From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753866AbcBARYw (ORCPT ); Mon, 1 Feb 2016 12:24:52 -0500 Received: from bombadil.infradead.org ([198.137.202.9]:47916 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752487AbcBARYv (ORCPT ); Mon, 1 Feb 2016 12:24:51 -0500 Date: Mon, 1 Feb 2016 18:24:44 +0100 From: Peter Zijlstra To: Will Deacon Cc: Paul McKenney , linux-kernel@vger.kernel.org, Davidlohr Bueso , Ingo Molnar , parri.andrea@gmail.com Subject: Re: [RFC][PATCH] locking/mcs: Fix ordering for mcs_spin_lock() Message-ID: <20160201172444.GX6357@twins.programming.kicks-ass.net> References: <20160201143724.GW6357@twins.programming.kicks-ass.net> <20160201165813.GH6828@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160201165813.GH6828@arm.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 Mon, Feb 01, 2016 at 04:58:13PM +0000, Will Deacon wrote: > Hi Peter, > > On Mon, Feb 01, 2016 at 03:37:24PM +0100, Peter Zijlstra wrote: > > Given the below patch; we've now got an unconditional full global > > barrier in, does this make the MCS spinlock RCsc ? > > > > The 'problem' is that this barrier can happen before we actually acquire > > the lock. That is, if we hit arch_mcs_spin_lock_contended() _that_ will > > be the acquire barrier and we end up with a SYNC in between unlock and > > lock -- ie. not an smp_mb__after_unlock_lock() equivalent. > > In which case, I don't think the lock will be RCsc with this change; > you'd need an smp_mb__after_unlock_lock() after > arch_mcs_spin_lock_contended(...) if you wanted the thing to be RCsc. Right, I think it works for TSO, but in general it makes my head hurt. > > This is non-critical because the MCS code isn't actually used and > > mostly serves as documentation, a stepping stone to the more complex > > things we've build on top of the idea. > > Although I wonder how useful this is as a documentation aid now that we > have the osq_lock. So the OSQ thing is horribly complex, pure MCS is a nice step-stone.