From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932933AbcFQQrP (ORCPT ); Fri, 17 Jun 2016 12:47:15 -0400 Received: from mx2.suse.de ([195.135.220.15]:60964 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754403AbcFQQrL (ORCPT ); Fri, 17 Jun 2016 12:47:11 -0400 Date: Fri, 17 Jun 2016 09:46:59 -0700 From: Davidlohr Bueso To: Waiman Long Cc: Peter Zijlstra , Ingo Molnar , linux-kernel@vger.kernel.org, x86@kernel.org, linux-alpha@vger.kernel.org, linux-ia64@vger.kernel.org, linux-s390@vger.kernel.org, linux-arch@vger.kernel.org, Jason Low , Dave Chinner , Scott J Norton , Douglas Hatch Subject: Re: [RFC PATCH-tip v2 1/6] locking/osq: Make lock/unlock proper acquire/release barrier Message-ID: <20160617164659.GC14591@linux-80c1.suse> References: <1465944489-43440-1-git-send-email-Waiman.Long@hpe.com> <1465944489-43440-2-git-send-email-Waiman.Long@hpe.com> <20160615165659.GC2094@linux-80c1.suse> <20160615171250.GO30921@twins.programming.kicks-ass.net> <20160615182724.GD2094@linux-80c1.suse> <20160615184007.GW30921@twins.programming.kicks-ass.net> <20160617011155.GA14591@linux-80c1.suse> <576408F7.8020901@hpe.com> <20160617162926.GB14591@linux-80c1.suse> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: <20160617162926.GB14591@linux-80c1.suse> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 17 Jun 2016, Davidlohr Bueso wrote: >On Fri, 17 Jun 2016, Waiman Long wrote: > >>On 06/16/2016 09:11 PM, Davidlohr Bueso wrote: >>>On Wed, 15 Jun 2016, Peter Zijlstra wrote: >>> >>>>Yeah, see a few patches further in this series, where he guards a >>>>variables with the osq_lock. >>> >>>So one problem I have with all this is that if we are hardening >>>osq_lock/unlock() >>>because of some future use that is specific to rwsems, then we >>>will immediately >>>be hurting mutexes for no good reason. >>> >> >>I am going to change it to use smp_acquire__after_ctrl_dep() as >>suggested by PeterZ. Is that a good enough compromise? I have also >>changed the xchg in the unlock side to xchg_release which could help >>performance in some archs. The thing is when developers see the name >>osq_lock/osq_unlock, they will naturally assume the proper barrriers >>are provided which is not currently the case. > >Oh, from your discussions with Boqun, I was under the impression that ->locked >was now going to be properly ordered in all cases now, which is why >I worry about mutexes. > >>Anyway, the change won't affect x86, it is probably ARM or PPC that >>may have an impact. > >Yes, that xchg() won't affect x86, but adding an smp_store_release(node->locked, 1) >or such will obviously. nm this last part, you're right, x86 smp_store_release is a nop.