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Date: Tue, 10 Oct 2017 10:32:18 +0100
From: Mark Rutland <mark.rutland@arm.com>
To: Michael Ellerman <mpe@ellerman.id.au>
Cc: linux-kernel@vger.kernel.org,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>, Shuah Khan <shuah@kernel.org>
Subject: Re: [PATCH 11/13] selftests/powerpc: kill off ACCESS_ONCE()
Message-ID: <20171010093218.GD27659@leverpostej>
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On Tue, Oct 10, 2017 at 11:45:04AM +1100, Michael Ellerman wrote:
> Mark Rutland <mark.rutland@arm.com> writes:
> 
> > For several reasons, it is desirable to use {READ,WRITE}_ONCE() in
> > preference to ACCESS_ONCE(), and new code is expected to use one of the
> > former. So far, there's been no reason to change most existing uses of
> > ACCESS_ONCE(), as these aren't currently harmful.
> >
> > However, for some features it is necessary to instrument reads and
> > writes separately, which is not possible with ACCESS_ONCE(). This
> > distinction is critical to correct operation.
> >
> > The bulk of the kernel code can be transformed via Coccinelle to use
> > {READ,WRITE}_ONCE(), though this only modifies users of ACCESS_ONCE(),
> > and not the implementation itself. As such, it has the potential to
> > break homebrew ACCESS_ONCE() macros seen in some user code in the kernel
> > tree (e.g. the virtio code, as fixed in commit ea9156fb3b71d9f7).
> >
> > To avoid fragility if/when that transformation occurs, and to align with
> > the preferred usage of {READ,WRITE}_ONCE(), this patch updates the DSCR
> > selftest code to use READ_ONCE() rather than ACCESS_ONCE(). There should
> > be no functional change as a result of this patch.
> >
> > Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> > Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> > Cc: Michael Ellerman <mpe@ellerman.id.au>
> > Cc: Paul Mackerras <paulus@samba.org>
> > Cc: Shuah Khan <shuah@kernel.org>
> > ---
> >  tools/testing/selftests/powerpc/dscr/dscr.h              | 2 +-
> >  tools/testing/selftests/powerpc/dscr/dscr_default_test.c | 2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> Acked-by: Michael Ellerman <mpe@ellerman.id.au>

Thanks!

Mark.