From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756231AbaICPM1 (ORCPT ); Wed, 3 Sep 2014 11:12:27 -0400 Received: from e37.co.us.ibm.com ([32.97.110.158]:54541 "EHLO e37.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932757AbaICPMR (ORCPT ); Wed, 3 Sep 2014 11:12:17 -0400 Date: Wed, 3 Sep 2014 08:03:06 -0700 From: "Paul E. McKenney" To: Peter Zijlstra Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com, <""@rjwysocki.net>, tianyu.lan@intel.com Subject: Re: [PATCH RFC tip/core/rcu] Eliminate deadlock between CPU hotplug and expedited grace periods Message-ID: <20140903150306.GU5001@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20140828194745.GA3761@linux.vnet.ibm.com> <20140901112059.GG27892@worktop.ger.corp.intel.com> <20140901160550.GL5001@linux.vnet.ibm.com> <20140901161735.GA5806@worktop.ger.corp.intel.com> <20140902163656.GC5001@linux.vnet.ibm.com> <20140903113112.GM4783@worktop.ger.corp.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140903113112.GM4783@worktop.ger.corp.intel.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14090315-7164-0000-0000-00000452BE98 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Sep 03, 2014 at 01:31:12PM +0200, Peter Zijlstra wrote: > On Tue, Sep 02, 2014 at 09:36:56AM -0700, Paul E. McKenney wrote: > > On Mon, Sep 01, 2014 at 06:17:35PM +0200, Peter Zijlstra wrote: > > > On Mon, Sep 01, 2014 at 09:05:50AM -0700, Paul E. McKenney wrote: > > > > > URGH.. I really hate that. The hotplug interface is already too > > > > > horrible, we should not add such hacks to it. > > > > > > > > We do have try_ interfaces to a number of other subsystems, so I don't > > > > believe that it qualifies as such a hack. > > > > > > We do indeed, but I'm not sure about adding this to the hotplug stuff. > > > > Looks pretty straightforward to me. > > > > > Also; not really understanding the problem doesn't help. > > > > The current implementation of synchronize_sched_expedited() > > calls get_online_cpus(). Some of the ACPI code needs to hold the > > acpi_ioremap_lock mutex across synchronize_sched_expedited(), and > > also needs to acquire this same mutex from a CPU hotplug notifier. > > This results in deadlock between the cpu_hotplug.lock mutex and the > > acpi_ioremap_lock mutex. > > > acpi_ioremap_lock cpu_hotplug_begin() > synchronize_sched() acpi_ioremap_lock > get_online_cpus() > > So yes, AB-BA. > > > Normal RCU grace periods avoid this by synchronizing on a lock acquired by > > the RCU CPU-hotplug notifiers, but this does not work for the expedited > > grace periods because the outgoing CPU can be running random tasks for > > quite some time after RCU's notifier executes. So the fix is just to > > drop back to a normal grace period when there is a CPU-hotplug operation > > in progress. > > So why are we 'normally' doing an expedited call here anyhow? Presumably because they set either the boot parameter or the sysfs variable that causes synchronize_sched() to so synchronize_sched_expedited(). > > > > > How about ripping that rcu_expedited stuff out instead? That's all > > > > > conditional anyhow, so might as well not do it. > > > > > > > > In what way is the expedited stuff conditional? > > > > > > synchronize_sched() conditionally calls synchronize_sched_expedited() > > > and its condition: rcu_expedited, gets set/cleared on pm notifiers and > > > nr_cpu_ids. > > > > There are also direct calls to both synchronize_sched_expedited() and > > synchronize_rcu_expedited(). > > But those are not within hotplug bits. Also weren't we removing them? I > thought we didn't appreciate spraying IPIs like they do? I hadn't heard anything about removing them, but making the expedited primitives a bit less IPI-happy is on my list. Thanx, Paul