Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against wakeup

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Apr 26, 2018 at 09:23:25PM +0530, Kohli, Gaurav wrote:
> On 4/26/2018 2:27 PM, Peter Zijlstra wrote:
> 
> > On Thu, Apr 26, 2018 at 10:41:31AM +0200, Peter Zijlstra wrote:
> > > diff --git a/kernel/kthread.c b/kernel/kthread.c
> > > index cd50e99202b0..4b6503c6a029 100644
> > > --- a/kernel/kthread.c
> > > +++ b/kernel/kthread.c
> > > @@ -177,12 +177,13 @@ void *kthread_probe_data(struct task_struct *task)
> > >   static void __kthread_parkme(struct kthread *self)
> > >   {
> > > -	__set_current_state(TASK_PARKED);
> > > -	while (test_bit(KTHREAD_SHOULD_PARK, &self->flags)) {
> > > +	for (;;) {
> > > +		__set_task_state(TASK_PARKED);
> > 		set_current_state(TASK_PARKED);
> > 
> > of course..
> 
> Hi Peter,
> 
> Maybe i am missing something , but still that race can come as we don't put task_parked on special state.
> 
> Controller                                                                       Hotplug
> 
>                                                                                  Loop
> 
>                                                                                  Task_Interruptible
> 
> Set SHOULD_PARK
> 
> wakeup -> Proceeds
> 
>                                                                                   Set Running
> 
>                                                                                   kthread_parkme
> 
>                                                                                   SET TASK_PARKED
> 
>                                                                                   schedule
> 
> Set TASK_RUNNING
> 
> Can you please correct ME, if I misunderstood this.

If that could happen, all wait-loops would be broken. However,
AFAICT that cannot happen, because ttwu_remote() and schedule()
serialize on rq->lock. See:


A						B

for (;;) {
	set_current_state(UNINTERRUPTIBLE);

						cond1 = true;
						wake_up_process(A)
						  lock(A->pi_lock)
						  smp_mb__after_spinlock()
						  if (A->state & TASK_NORMAL)
						    A->on_rq && ttwu_remote()
	if (cond1) // true
		break;
	schedule();
}
__set_current_state(RUNNING);

for (;;) {
	set_current_state(UNINTERRUPTIBLE);
	if (cond2)
		break;

	schedule();
	  lock(rq->lock)
	  smp_mb__after_spinlock();
	  deactivate_task(A);
	  <sched-out>
	  unlock(rq->lock);
						      rq = __task_rq_lock(A)
						      if (A->on_rq) // false
						        A->state = TASK_RUNNING;
						      __task_rq_unlock(rq)


Either A's schedule() must observe RUNNING (not shown) or B must
observe !A->on_rq (shown) and not issue the store.