acquire when CLANG_LTO=y

On Wed, Jul 1, 2020 at 12:19 PM Will Deacon <will@...nel.org> wrote:
> On Tue, Jun 30, 2020 at 09:25:03PM +0200, Arnd Bergmann wrote:
> > On Tue, Jun 30, 2020 at 7:39 PM Will Deacon <will@...nel.org> wrote:
> > Once we make gcc-4.9 the minimum version,
> > this could be further improved to
> >
> >        __auto_type __x = &(x);
>
> Is anybody working on moving to 4.9? I've seen the mails from Linus
> championing it, but I thought there was a RHEL in support that people
> might care about?

I don't think there was a serious discussion about it so far, and
we only just moved to gcc-4.8.

I think moving to gnu11 (gcc-4.9 or clang) instead of gnu99 has other
benefits as well, so we may well want to do it anyway when something
else comes up.

For __auto_type(), we could do it like

#if (clang or gcc-4.9+)
#define auto_typeof(x) __auto_type
#else
#define auto_typeof(x) typeof(x)
#endif

which could be used in a lot of macros.

     Arnd