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Date:	Wed, 18 Mar 2009 09:40:44 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	balbir@...ux.vnet.ibm.com
CC:	bharata@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Paul Menage <menage@...gle.com>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH -tip] cpuacct: Make cpuacct hierarchy walk in	cpuacct_charge()
 safe when rcupreempt is used.

Balbir Singh wrote:
> * Li Zefan <lizf@...fujitsu.com> [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.

> 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.
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