From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751388AbcBKPQR (ORCPT ); Thu, 11 Feb 2016 10:16:17 -0500 Received: from bombadil.infradead.org ([198.137.202.9]:45812 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750955AbcBKPQP (ORCPT ); Thu, 11 Feb 2016 10:16:15 -0500 Date: Thu, 11 Feb 2016 16:16:10 +0100 From: Peter Zijlstra To: Alfredo Alvarez Fernandez Cc: mingo@redhat.com, sasha.levin@oracle.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/3] tools/liblockdep: add userspace version of READ_ONCE Message-ID: <20160211151610.GL6357@twins.programming.kicks-ass.net> References: <1455147212-2389-1-git-send-email-alfredoalvarezernandez@gmail.com> <1455147212-2389-2-git-send-email-alfredoalvarezernandez@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1455147212-2389-2-git-send-email-alfredoalvarezernandez@gmail.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Feb 11, 2016 at 12:33:30AM +0100, Alfredo Alvarez Fernandez wrote: > From: Alfredo Alvarez Fernandez > > This was added to the kernel code in 1658d35ead5d ("list: Use > READ_ONCE() when testing for empty lists") > There's nothing special we need to do about it in userspace. > > Signed-off-by: Alfredo Alvarez Fernandez > --- > tools/lib/lockdep/uinclude/linux/compiler.h | 1 + > 1 file changed, 1 insertion(+) > > diff --git a/tools/lib/lockdep/uinclude/linux/compiler.h b/tools/lib/lockdep/uinclude/linux/compiler.h > index 6386dc3..fd3e56a 100644 > --- a/tools/lib/lockdep/uinclude/linux/compiler.h > +++ b/tools/lib/lockdep/uinclude/linux/compiler.h > @@ -3,6 +3,7 @@ > > #define __used __attribute__((__unused__)) > #define unlikely > +#define READ_ONCE(x) (x) > #define WRITE_ONCE(x, val) x=(val) I would argue we'd still very much want the volatile cast for both READ and WRITE_ONCE(). Why do these things have different semantics between user and kernel space?