linux-kernel - Re: [PATCH v2 09/14] Keep nr

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Date:	Thu, 10 Nov 2011 13:27:21 +0300
From:	Andrew Wagin <avagin@...il.com>
To:	Glauber Costa <glommer@...allels.com>
Cc:	linux-kernel@...r.kernel.org, paul@...lmenage.org,
	lizf@...fujitsu.com, daniel.lezcano@...e.fr,
	a.p.zijlstra@...llo.nl, jbottomley@...allels.com, pjt@...gle.com,
	fweisbec@...il.com
Subject: Re: [PATCH v2 09/14] Keep nr_iowait per cgroup

This code has a race condition.
A task may be moved in another group between  task_group_nr_iowait_inc
and task_group_nr_iowait_dec.

# dd if=/dev/sda of=//dev/null bs=4095 & pid=$!
# echo $pid > /cgroup/1/tasks
# kill $pid
# cat /cgroup/1/cpu.proc.stat  | grep procs_blocked
procs_blocked 18446744073709551615

> +static inline void task_group_nr_iowait_inc(struct task_struct *p, int cpu)
> +{
> +
> +       atomic_inc(&per_cpu(kernel_cpustat, cpu).nr_iowait);
> +
> +#ifdef CONFIG_CGROUP_SCHED
> +       if (static_branch(&sched_cgroup_enabled)) {
> +               struct kernel_cpustat *kcpustat;
> +               struct task_group *tg;
> +
> +               rcu_read_lock();
> +               tg = task_group(p);
> +               while (tg && (tg != &root_task_group)) {
> +                       kcpustat = per_cpu_ptr(tg->cpustat, cpu);
> +                       atomic_inc(&kcpustat->nr_iowait);
> +                       tg = tg->parent;
> +               }
> +               rcu_read_unlock();
> +       }
> +#endif
> +}
> +
> +static inline void task_group_nr_iowait_dec(struct task_struct *p, int cpu)
> +{
> +
> +       atomic_dec(&per_cpu(kernel_cpustat, cpu).nr_iowait);
> +
> +#ifdef CONFIG_CGROUP_SCHED
> +       if (static_branch(&sched_cgroup_enabled)) {
> +               struct kernel_cpustat *kcpustat;
> +               struct task_group *tg;
> +
> +               rcu_read_lock();
> +               tg = task_group(p);
> +               while (tg && (tg != &root_task_group)) {
> +                       kcpustat = per_cpu_ptr(tg->cpustat, cpu);
> +                       atomic_dec(&kcpustat->nr_iowait);
> +                       tg = tg->parent;
> +               }
> +               rcu_read_unlock();
> +       }
> +#endif
> +}
> +
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