Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751735AbdIEITY (ORCPT ); Tue, 5 Sep 2017 04:19:24 -0400 Received: from Galois.linutronix.de ([146.0.238.70]:44262 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751635AbdIEITW (ORCPT ); Tue, 5 Sep 2017 04:19:22 -0400 Date: Tue, 5 Sep 2017 10:19:13 +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: <20170904140353.k5mo3f4wela5nxqe@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> 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 Content-Length: 945 Lines: 27 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. 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. Thanks, tglx