From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754368Ab3A1V2W (ORCPT ); Mon, 28 Jan 2013 16:28:22 -0500 Received: from mail-qe0-f47.google.com ([209.85.128.47]:60482 "EHLO mail-qe0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753953Ab3A1V2T (ORCPT ); Mon, 28 Jan 2013 16:28:19 -0500 Date: Mon, 28 Jan 2013 13:28:14 -0800 From: Tejun Heo To: Kent Overstreet Cc: Oleg Nesterov , srivatsa.bhat@linux.vnet.ibm.com, rusty@rustcorp.com.au, linux-kernel@vger.kernel.org Subject: Re: [PATCH] generic dynamic per cpu refcounting Message-ID: <20130128212814.GL22465@mtj.dyndns.org> References: <20130125191139.GA19247@redhat.com> <20130128181528.GA26407@google.com> <20130128182737.GC22465@mtj.dyndns.org> <20130128184933.GC26407@google.com> <20130128185552.GD22465@mtj.dyndns.org> <20130128202214.GD26407@google.com> <20130128205540.GE26407@google.com> <20130128211832.GK22465@mtj.dyndns.org> <20130128212407.GF26407@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130128212407.GF26407@google.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 28, 2013 at 01:24:07PM -0800, Kent Overstreet wrote: > > set dying; > > synchronize_sched(); > > collect percpu refs into global atomic_t; > > put the base ref; > > After you set state := dying, percpu_ref_put() decrements the atomic_t, > but it can't check if it's 0 yet because the thread that's collecting > the percpu refs might not be done yet. > > So percpu_ref_put can't check for ref == 0 until after state == dead. > But the put in your example might have made ref 0. When did you set > state to dead? But at that point, the operation is already global, so there gotta be a lighter way to synchronize stuff than going through full grace period. ie. You can add a bias value before marking dead so that the counter never reaches zero before all percpu counters are collected and then unbias it right before putting the base ref, that way the only way you can hit zero ref is all refs are actually zero. Thanks. -- tejun