Messages in this thread | | | Date | Wed, 2 Jun 2021 16:26:29 +0100 | From | Will Deacon <> | Subject | Re: [PATCH 2/2] sched: Plug race between SCA, hotplug and migration_cpu_stop() |
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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
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