From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752882AbcGYRFL (ORCPT ); Mon, 25 Jul 2016 13:05:11 -0400 Received: from mail-oi0-f53.google.com ([209.85.218.53]:32916 "EHLO mail-oi0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752476AbcGYRFI (ORCPT ); Mon, 25 Jul 2016 13:05:08 -0400 MIME-Version: 1.0 In-Reply-To: <20160725170116.GA24149@redhat.com> References: <20160715134523.GB28589@redhat.com> <20160715153840.GI7094@linux.vnet.ibm.com> <20160715164938.GA4294@redhat.com> <20160715180140.GM7094@linux.vnet.ibm.com> <20160716171007.GA30000@redhat.com> <20160719205018.GB7094@linux.vnet.ibm.com> <20160720171602.GA5059@redhat.com> <20160720213144.GM7094@linux.vnet.ibm.com> <20160721173435.GB22488@redhat.com> <20160722032641.GE7094@linux.vnet.ibm.com> <20160725170116.GA24149@redhat.com> From: John Stultz Date: Mon, 25 Jul 2016 10:05:06 -0700 Message-ID: Subject: Re: [PATCH] rcu_sync: simplify the state machine, introduce __rcu_sync_enter() To: Oleg Nesterov Cc: "Paul E. McKenney" , Peter Zijlstra , Ingo Molnar , lkml , Tejun Heo , Dmitry Shmidt , Rom Lemarchand , Colin Cross , Todd Kjos Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jul 25, 2016 at 10:01 AM, Oleg Nesterov wrote: > Paul, sorry for delay. > > On 07/21, Paul E. McKenney wrote: >> >> On Thu, Jul 21, 2016 at 07:34:36PM +0200, Oleg Nesterov wrote: >> > On 07/20, Paul E. McKenney wrote: >> > > >> > > On Wed, Jul 20, 2016 at 07:16:03PM +0200, Oleg Nesterov wrote: >> > > >> > > > Now, suppose we add the additional enter/exit's: >> > > > >> > > > freeze_super(sb) >> > > > { >> > > > // this doesn't block >> > > > __rcu_sync_enter(SEM3); >> > > > __rcu_sync_enter(SEM2); >> > > > __rcu_sync_enter(SEM1); >> > > > >> > > > down_write(&sb->s_umount); >> > > > if (NEED_TO_FREEZE) { >> > > > percpu_down_write(SEM1); >> > > >> > > The above waits for the grace period initiated by __rcu_sync_enter(), >> > > correct? Presumably "yes", because it will invoke rcu_sync_enter(), which >> > > will see the state as GP_ENTER, and will thus wait. >> > >> > But if down_write() blocks and/or NEED_TO_FREEZE takes some time it >> > could already see the GP_PASSED state, or at least it can sleep less. >> > >> > > But your point is that if !NEED_TO_FREEZE, we will get here without >> > > waiting for a grace period. >> > > >> > > But why aren't the __rcu_sync_enter() and rcu_sync_exit() calls inside >> > > the "if" statement? >> > >> > Yes, if we do __rcu_sync_enter() inside "if", then rcu_sync_exit() can't >> > hit GP_ENTER. >> > >> > But why we should disallow this use-case? It does not complicate the code >> > at all. >> >> I do agree that it doesn't complicate the current implementation. >> But it relies on a global lock, so I am not at all confident that this >> implementation is the final word. > > Hmm. which global lock? Or did you mean freeze_super(), not rcu_sync? > >> And speaking of global locks, failing to discourage the pattern above >> means that the code is unnecessarily acquiring three global locks, >> which doesn't seem like a good thing to me. > > Well, I do not agree, but this wasn't written by me. Just in case, all these > locks above are not really global, they are per-sb, but this is minor. > > And the patches which changed sb->s_writers to use percpu_rw_semaphore/rcu_sync > didn't change this logic. > > Except the old implementation was buggy, and the readers were slower than now. > >> I agree that there are use cases for beginning-of-time __rcu_sync_enter() >> or whatever we end up naming it. > > OK, at least iiuc you agree that cgroup_init() can use __rcu_sync_enter(). > As for other potential use-cases, we will disccuss this later. I will have > to CC you anyway ;) > > So I'll send v2 with renames after I test it. Thanks again. Can you also make clear which patches of PeterZ's I should be adding as well for testing? I've lost the plot as to what goes with what.. thanks -john