Re: [PATCH 2/2] sched: Plug race between SCA, hotplug and migration_cpu_stop()

[Will Deacon]

On Tue, Jun 01, 2021 at 05:59:56PM +0100, Valentin Schneider wrote:
> On 26/05/21 21:57, Valentin Schneider wrote:
> > +		dest_cpu = arg->dest_cpu;
> > +		if (task_on_rq_queued(p)) {
> > +			/*
> > +			 * A hotplug operation could have happened between
> > +			 * set_cpus_allowed_ptr() and here, making dest_cpu no
> > +			 * longer allowed.
> > +			 */
> > +			if (!is_cpu_allowed(p, dest_cpu))
> > +				dest_cpu = select_fallback_rq(cpu_of(rq), p);
> > +			/*
> > +			 * dest_cpu can be victim of hotplug between is_cpu_allowed()
> > +			 * and here. However, per the synchronize_rcu() in
> > +			 * sched_cpu_deactivate(), it can't have gone lower than
> > +			 * CPUHP_AP_ACTIVE, so it's safe to punt it over and let
> > +			 * balance_push() route it elsewhere.
> > +			 */
> > +			update_rq_clock(rq);
> > +			rq = move_queued_task(rq, &rf, p, dest_cpu);
> 
> So, while digesting this I started having doubts vs pcpu kthreads since
> they're allowed on online CPUs. The bogus scenario here would be picking a
> !active && online CPU, and see it go !online before the move_queued_task().
> 
> Now, to transition from online -> !online, we have to go through
> take_cpu_down() which is issued via a stop_machine() call. This means the
> transition can't happen until all online CPUs are running the stopper task
> and reach MULTI_STOP_RUN.
> 
> migration_cpu_stop() being already a stopper callback should thus make it
> "atomic" vs takedown_cpu(), meaning the above should be fine.

I'd be more inclined to agree with your reasoning if migration_cpu_stop()
couldn't itself call stop_one_cpu_nowait() to queue more work for the
stopper thread. What guarantees that takedown_cpu() can't queue its stopper
work in the middle of that?

Will