On Wed, Jan 13, 2021 at 02:16:10PM +0000, Valentin Schneider wrote:
> On 13/01/21 21:28, Lai Jiangshan wrote:
> > On Tue, Jan 12, 2021 at 10:51 PM Peter Zijlstra <[email protected]>
> > wrote:
> >> @@ -4972,9 +4977,11 @@ static void rebind_workers(struct worker
> >> * of all workers first and then clear UNBOUND. As we're called
> >> * from CPU_ONLINE, the following shouldn't fail.
> >> */
> >> - for_each_pool_worker(worker, pool)
> >> + for_each_pool_worker(worker, pool) {
> >> WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task,
> >> pool->attrs->cpumask) <
> >> 0);
> >> + kthread_set_per_cpu(worker->task, true);
> >
> > Will the schedule break affinity in the middle of these two lines due to
> > patch4 allowing it and result in Paul's reported splat.
> >
>
> You might be right; at this point we would still have BALANCE_PUSH set,
> so something like the below could happen
>
> rebind_workers()
> set_cpus_allowed_ptr()
> affine_move_task()
> task_running() => stop_one_cpu()
>
> ... // Stopper migrates the kworker here in the meantime
>
> switch_to(<pcpu kworker>) // Both cpuhp thread and kworker should be
> enqueued
> // here, so one or the other could be picked
> balance_switch()
> balance_push()
> ^-- no KTHREAD_IS_PER_CPU !
>
> This should however trigger the WARN_ON_ONCE() in kthread_set_per_cpu()
> *before* the one in process_one_work(), which I haven't seen in Paul's
> mails.
The 56 instances of one-hour SRCU-P scenarios hit the WARN_ON_ONCE()
in process_one_work() once, but there is no sign of a WARN_ON_ONCE()
from kthread_set_per_cpu(). But to your point, this does appear to be
a rather low-probability race condition, once per some tens of hours
of SRCU-P.
Is there a more focused check for the race condition above?
Thanx, Paul
