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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Thu, 2 Jul 2020 13:39:10 +0200
Message-ID: <CAK8P3a3VrEkDf8t1CCsY+g7rtXkqBsiPOr97b1yRbz8NpjdfFA@mail.gmail.com>
Subject: Re: [PATCH 04/18] alpha: Override READ_ONCE() with barriered implementation
To:     Will Deacon <will@kernel.org>
Cc:     Mark Rutland <mark.rutland@arm.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Sami Tolvanen <samitolvanen@google.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Kees Cook <keescook@chromium.org>,
        Marco Elver <elver@google.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Matt Turner <mattst88@gmail.com>,
        Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
        Richard Henderson <rth@twiddle.net>,
        Peter Zijlstra <peterz@infradead.org>,
        Alan Stern <stern@rowland.harvard.edu>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        alpha <linux-alpha@vger.kernel.org>,
        virtualization@lists.linux-foundation.org,
        Android Kernel Team <kernel-team@android.com>
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On Thu, Jul 2, 2020 at 1:18 PM Will Deacon <will@kernel.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@kernel.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