Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754093AbbEAPWG (ORCPT ); Fri, 1 May 2015 11:22:06 -0400 Received: from e35.co.us.ibm.com ([32.97.110.153]:55205 "EHLO e35.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753902AbbEAPWD (ORCPT ); Fri, 1 May 2015 11:22:03 -0400 Date: Fri, 1 May 2015 08:21:57 -0700 From: "Paul E. McKenney" To: Jason Low Cc: Waiman Long , Peter Zijlstra , Ingo Molnar , Thomas Gleixner , linux-kernel@vger.kernel.org, Andrew Morton , Oleg Nesterov , Frederic Weisbecker , Mel Gorman , Rik van Riel , Steven Rostedt , Preeti U Murthy , Mike Galbraith , Davidlohr Bueso , Aswin Chandramouleeswaran , Scott J Norton Subject: Re: [PATCH v2 2/5] sched, numa: Document usages of mm->numa_scan_seq Message-ID: <20150501152157.GF5381@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1430251224-5764-1-git-send-email-jason.low2@hp.com> <1430251224-5764-3-git-send-email-jason.low2@hp.com> <55411F91.6050101@hp.com> <1430333101.8722.32.camel@j-VirtualBox> <55427794.30808@hp.com> <1430428387.2475.47.camel@j-VirtualBox> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1430428387.2475.47.camel@j-VirtualBox> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15050115-0013-0000-0000-00000A6E2009 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2039 Lines: 38 On Thu, Apr 30, 2015 at 02:13:07PM -0700, Jason Low wrote: > On Thu, 2015-04-30 at 14:42 -0400, Waiman Long wrote: > > > I do have a question of what kind of tearing you are talking about. Do > > you mean the tearing due to mm being changed in the middle of the > > access? The reason why I don't like this kind of construct is that I am > > not sure if > > the address translation p->mm->numa_scan_seq is being done once or > > twice. I looked at the compiled code and the translation is done only once. > > > > Anyway, the purpose of READ_ONCE and WRITE_ONCE is not for eliminating > > data tearing. They are to make sure that the compiler won't compile away > > data access and they are done in the order they appear in the program. I > > don't think it is a good idea to associate tearing elimination with > > those macros. So I would suggest removing the last sentence in your comment. > > Yes, I can remove the last sentence in the comment since the main goal > was to document that we're access this field without exclusive access. > > In terms of data tearing, an example would be the write operation gets > split into multiple stores (though this is architecture dependent). The > idea was that since we're modifying a seq variable without the write > lock, we want to remove any forms of optimizations as mentioned above or > unpredictable behavior, since READ_ONCE/WRITE_ONCE adds no overhead. Just to be clear... READ_ONCE() and WRITE_ONCE() do not avoid data tearing in cases where the thing read or written is too big for a machine word. If the thing read/written does fit into a machine word and if the location read/written is properly aligned, I would be quite surprised if either READ_ONCE() or WRITE_ONCE() resulted in any sort of tearing. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/