Re: [PATCH v3 6/6] freezer,sched: Rewrite core freezer logic

[Peter Zijlstra]

On Thu, Oct 27, 2022 at 01:58:09PM +0800, Chen Yu wrote:

> > It's a very narrow race between schedule() and task_call_func().
> > 
> >   CPU0						CPU1
> > 
> >   __schedule()
> >     rq_lock();
> >     prev_state = READ_ONCE(prev->__state);
> >     if (... && prev_state) {
> >       deactivate_tasl(rq, prev, ...)
> >         prev->on_rq = 0;
> > 
> > 						task_call_func()
> > 						  raw_spin_lock_irqsave(p->pi_lock);
> > 						  state = READ_ONCE(p->__state);
> > 						  smp_rmb();
> > 						  if (... || p->on_rq) // false!!!
> > 						    rq = __task_rq_lock()
> > 
> > 						  ret = func();
> > 
> >     next = pick_next_task();
> >     rq = context_switch(prev, next)
> >       prepare_lock_switch()
> >         spin_release(&__rq_lockp(rq)->dep_map...)
> > 
> > 
> > 
> > So while the task is on it's way out, it still holds rq->lock for a
> > little while, and right then task_call_func() comes in and figures it
> > doesn't need rq->lock anymore (because the task is already dequeued --
> > but still running there) and then the __set_task_frozen() thing observes
> > it's holding rq->lock and yells murder.
> > 
> > Could you please give the below a spin?
> > 
> > ---
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index cb2aa2b54c7a..f519f44cd4c7 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -4200,6 +4200,37 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
> >  	return success;
> >  }
> >  
> > +static bool __task_needs_rq_lock(struct task_struct *p)
> > +{
> > +	unsigned int state = READ_ONCE(p->__state);
> > +
> > +	/*
> > +	 * Since pi->lock blocks try_to_wake_up(), we don't need rq->lock when
> > +	 * the task is blocked. Make sure to check @state since ttwu() can drop
> > +	 * locks at the end, see ttwu_queue_wakelist().
> > +	 */
> > +	if (state == TASK_RUNNING || state == TASK_WAKING)
> > +		return true;
> > +
> > +	/*
> > +	 * Ensure we load p->on_rq after p->__state, otherwise it would be
> > +	 * possible to, falsely, observe p->on_rq == 0.
> > +	 *
> > +	 * See try_to_wake_up() for a longer comment.
> > +	 */
> > +	smp_rmb();
> > +	if (p->on_rq)
> > +		return true;
> > +
> > +#ifdef CONFIG_SMP
> > +	smp_rmb();
> > +	if (p->on_cpu)
> > +		return true;
> > +#endif
> Should we also add p->on_cpu check to return 0 in __set_task_frozen()?
> Otherwise it might still warn that p is holding the lock?

With this, I don't think __set_task_frozen() should ever see
'p->on_cpu && !p->on_rq'. By forcing task_call_func() to acquire
rq->lock that window is closed. That is, this window only exits in
__schedule() while it holds rq->lock, since we're now serializing
against that, we should no longer observe it.