Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934007AbaFSRF4 (ORCPT ); Thu, 19 Jun 2014 13:05:56 -0400 Received: from e39.co.us.ibm.com ([32.97.110.160]:45171 "EHLO e39.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933127AbaFSRFz (ORCPT ); Thu, 19 Jun 2014 13:05:55 -0400 Date: Thu, 19 Jun 2014 10:05:49 -0700 From: "Paul E. McKenney" To: Tejun Heo Cc: Lai Jiangshan , cl@linux-foundation.org, kmo@daterainc.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH 6/6] percpu-refcount: implement percpu_ref_reinit() and percpu_ref_is_zero() Message-ID: <20140619170549.GC4904@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1403053685-28240-1-git-send-email-tj@kernel.org> <1403053685-28240-7-git-send-email-tj@kernel.org> <53A1097F.3060400@cn.fujitsu.com> <20140618153222.GA11042@htj.dyndns.org> <53A243B8.4010501@cn.fujitsu.com> <20140619020727.GC20100@mtj.dyndns.org> <20140619022708.GB4669@linux.vnet.ibm.com> <20140619133624.GI11042@htj.dyndns.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140619133624.GI11042@htj.dyndns.org> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14061917-9332-0000-0000-00000124AA2E Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 19, 2014 at 09:36:24AM -0400, Tejun Heo wrote: > Hey, Paul. > > On Wed, Jun 18, 2014 at 07:27:08PM -0700, Paul E. McKenney wrote: > > Yep, smp_load_acquire() orders its load against later loads and stores, > > so it really does need a memory barrier on weakly ordered systems. > > Yeap. > > > This is the "publish" operation for dynamically allocated per-CPU > > references? If so, agreed, you should be able to rely on dependency > > ordering. Make sure to comment the smp_read_barrier_depends(). ;-) > > Definitely, there aren't many things which are more frustrating than > barriers w/o comments explaining their pairing. I'm pairing > store_release with read_barrier_depends as that's what RCU is doing. > Is this the preferred way now? I like the new store_release and > load_acquire as they document what's being barriered better but as Lai > suggested in another reply it does seem a bit unbalanced. I wonder > whether load_acquire_depends would make sense. If you mean what I think you mean by load_acquire_depends(), it is spelled "rcu_dereference()" or, in this case, where you are never removing anything that has been added, "rcu_dereference_raw()". Because you are never removing anything, you don't need rcu_read_lock() or rcu_read_unlock(), thus you don't want lockdep yelling at you about not having RCU read-side critical sections, thus rcu_dereference_raw(). Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/