From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751984AbYKLEy1 (ORCPT ); Tue, 11 Nov 2008 23:54:27 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751207AbYKLEyR (ORCPT ); Tue, 11 Nov 2008 23:54:17 -0500 Received: from e28smtp07.in.ibm.com ([59.145.155.7]:40042 "EHLO e28smtp07.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751144AbYKLEyQ (ORCPT ); Tue, 11 Nov 2008 23:54:16 -0500 Message-ID: <491A6163.4040100@linux.vnet.ibm.com> Date: Wed, 12 Nov 2008 10:23:55 +0530 From: Balbir Singh Reply-To: balbir@linux.vnet.ibm.com Organization: IBM User-Agent: Thunderbird 2.0.0.17 (X11/20080925) MIME-Version: 1.0 To: KAMEZAWA Hiroyuki CC: "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , "menage@google.com" , "nishimura@mxp.nes.nec.co.jp" , "lizf@cn.fujitsu.com" , "akpm@linux-foundation.org" Subject: Re: [PATCH] [BUGFIX]cgroup: fix potential deadlock in pre_destroy. References: <20081112133002.15c929c3.kamezawa.hiroyu@jp.fujitsu.com> In-Reply-To: <20081112133002.15c929c3.kamezawa.hiroyu@jp.fujitsu.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org KAMEZAWA Hiroyuki wrote: > Balbir, Paul, Li, How about this ? > = > As Balbir pointed out, memcg's pre_destroy handler has potential deadlock. > > It has following lock sequence. > > cgroup_mutex (cgroup_rmdir) > -> pre_destroy > -> mem_cgroup_pre_destroy > -> force_empty > -> lru_add_drain_all-> > -> schedule_work_on_all_cpus > -> get_online_cpus -> cpuhotplug.lock. > > But, cpuset has following. > cpu_hotplug.lock (call notifier) > -> cgroup_mutex. (within notifier) > > Then, this lock sequence should be fixed. > > Considering how pre_destroy works, it's not necessary to holding > cgroup_mutex() while calling it. > > As side effect, we don't have to wait at this mutex while memcg's force_empty > works.(it can be long when there are tons of pages.) > > Note: memcg is an only user of pre_destroy, now. > I thought about this and it seems promising. My concern is that with cgroup_mutex given, the state of cgroup within pre-destroy will be unpredictable. I suspect, if pre-destory really needs cgroup_mutex, we can hold it within pre-destroy. BTW, your last check, does not seem right + if (atomic_read(&cgrp->count) + || list_empty(&cgrp->children) Why should list_empty() result in EBUSY, shouldn't it be !list_empty()? + || cgroup_has_css_refs(cgrp)) { -- Balbir