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Date: Wed, 18 Mar 2009 08:48:32 +0530
From: Bharata B Rao <bharata@linux.vnet.ibm.com>
To: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>, linux-kernel@vger.kernel.org,
       Dhaval Giani <dhaval@linux.vnet.ibm.com>,
       Paul Menage <menage@google.com>, Ingo Molnar <mingo@elte.hu>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Subject: Re: [PATCH -tip] cpuacct: Make cpuacct hierarchy walk in
	cpuacct_charge() safe when rcupreempt is used.
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On Tue, Mar 17, 2009 at 06:42:51PM +0530, Balbir Singh wrote:
> * Bharata B Rao <bharata@linux.vnet.ibm.com> [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 <balbir@linux.vnet.ibm.com>
> Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>

So I guess this ack is not for this patch but for the per-cgroup
stime/utime cpuacct controller statistics patch.

Regards,
Bharata.