Re: [PATCH -tip] cpuacct: Make cpuacct hierarchy walk incpuacct_charge() safe when rcupreempt is used.

[Balbir Singh]

* Li Zefan <lizf@xxxxxxxxxxxxxx> [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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/