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

[Peter Zijlstra]

On Tue, Aug 25, 2015 at 12:05:27PM +0200, 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.

Another problem, migration_call() will have rq->lock held, so you're
proposing to acquire pi_lock while holding rq->lock, this is an
inversion from the regular nesting order.


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