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SubjectRe: [PATCH -tip] cpuacct: Make cpuacct hierarchy walk in cpuacct_charge() safe when rcupreempt is used.
* Li Zefan <lizf@cn.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
task_subsys_state() and that is RCU safe, we should be OK.

--
Balbir


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