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

[Kohli, Gaurav]

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

ÂÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ Loop

ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂTask_Interruptible

Set SHOULD_PARK

wakeup -> Proceeds

ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ Â Set Running

ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ Â kthread_parkme

ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ Â SET TASK_PARKED

ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ ÂÂÂ Â schedule

Set TASK_RUNNING

Can you please correct ME, if I misunderstood this.

> > +		if (!test_bit(KTHREAD_SHOULD_PARK, &self->flags))
> > +			break;
> >   		if (!test_and_set_bit(KTHREAD_IS_PARKED, &self->flags))
> >   			complete(&self->parked);
> >   		schedule();
> > -		__set_current_state(TASK_PARKED);
> >   	}
> >   	clear_bit(KTHREAD_IS_PARKED, &self->flags);
> >   	__set_current_state(TASK_RUNNING);
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