From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751288AbaKXUq0 (ORCPT ); Mon, 24 Nov 2014 15:46:26 -0500 Received: from e37.co.us.ibm.com ([32.97.110.158]:38427 "EHLO e37.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750821AbaKXUqV (ORCPT ); Mon, 24 Nov 2014 15:46:21 -0500 Date: Mon, 24 Nov 2014 12:46:14 -0800 From: "Paul E. McKenney" To: Linus Torvalds Cc: Christian Borntraeger , Alexei Starovoitov , David Howells , "linux-kernel@vger.kernel.org" , "linux-arch@vger.kernel.org" , linux-mips , linux-x86_64@vger.kernel.org, linux-s390 , Paolo Bonzini , Ingo Molnar , Catalin Marinas , Will Deacon Subject: Re: [PATCH/RFC 7/7] kernel: Force ACCESS_ONCE to work only on scalar types Message-ID: <20141124204614.GU5050@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1416834210-61738-1-git-send-email-borntraeger@de.ibm.com> <1416834210-61738-8-git-send-email-borntraeger@de.ibm.com> <15567.1416835858@warthog.procyon.org.uk> <547381D7.2070404@de.ibm.com> <20141124194200.GR5050@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14112420-0025-0000-0000-000006598985 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Nov 24, 2014 at 12:19:11PM -0800, Linus Torvalds wrote: > On Mon, Nov 24, 2014 at 11:42 AM, Paul E. McKenney > wrote: > > > > OK, how about the following? > > Ugh. Disgusting. > > Why the heck isn't it just "sizeof(*__vp) <= sizeof(long)"? > > If the architecture has a 3-byte scalar type, then it probably has a > 3-byte load. Because I was allowing for the possibility of a 3-byte struct, which as you point out below... > > It complains if the variable is too large, for example, long long on > > 32-bit systems or large structures. It is OK loading from and storing > > to small structures as well, which I am having a hard time thinking of > > as a disadvantage. > > .. but that's *exactly* the gcc bug in question. It's a word-sized > struct that gcc loads twice. ...was a stupid thought anyway. OK, how about the attempt below? The initialization of __p complains for structures and unions, but gets optimized out. Thanx, Paul ------------------------------------------------------------------------- #define get_scalar_volatile_pointer(x) ({ \ typeof(x) __maybe_unused __p = 0; \ volatile typeof(x) *__vp = &(x); \ BUILD_BUG_ON(sizeof(*__vp) > sizeof(long)); \ __vp; }) #define ACCESS_ONCE(x) (*get_scalar_volatile_pointer(x))