Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754709AbcDNIxV (ORCPT ); Thu, 14 Apr 2016 04:53:21 -0400 Received: from foss.arm.com ([217.140.101.70]:40549 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752025AbcDNIxT (ORCPT ); Thu, 14 Apr 2016 04:53:19 -0400 Date: Thu, 14 Apr 2016 09:53:17 +0100 From: Will Deacon To: Jason Low Cc: Peter Zijlstra , Linus Torvalds , "linux-kernel@vger.kernel.org" , "mingo@redhat.com" , "paulmck@linux.vnet.ibm.com" , "Rudd, Terry (HP Cloud Systems Linux R&D)" , "Long, Wai Man" , "boqun.feng@gmail.com" , "dave@stgolabs.net" , kbuild test robot , "kbuild-all@01.org" , jason.low2@hpe.com Subject: Re: [PATCH v2] MCS spinlock: Use smp_cond_load_acquire() Message-ID: <20160414085316.GC23207@arm.com> References: <1460516537.2709.7.camel@j-VirtualBox> <20160413174302.GC21722@arm.com> <1460580544.2709.31.camel@j-VirtualBox> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1460580544.2709.31.camel@j-VirtualBox> 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 Content-Length: 1220 Lines: 27 On Wed, Apr 13, 2016 at 01:49:04PM -0700, Jason Low wrote: > On Wed, 2016-04-13 at 10:43 -0700, Will Deacon wrote: > > On Tue, Apr 12, 2016 at 08:02:17PM -0700, Jason Low wrote: > > > For qspinlocks on ARM64, we would like to use WFE instead > > > of purely spinning. Qspinlocks internally have lock > > > contenders spin on an MCS lock. > > > > > > Update arch_mcs_spin_lock_contended() such that it uses > > > the new smp_cond_load_acquire() so that ARM64 can also > > > override this spin loop with its own implementation using WFE. > > > > > > On x86, it can also cheaper to use this than spinning on > > > smp_load_acquire(). > > > > > > Signed-off-by: Jason Low > > > > FWIW, we just override arch_mcs_spin_lock_contended entirely for arch/arm/ > > and use wfe there so we could do the same for arm64 in mainline already. > > Right, I was also thinking about that, although when we use > smp_cond_load_acquire() in the generic implementation, would we just end > up overriding it for the arch/arm64 version with the same thing? :) Sure, and we can convert to smp_cond_load_acquire then. I was just thinking that you can avoid the immediate dependency on Peter's stuff, that's all. Will