From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752921AbbGXLbF (ORCPT ); Fri, 24 Jul 2015 07:31:05 -0400 Received: from foss.arm.com ([217.140.101.70]:44567 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750740AbbGXLbB (ORCPT ); Fri, 24 Jul 2015 07:31:01 -0400 Date: Fri, 24 Jul 2015 12:31:01 +0100 From: Will Deacon To: "Paul E. McKenney" Cc: Peter Zijlstra , "linux-arch@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Benjamin Herrenschmidt Subject: Re: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock_lock() Message-ID: <20150724113101.GE30410@arm.com> References: <20150713230405.GB3717@linux.vnet.ibm.com> <20150714100429.GC15448@arm.com> <20150714124540.GC3717@linux.vnet.ibm.com> <20150714125146.GJ16213@arm.com> <20150714140014.GD3717@linux.vnet.ibm.com> <20150714141202.GN16213@arm.com> <20150714193144.GP3717@linux.vnet.ibm.com> <20150715013820.GA21971@linux.vnet.ibm.com> <20150715105135.GE1005@arm.com> <20150715131221.GY3717@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150715131221.GY3717@linux.vnet.ibm.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Paul, On Wed, Jul 15, 2015 at 02:12:21PM +0100, Paul E. McKenney wrote: > On Wed, Jul 15, 2015 at 11:51:35AM +0100, Will Deacon wrote: > > On Wed, Jul 15, 2015 at 02:38:20AM +0100, Paul E. McKenney wrote: > > > On Tue, Jul 14, 2015 at 12:31:44PM -0700, Paul E. McKenney wrote: > > > > On Tue, Jul 14, 2015 at 03:12:16PM +0100, Will Deacon wrote: > > > > > On Tue, Jul 14, 2015 at 03:00:14PM +0100, Paul E. McKenney wrote: > > > > > > On Tue, Jul 14, 2015 at 01:51:46PM +0100, Will Deacon wrote: > > > > > > > On Tue, Jul 14, 2015 at 01:45:40PM +0100, Paul E. McKenney wrote: > > > > > > > > On Tue, Jul 14, 2015 at 11:04:29AM +0100, Will Deacon wrote: > > > > > > > > > Given that RCU is currently the only user of this barrier, how would you > > > > > > > > > feel about making the barrier local to RCU and not part of the general > > > > > > > > > memory-barrier API? > > > > > > > > > > > > > > > > In theory, no objection. Your thought is to leave the definitions where > > > > > > > > they are, mark them as being used only by RCU, and removing mention from > > > > > > > > memory-barriers.txt? Or did you have something else in mind? > > > > > > > > > > > > > > Actually, I was thinking of defining them in an RCU header file with an > > > > > > > #ifdef CONFIG_POWERPC for the smb_mb() version. Then you could have a big > > > > > > > comment describing the semantics, or put that in an RCU Documentation file > > > > > > > instead of memory-barriers.txt. > > > > > > > > > > > > > > That *should* then mean we notice anybody else trying to use the barrier, > > > > > > > because they'd need to send patches to either add something equivalent > > > > > > > or move the definition out again. > > > > > > > > > > > > My concern with this approach is that someone putting together a new > > > > > > architecture might miss this. That said, this approach certainly would > > > > > > work for the current architectures. > > > > > > > > > > I don't think they're any more likely to miss it than with the current > > > > > situation where the generic code defines the macro as a NOP unless you > > > > > explicitly override it. > > > > > > > > Fair enough... > > > > > > Like this? > > > > Precisely! Thanks for cooking the patch -- this lays all my worries to > > rest, so: > > > > Acked-by: Will Deacon > > Thank you! [...] > > > commit 695c05d4b9666c50b40a1c022678b5f6e2e3e771 > > > Author: Paul E. McKenney > > > Date: Tue Jul 14 18:35:23 2015 -0700 > > > > > > rcu,locking: Privatize smp_mb__after_unlock_lock() > > > > > > RCU is the only thing that uses smp_mb__after_unlock_lock(), and is > > > likely the only thing that ever will use it, so this commit makes this > > > macro private to RCU. > > > > > > Signed-off-by: Paul E. McKenney > > > Cc: Will Deacon > > > Cc: Peter Zijlstra > > > Cc: Benjamin Herrenschmidt > > > Cc: "linux-arch@vger.kernel.org" Are you planning to queue this somewhere? I think it makes sense regardless of whether we change PowerPc or not and ideally it would be merged around the same time as my relaxed atomics series. Will