From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1763332AbZCQNNk (ORCPT ); Tue, 17 Mar 2009 09:13:40 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754071AbZCQNNb (ORCPT ); Tue, 17 Mar 2009 09:13:31 -0400 Received: from e28smtp06.in.ibm.com ([59.145.155.6]:53986 "EHLO e28smtp06.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753997AbZCQNNa (ORCPT ); Tue, 17 Mar 2009 09:13:30 -0400 Date: Tue, 17 Mar 2009 18:42:51 +0530 From: Balbir Singh To: Bharata B Rao Cc: Li Zefan , linux-kernel@vger.kernel.org, Dhaval Giani , Paul Menage , Ingo Molnar , Peter Zijlstra , KAMEZAWA Hiroyuki Subject: Re: [PATCH -tip] cpuacct: Make cpuacct hierarchy walk in cpuacct_charge() safe when rcupreempt is used. Message-ID: <20090317131251.GU16897@balbir.in.ibm.com> Reply-To: balbir@linux.vnet.ibm.com References: <20090317061754.GD3314@in.ibm.com> <49BF42FB.4030103@cn.fujitsu.com> <20090317073649.GH3314@in.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <20090317073649.GH3314@in.ibm.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Bharata B Rao [2009-03-17 13:06:49]: > On Tue, Mar 17, 2009 at 02:28:11PM +0800, Li Zefan wrote: > > Bharata B Rao wrote: > > > cpuacct: Make cpuacct hierarchy walk in cpuacct_charge() safe when > > > rcupreempt is used. > > > > > > cpuacct_charge() obtains task's ca and does a hierarchy walk upwards. > > > This can race with the task's movement between cgroups. This race > > > can cause an access to freed ca pointer in cpuacct_charge(). This will not > > > > Actually it can also end up access invalid tsk->cgroups. ;) > > > > get tsk->cgroups (cg) > > (move tsk to another cgroup) or (tsk exiting) > > -> kfree(tsk->cgroups) > > get cg->subsys[..] > > Ok :) Here is the patch again with updated description. > > cpuacct: Make cpuacct hierarchy walk in cpuacct_charge() safe when > rcupreempt is used. > > cpuacct_charge() obtains task's ca and does a hierarchy walk upwards. > This can race with the task's movement between cgroups. This race > can cause an access to freed ca pointer in cpuacct_charge() or access > to invalid cgroups pointer of the task. This will not happen with rcu or > tree rcu as cpuacct_charge() is called with preemption disabled. However if > rcupreempt is used, the race is seen. Thanks to Li Zefan for explaining this. > > Fix this race by explicitly protecting ca and the hierarchy walk with > rcu_read_lock(). > Looks good and works very well (except for the batch issue that you pointed out, it takes up to batch values before updates are seen). I'd like to get the patches in -tip and see the results, I would recommend using percpu_counter_sum() while reading the data as an enhancement to this patch. If user space does not overwhelm with a lot of reads, sum would work out better. Tested-by: Balbir Singh Acked-by: Balbir Singh -- Balbir