linux-kernel - Re: [PATCH 11/13] selftests/powerpc: kill off ACCESS

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Message-ID: <87mv4zygof.fsf@concordia.ellerman.id.au>
Date:   Tue, 10 Oct 2017 11:45:04 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org
Cc:     Mark Rutland <mark.rutland@....com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Shuah Khan <shuah@...nel.org>
Subject: Re: [PATCH 11/13] selftests/powerpc: kill off ACCESS_ONCE()

Mark Rutland <mark.rutland@....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@....com>
> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> Cc: Michael Ellerman <mpe@...erman.id.au>
> Cc: Paul Mackerras <paulus@...ba.org>
> Cc: Shuah Khan <shuah@...nel.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@...erman.id.au>

cheers