From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752884AbcGYRBW (ORCPT ); Mon, 25 Jul 2016 13:01:22 -0400 Received: from mx1.redhat.com ([209.132.183.28]:53654 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751036AbcGYRBU (ORCPT ); Mon, 25 Jul 2016 13:01:20 -0400 Date: Mon, 25 Jul 2016 19:01:17 +0200 From: Oleg Nesterov To: "Paul E. McKenney" Cc: Peter Zijlstra , mingo@kernel.org, linux-kernel@vger.kernel.org, tj@kernel.org, john.stultz@linaro.org, dimitrysh@google.com, romlem@google.com, ccross@google.com, tkjos@google.com Subject: Re: [PATCH] rcu_sync: simplify the state machine, introduce __rcu_sync_enter() Message-ID: <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160722032641.GE7094@linux.vnet.ibm.com> User-Agent: Mutt/1.5.24 (2015-08-30) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Mon, 25 Jul 2016 17:01:20 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. Oleg.