linux-kernel - Re: [PATCH -tip] cpuacct: Make cpuacct hierarchy walk in cpuacct

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Date:	Wed, 18 Mar 2009 08:29:33 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Li Zefan <lizf@...fujitsu.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.

* Li Zefan <lizf@...fujitsu.com> [2009-03-18 09:40:44]:

> 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.

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
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