From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751376AbdIELEP (ORCPT ); Tue, 5 Sep 2017 07:04:15 -0400 Received: from Galois.linutronix.de ([146.0.238.70]:44874 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750762AbdIELEM (ORCPT ); Tue, 5 Sep 2017 07:04:12 -0400 Date: Tue, 5 Sep 2017 13:04:05 +0200 (CEST) From: Thomas Gleixner To: Michal Hocko cc: Johannes Weiner , Artem Savkov , "Paul E. McKenney" , LKML , linux-mm@kvack.org Subject: Re: possible circular locking dependency mmap_sem/cpu_hotplug_lock.rw_sem In-Reply-To: <20170905102336.bqxb7tltnt3lphkq@dhcp22.suse.cz> Message-ID: References: <20170807140947.nhfz2gel6wytl6ia@shodan.usersys.redhat.com> <20170830141543.qhipikpog6mkqe5b@dhcp22.suse.cz> <20170830154315.sa57wasw64rvnuhe@dhcp22.suse.cz> <20170904140353.k5mo3f4wela5nxqe@dhcp22.suse.cz> <20170905102336.bqxb7tltnt3lphkq@dhcp22.suse.cz> User-Agent: Alpine 2.20 (DEB 67 2015-01-07) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 5 Sep 2017, Michal Hocko wrote: > On Tue 05-09-17 10:19:13, Thomas Gleixner wrote: > > On Mon, 4 Sep 2017, Michal Hocko wrote: > > > > > Thomas, Johannes, > > > could you double check my thinking here? I will repost the patch to > > > Andrew if you are OK with this. > > > > + /* > > > > + * The only protection from memory hotplug vs. drain_stock races is > > > > + * that we always operate on local CPU stock here with IRQ disabled > > > > + */ > > > > local_irq_save(flags); > > > > > > > > stock = this_cpu_ptr(&memcg_stock); > > > > @@ -1807,26 +1811,27 @@ static void drain_all_stock(struct mem_cgroup *root_memcg) > > > > if (!mutex_trylock(&percpu_charge_mutex)) > > > > return; > > > > /* Notify other cpus that system-wide "drain" is running */ > > > > - get_online_cpus(); > > > > curcpu = get_cpu(); > > > > The problem here is that this does only protect you against a CPU being > > unplugged, but not against a CPU coming online concurrently. > > Yes but same as the drain_all_pages we do not have any cpu up specific > intialization so there is no specific action to race against AFAICS. > > > I have no idea > > whether that might be a problem, but at least you should put a comment in > > which explains why it is not. > > What about this? Looks good. > --- > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 5c70f47abb3d..ff9b0979ccc3 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -1810,7 +1810,12 @@ static void drain_all_stock(struct mem_cgroup *root_memcg) > /* If someone's already draining, avoid adding running more workers. */ > if (!mutex_trylock(&percpu_charge_mutex)) > return; > - /* Notify other cpus that system-wide "drain" is running */ > + /* > + * Notify other cpus that system-wide "drain" is running > + * We do not care about races with the cpu hotplug because cpu down > + * as well as workers from this path always operate on the local > + * per-cpu data. CPU up doesn't touch memcg_stock at all. > + */ > curcpu = get_cpu(); > for_each_online_cpu(cpu) { > struct memcg_stock_pcp *stock = &per_cpu(memcg_stock, cpu); > > -- > Michal Hocko > SUSE Labs >