linux-kernel - Re: [PATCH 01/13] compiler.h: Split {READ,WRITE}

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Message-ID: <CAK8P3a21KdGKMDDPs3jc9XEg3=LbzFnGwVm+xDTB+EqGXiZorA@mail.gmail.com>
Date:   Mon, 11 Nov 2019 10:32:46 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Christian Borntraeger <borntraeger@...ibm.com>
Cc:     Will Deacon <will@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Yunjae Lee <lyj7694@...il.com>,
        SeongJae Park <sj38.park@...il.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 Ellerman <mpe@...erman.id.au>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Joe Perches <joe@...ches.com>,
        Boqun Feng <boqun.feng@...il.com>,
        alpha <linux-alpha@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH 01/13] compiler.h: Split {READ,WRITE}_ONCE definitions out
 into rwonce.h

On Mon, Nov 11, 2019 at 9:10 AM Christian Borntraeger
<borntraeger@...ibm.com> wrote:
> On 08.11.19 20:57, Arnd Bergmann wrote:
> > On Fri, Nov 8, 2019 at 6:01 PM Will Deacon <will@...nel.org> wrote:
> >>
> >> In preparation for allowing architectures to define their own
> >> implementation of the 'READ_ONCE()' macro, move the generic
> >> '{READ,WRITE}_ONCE()' definitions out of the unwieldy 'linux/compiler.h'
> >> and into a new 'rwonce.h' header under 'asm-generic'.
> >
> > Adding Christian Bornträger to Cc, he originally added the
> > READ_ONCE()/WRITE_ONCE()
> > code.
> >
> > I wonder if it would be appropriate now to revert back to a much simpler version
> > of these helpers for any modern compiler. As I understand, only gcc-4.6 and
> > gcc4.7 actually need the song-and-dance version with the union and switch/case,
> > while for others, we can might be able back to a macro doing a volatile access.
>
> As far as I know this particular issue with  volatile access on aggregate types
> was fixed in gcc 4.8. On the other hand we know that the current construct will
> work on all compilers. Not so sure about the orignal ACCESS_ONCE implementation.

I've seen problems with clang on the current version, leading to unnecessary
temporaries being spilled to the stack in some cases, so I think it would still
help to simplify it.

We probably don't want the exact ACCESS_ONCE() implementation back
that existed before, but rather something that implements the stricter
READ_ONCE() and WRITE_ONCE(). I'd probably also want to avoid the
__builtin_memcpy() exception for odd-sized accesses and instead have
a separate way to do those.

      Arnd