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

[Li Zefan <lizf@cn.fujitsu.com>]

>>> +/*
>>> + * 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.