From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752265AbYKLE4l (ORCPT ); Tue, 11 Nov 2008 23:56:41 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751217AbYKLE4d (ORCPT ); Tue, 11 Nov 2008 23:56:33 -0500 Received: from fgwmail6.fujitsu.co.jp ([192.51.44.36]:49385 "EHLO fgwmail6.fujitsu.co.jp" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751213AbYKLE4c (ORCPT ); Tue, 11 Nov 2008 23:56:32 -0500 Date: Wed, 12 Nov 2008 13:55:48 +0900 From: KAMEZAWA Hiroyuki To: balbir@linux.vnet.ibm.com 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. Message-Id: <20081112135548.74503b7b.kamezawa.hiroyu@jp.fujitsu.com> In-Reply-To: <491A6163.4040100@linux.vnet.ibm.com> References: <20081112133002.15c929c3.kamezawa.hiroyu@jp.fujitsu.com> <491A6163.4040100@linux.vnet.ibm.com> Organization: FUJITSU Co. LTD. X-Mailer: Sylpheed 2.5.0 (GTK+ 2.10.14; i686-pc-mingw32) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 12 Nov 2008 10:23:55 +0530 Balbir Singh wrote: > 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. > I agree. > 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)) { > Oh, my bad... will fix soon. Thanks, -Kame