Re: [PATCH] sched: fix tsk->pi_lock isn't held when do_set_cpus_allowed()

[Peter Zijlstra]

On Tue, Aug 25, 2015 at 03:59:54PM +0800, Wanpeng Li wrote:
> +++ b/kernel/cpuset.c
> @@ -2376,8 +2376,12 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)
>  
>  void cpuset_cpus_allowed_fallback(struct task_struct *tsk)
>  {
> +	unsigned long flags;
> +
>  	rcu_read_lock();
> +	raw_spin_lock_irqsave(&tsk->pi_lock, flags);
>  	do_set_cpus_allowed(tsk, task_cs(tsk)->effective_cpus);
> +	raw_spin_lock_irqsave(&tsk->pi_lock, flags);
>  	rcu_read_unlock();

Aside from the double lock thing that was already pointed out, I think
this is wrong, because the select_task_rq() call can already have
pi_lock held.

Taking it again would result in a deadlock.

Consider for instance:

try_to_wake_up()
  raw_spin_lock_irqsave(->pi_lock)
  select_task_rq()
    select_ballback_rq()
      cpuset_cpus_allowed_fallback()
        raw_spin_lock_irqsave(->pi_lock)


The problem is with the migration path and should be fixed there.
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