Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933778AbaFSQzk (ORCPT ); Thu, 19 Jun 2014 12:55:40 -0400 Received: from e39.co.us.ibm.com ([32.97.110.160]:39101 "EHLO e39.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932232AbaFSQzj (ORCPT ); Thu, 19 Jun 2014 12:55:39 -0400 Date: Thu, 19 Jun 2014 09:55:02 -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 v2 6/6] percpu-refcount: implement percpu_ref_reinit() and percpu_ref_is_zero() Message-ID: <20140619165501.GB4904@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> <20140619022055.GD20100@mtj.dyndns.org> <53A25286.1030003@cn.fujitsu.com> <20140619133104.GH11042@htj.dyndns.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140619133104.GH11042@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: 14061916-9332-0000-0000-000001249D07 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jun 19, 2014 at 09:31:04AM -0400, Tejun Heo wrote: > On Thu, Jun 19, 2014 at 11:01:26AM +0800, Lai Jiangshan wrote: > > > + /* > > > + * Restore per-cpu operation. smp_store_release() is paired with > > > + * smp_load_acquire() in __pcpu_ref_alive() and guarantees that the > > > > s/smp_load_acquire()/smp_read_barrier_depends()/ > > Will update. > > > s/smp_store_release()/smp_mb()/ if you accept my next comment. > > > > > + * zeroing is visible to all percpu accesses which can see the > > > + * following PCPU_REF_DEAD clearing. > > > + */ > > > + for_each_possible_cpu(cpu) > > > + *per_cpu_ptr(pcpu_count, cpu) = 0; > > > + > > > + smp_store_release(&ref->pcpu_count_ptr, > > > + ref->pcpu_count_ptr & ~PCPU_REF_DEAD); > > > > I think it would be better if smp_mb() is used. > > smp_wmb() would be better here. We don't need the reader side. > > > it is documented that smp_read_barrier_depends() and smp_mb() are paired. > > Not smp_read_barrier_depends() and smp_store_release(). Well, sounds like the documentation needs an update, then. ;-) For example, current rcu_assign_pointer() is a wrapper around smp_store_release(). > I don't know. I thought about doing that but the RCU accessors are > pairing store_release with read_barrier_depends, so I don't think the > particular paring is problematic and store_release is better at > documenting what's being barriered. Which Tejun noted as well. 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/