From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759407AbZCSJnS (ORCPT ); Thu, 19 Mar 2009 05:43:18 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755013AbZCSJnA (ORCPT ); Thu, 19 Mar 2009 05:43:00 -0400 Received: from e37.co.us.ibm.com ([32.97.110.158]:47573 "EHLO e37.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754055AbZCSJm7 (ORCPT ); Thu, 19 Mar 2009 05:42:59 -0400 Date: Thu, 19 Mar 2009 15:13:17 +0530 From: Bharata B Rao To: Peter Zijlstra Cc: Li Zefan , linux-kernel@vger.kernel.org, Dhaval Giani , Balbir Singh , Paul Menage , Ingo Molnar , KAMEZAWA Hiroyuki Subject: Re: [PATCH -tip] cpuacct: Make cpuacct hierarchy walk in cpuacct_charge() safe when rcupreempt is used. Message-ID: <20090319094317.GC3303@in.ibm.com> Reply-To: bharata@linux.vnet.ibm.com References: <20090317061754.GD3314@in.ibm.com> <49BF42FB.4030103@cn.fujitsu.com> <20090317073649.GH3314@in.ibm.com> <1237454421.7867.27.camel@twins> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1237454421.7867.27.camel@twins> 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 On Thu, Mar 19, 2009 at 10:20:21AM +0100, Peter Zijlstra wrote: > On Tue, 2009-03-17 at 13:06 +0530, Bharata B Rao wrote: > > 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(). > > > > Signed-off-by: Bharata B Rao > > I would ditch the comment, it doesn't add anything. > > The simple rule is: if you want RCU-safe, use rcu_read_lock(). > preempt/irq disable isn't sufficient -- hasn't been for a long long > while. > > After that, > > Acked-by: Peter Zijlstra > Ok. Removed the comment. Here is the updated patch. 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(). Signed-off-by: Bharata B Rao Acked-by: Peter Zijlstra Acked-by: Balbir Singh Tested-by: Balbir Singh --- kernel/sched.c | 3 +++ 1 file changed, 3 insertions(+) --- a/kernel/sched.c +++ b/kernel/sched.c @@ -9894,6 +9894,8 @@ static void cpuacct_charge(struct task_s return; cpu = task_cpu(tsk); + + rcu_read_lock(); ca = task_ca(tsk); do { @@ -9901,6 +9903,7 @@ static void cpuacct_charge(struct task_s *cpuusage += cputime; ca = ca->parent; } while (ca); + rcu_read_unlock(); } struct cgroup_subsys cpuacct_subsys = {