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Subject: Re: [PATCH 04/10] Display /proc/stat information per cgroup
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Glauber Costa <glommer@parallels.com>
Cc: linux-kernel@vger.kernel.org, paul@paulmenage.org, lizf@cn.fujitsu.com,
        daniel.lezcano@free.fr, jbottomley@parallels.com
Date: Wed, 05 Oct 2011 14:38:14 +0200
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On Wed, 2011-10-05 at 16:10 +0400, Glauber Costa wrote:
> On 10/05/2011 12:56 PM, Peter Zijlstra wrote:
> > On Sun, 2011-10-02 at 23:21 +0400, Glauber Costa wrote:
> >> +struct kernel_stat *task_group_kstat(struct task_struct *p)
> >> +{
> >> +       struct task_group *tg;
> >> +       struct kernel_stat *kstat;
> >> +
> >> +       rcu_read_lock();
> >> +       tg = task_group(p);
> >> +       kstat = tg->cpustat;
> >> +       rcu_read_unlock();
> >> +       return kstat;
> >> +}
> >
> > Who keeps tg alive and kicking while you poke at its (cpustat) member?
> 
> * All calls to this function currently pass current as a parameter 
> (Okay, maybe it is too generic and we should pass nothing at all, and 
> grab current within it)
> * rcu_read_lock() guarantees that current will exist during this call, 
> and task_group won't change. (right?)

The thing I worry about is:

A (pid n)				B

kstat = task_group_kstat()
					echo n > /cgroup/something-else/pid
					rmdir /cgroup/group-that-had-A
<timer interrupt>
  RCU complete
  <softirq>
    kfree(tg) etc..

kstat->foo++; <-- *BOOM*


The only way to avoid someone moving you around is by holding some
cgroup lock, task->alloc_lock, task->pi_lock or the rq->lock where task
runs. Alternatively keep rcu_read_lock() around the entire kstat usage.