From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754402Ab3A1VYN (ORCPT ); Mon, 28 Jan 2013 16:24:13 -0500 Received: from mail-pb0-f50.google.com ([209.85.160.50]:52018 "EHLO mail-pb0-f50.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754333Ab3A1VYK (ORCPT ); Mon, 28 Jan 2013 16:24:10 -0500 Date: Mon, 28 Jan 2013 13:24:07 -0800 From: Kent Overstreet To: Tejun Heo 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: <20130128212407.GF26407@google.com> References: <20130125180941.GA16896@redhat.com> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130128211832.GK22465@mtj.dyndns.org> 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 01:18:32PM -0800, Tejun Heo wrote: > Hello, Kent. > > On Mon, Jan 28, 2013 at 12:55:40PM -0800, Kent Overstreet wrote: > > > I don't understand why we need two stages. What prevents the killing > > > thread from fetching percpu counters after dying passes one > > > synchronize_sched()? > > > > It does. The second synchronize_sched() is needed after we set state := > > dead, and before we drop the initial ref. Otherwise the ref could hit 0 > > before percpu_ref_put knows to check for it. > > Still a bit confused. Why do we need to make the two steps separate? > What prevents us from doing the following? > > 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?