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

From: Li Zefan
Date: Mon Mar 16 2009 - 03:13:43 EST


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

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