Re: [RFC PATCH -tip] cpuacct: per-cgroup utime/stime statistics -v2

[Bharata B Rao]

On Mon, Mar 16, 2009 at 03:13:38PM +0800, Li Zefan wrote:
> >>> +/*
> >>> + * Account the system/user time to the task's accounting group.
> >>> + */
> >>> +static void cpuacct_update_stats(struct task_struct *tsk,
> >>> +		enum cpuacct_stat_index idx, cputime_t val)
> >>> +{
> >>> +	struct cpuacct *ca;
> >>> +
> >>> +	if (unlikely(!cpuacct_subsys.active))
> >>> +		return;
> >>> +
> >>> +	ca = task_ca(tsk);
> >>> +
> >>> +	do {
> >>> +		percpu_counter_add(&ca->cpustat[idx], val);
> >>> +		ca = ca->parent;
> >>> +	} while (ca);
> >>> +}
> >>> +
> >> IIUC, to make sure accessing "ca" to be safe, we need some condition.
> >> (task_lock() or some other.....
> > 
> > task_lock() protects tsk->cgroups->subsys[]. So can we hold task_lock()
> > to protect this walk ? But we do this cpuacct hierarchy walk for the
> > current task here. So can a current task's ca or ca's parents disappear
> > from under us ?
> > 
> 
> task_ca() should be protected by task_lock() or rcu_read_lock(), otherwise
> there is a very small race:
> 
>   ca = task_ca(tsk)
> 				move @tsk to another cgroup
> 				rmdir old_cgrp (thus ca is freed)
>   ca->cpustat <--- accessing freed memory
> 
> As KAMEZAWA-san said all updates are called under preempt-disabled, and
> classic and tree rcu's rcu_read_lock does preempt disable only, so above
> code is ok, except for rcupreempt.

So I will protect task_ca() and ca hierarchy walk with explicit
rcu_read_lock() to be fully safe.

By the same logic, hierarchy walk in cpuacct_charge() is also
not safe with rcupreempt. It is under preempt disabled section due
to rq->lock. Does cpuacct_charge() also need a fix then ?

Regards,
Bharata.
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