linux-kernel - Re: [PATCH 04/18] alpha: Override READ_ONCE() with barriered implementation

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Message-ID: <CAK8P3a3VrEkDf8t1CCsY+g7rtXkqBsiPOr97b1yRbz8NpjdfFA@mail.gmail.com>
Date:   Thu, 2 Jul 2020 13:39:10 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Will Deacon <will@...nel.org>
Cc:     Mark Rutland <mark.rutland@....com>,
        "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>,
        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 04/18] alpha: Override READ_ONCE() with barriered implementation

On Thu, Jul 2, 2020 at 1:18 PM Will Deacon <will@...nel.org> wrote:
> On Thu, Jul 02, 2020 at 12:08:41PM +0200, Arnd Bergmann wrote:
> > On Thu, Jul 2, 2020 at 11:48 AM Will Deacon <will@...nel.org> wrote:
> > > On Thu, Jul 02, 2020 at 10:32:39AM +0100, Mark Rutland wrote:

> Not sure I follow you here, but I can confirm that what you're worried
> about doesn't happen for the usual case of a pointer-to-volatile scalar.
>
> For example, ignoring dependency ordering:
>
> unsigned long foo(volatile unsigned long *p)
> {
>         return smp_load_acquire(p) + 1;
> }
>
> Ends up looking like:
>
>         unsigned long ___p1 = *(const volatile unsigned long *)p;
>         smp_mb();
>         (volatile unsigned long)___p1;
>
> My understanding is that casting a non-pointer type to volatile doesn't
> do anything, so we're good.

Right, I mixed up the correct

        (typeof(*p))___p;

with the incorrect

       *typeof(p)&___p;

which would dereference a volatile pointer and cause the
problem.

The code is all fine then.

    Arnd