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

[Wanpeng Li]

On 8/25/15 6:05 PM, Peter Zijlstra wrote:
> 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.

Indeed, it should be fixed in migration path. I will try to fight it out 
and post a patch. :)

Regards,
Wanpeng Li

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