From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752413AbaIDJxk (ORCPT ); Thu, 4 Sep 2014 05:53:40 -0400 Received: from gate.crashing.org ([63.228.1.57]:55031 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750902AbaIDJxg (ORCPT ); Thu, 4 Sep 2014 05:53:36 -0400 Message-ID: <1409824374.4246.62.camel@pasglop> Subject: Re: bit fields && data tearing From: Benjamin Herrenschmidt To: David Laight Cc: Peter Hurley , Jakub Jelinek , "linux-arch@vger.kernel.org" , Tony Luck , "linux-ia64@vger.kernel.org" , Oleg Nesterov , "linux-kernel@vger.kernel.org" , Paul Mackerras , "Paul E. McKenney" , "linuxppc-dev@lists.ozlabs.org" , Miroslav Franc , Richard Henderson Date: Thu, 04 Sep 2014 19:52:54 +1000 In-Reply-To: <063D6719AE5E284EB5DD2968C1650D6D17487172@AcuExch.aculab.com> References: <20140712181328.GA8738@redhat.com> <54079B70.4050200@hurleysoftware.com> <1409785893.30640.118.camel@pasglop> <063D6719AE5E284EB5DD2968C1650D6D17487172@AcuExch.aculab.com> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.12.2 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2014-09-04 at 08:43 +0000, David Laight wrote: > From: Benjamin Herrenschmidt > > On Wed, 2014-09-03 at 18:51 -0400, Peter Hurley wrote: > > > > > Apologies for hijacking this thread but I need to extend this discussion > > > somewhat regarding what a compiler might do with adjacent fields in a structure. > > > > > > The tty subsystem defines a large aggregate structure, struct tty_struct. > > > Importantly, several different locks apply to different fields within that > > > structure; ie., a specific spinlock will be claimed before updating or accessing > > > certain fields while a different spinlock will be claimed before updating or > > > accessing certain _adjacent_ fields. > > > > > > What is necessary and sufficient to prevent accidental false-sharing? > > > The patch below was flagged as insufficient on ia64, and possibly ARM. > > > > We expect native aligned scalar types to be accessed atomically (the > > read/modify/write of a larger quantity that gcc does on some bitfield > > cases has been flagged as a gcc bug, but shouldn't happen on normal > > scalar types). > > That isn't true on all architectures for items smaller than a machine word. > At least one has to do rmw for byte accesses. Yeah correct, alpha and bytes right ? Is there any other ? That's why I suggested int. > David > > > I am not 100% certain of "bool" here, I assume it's treated as a normal > > scalar and thus atomic but if unsure, you can always use int. > > > > Another option is to use the atomic bitops and make these bits in a > > bitmask but that is probably unnecessary if you have locks already. > > > > Cheers, > > Ben.