From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752733AbbJLAkq (ORCPT ); Sun, 11 Oct 2015 20:40:46 -0400 Received: from e34.co.us.ibm.com ([32.97.110.152]:55450 "EHLO e34.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752562AbbJLAko (ORCPT ); Sun, 11 Oct 2015 20:40:44 -0400 X-IBM-Helo: d03dlp01.boulder.ibm.com X-IBM-MailFrom: paulmck@linux.vnet.ibm.com X-IBM-RcptTo: linux-doc@vger.kernel.org;linux-kernel@vger.kernel.org Date: Sun, 11 Oct 2015 17:40:44 -0700 From: "Paul E. McKenney" To: Boqun Feng Cc: Peter Zijlstra , Oleg Nesterov , linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Jonathan Corbet , Michal Hocko , David Howells , Linus Torvalds , Will Deacon Subject: Re: [PATCH] Documentation: Remove misleading examples of the barriers in wake_*() Message-ID: <20151012004044.GZ3910@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1441674841-11498-1-git-send-email-boqun.feng@gmail.com> <20150909192822.GM4029@linux.vnet.ibm.com> <20150910021612.GC18494@fixme-laptop.cn.ibm.com> <20150910175557.GA20640@redhat.com> <20150917130125.GL3816@twins.programming.kicks-ass.net> <20150924132121.GA1814@fixme-laptop.cn.ibm.com> <20151006160650.GT3604@twins.programming.kicks-ass.net> <20151011152640.GC27351@fixme-laptop.cn.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151011152640.GC27351@fixme-laptop.cn.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15101200-0017-0000-0000-00000E904C51 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Oct 11, 2015 at 11:26:40PM +0800, Boqun Feng wrote: > On Tue, Oct 06, 2015 at 06:06:50PM +0200, Peter Zijlstra wrote: > > On Thu, Sep 24, 2015 at 09:21:22PM +0800, Boqun Feng wrote: > > > > Included in it are some of the details on this subject, because a wakeup > > > > has two prior states that are of importance, the tasks own prior state > > > > and the wakeup state, both should be considered in the 'program order' > > > > flow. > > > > > > > > > > Great and very helpful ;-) > > > > > > > So maybe we can reduce the description in memory-barriers to this > > > > 'split' program order guarantee, where a woken task must observe both > > > > its own prior state and its wakee state. > > > ^^^^^ > > > I think you mean "waker" here, right? > > > > Yes. > > > > > And the waker is not necessarily the same task who set the @cond to > > > true, right? > > > > It should be. > > > > > If so, I feel like it's really hard to *use* this 'split' > > > program order guarantee in other places than sleep/wakeup itself. Could > > > you give an example? Thank you. > > > > It was not meant to be used in any other scenario; the 'split' PO really > > is part of the whole sleep/wakeup. It does not apply to anything else. > > Got it. So at this point, I think it's better to remove the entire > "Sleep and wake-up functions" section in memory-barriers.txt. Because > this order guarantee is not for other users except sleep/wakeup. Any > concern, Paul? The concern I have with just removing it is that it is all too easy for people to assume that they provide ordering. So we should at least have a section stating clearly that ordering is not guaranteed without help from locks, release-acquire, explicit memory barriers, etc. Thanx, Paul