From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755789AbdJJApH (ORCPT ); Mon, 9 Oct 2017 20:45:07 -0400 Received: from ozlabs.org ([103.22.144.67]:44501 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754707AbdJJApF (ORCPT ); Mon, 9 Oct 2017 20:45:05 -0400 From: Michael Ellerman To: Mark Rutland , linux-kernel@vger.kernel.org Cc: Mark Rutland , Benjamin Herrenschmidt , Paul Mackerras , Shuah Khan Subject: Re: [PATCH 11/13] selftests/powerpc: kill off ACCESS_ONCE() In-Reply-To: <1507573730-8083-12-git-send-email-mark.rutland@arm.com> References: <1507573730-8083-1-git-send-email-mark.rutland@arm.com> <1507573730-8083-12-git-send-email-mark.rutland@arm.com> Date: Tue, 10 Oct 2017 11:45:04 +1100 Message-ID: <87mv4zygof.fsf@concordia.ellerman.id.au> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Mark Rutland 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 > Cc: Benjamin Herrenschmidt > Cc: Michael Ellerman > Cc: Paul Mackerras > Cc: Shuah Khan > --- > 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 cheers