Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757358AbZCRC77 (ORCPT ); Tue, 17 Mar 2009 22:59:59 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755421AbZCRC7t (ORCPT ); Tue, 17 Mar 2009 22:59:49 -0400 Received: from e28smtp07.in.ibm.com ([59.145.155.7]:57483 "EHLO e28smtp07.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754937AbZCRC7t (ORCPT ); Tue, 17 Mar 2009 22:59:49 -0400 Date: Wed, 18 Mar 2009 08:29:33 +0530 From: Balbir Singh To: Li Zefan Cc: bharata@linux.vnet.ibm.com, 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: <20090318025933.GW16897@balbir.in.ibm.com> Reply-To: balbir@linux.vnet.ibm.com References: <20090317061754.GD3314@in.ibm.com> <49BF42FB.4030103@cn.fujitsu.com> <20090317124031.GT16897@balbir.in.ibm.com> <49C0511C.9030508@cn.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <49C0511C.9030508@cn.fujitsu.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 Content-Length: 1612 Lines: 46 * Li Zefan [2009-03-18 09:40:44]: > Balbir Singh wrote: > > * Li Zefan [2009-03-17 14:28:11]: > > > >> 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[..] > >> > > > > That problem should only occur if we dereference tsk->cgroups > > separately and then use that to dereference cg->subsys. Since we use > > Do you mean tsk->cgroups->subsys is safe and > cg = tsk->cgroups;...; cg->subsys is unsafe ? > This is wrong. Without rcu_read_lock/unlock they are unsafe, even with the lock, we need to use rcu_dereference() to make sure we get consistent values. > > > task_subsys_state() and that is RCU safe, we should be OK. > > > > Yes, it's RCU safe, which means it's unsafe without rcu_read_lock/unlock. > Yes, I meant under rcu_read_lock/unlock. -- Balbir -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/