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

[Kohli, Gaurav]

On 4/24/2018 11:56 PM, Peter Zijlstra wrote:

> On Tue, Apr 24, 2018 at 02:58:25PM +0530, Gaurav Kohli wrote:
> > The control cpu thread which initiates hotplug calls kthread_park()
> > for hotplug thread and sets KTHREAD_SHOULD_PARK. After this control
> > thread wakes up the hotplug thread. There is a chance that wakeup
> > code sees the hotplug thread (running on AP core) in INTERRUPTIBLE
> > state, but sets its state to RUNNING after hotplug thread has entered
> > kthread_parkme() and changed its state to TASK_PARKED. This can result
> > in panic later on in kthread_unpark(), as it sees KTHREAD_IS_PARKED
> > flag set but fails to rebind the kthread, due to it being not in
> > TASK_PARKED state. Fix this, by serializing wakeup state change,
> > against state change before parking the kthread.
> >
> > Below is the possible race:
> >
> > Control thread				      Hotplug Thread
> >
> > kthread_park()
> > set KTHREAD_SHOULD_PARK
> > 					      smpboot_thread_fn
> > 					      set_current_state(TASK_INTERRUPTIBLE);
> > 					      kthread_parkme
> >
> > wake_up_process()
> >
> > raw_spin_lock_irqsave(&p->pi_lock, flags);
> > if (!(p->state & state)) -> this will fail
> >              goto out;
> >
> > 					      __kthread_parkme
> > 					       __set_current_state(TASK_PARKED);
> >
> > if (p->on_rq && ttwu_remote(p, wake_flags))
> >      ttwu_remote()
> >          p->state = TASK_RUNNING;
> > 						schedule();
> >
> > So to avoid this race, take pi_lock to serial state changes.
> >
> > Suggested-by: Pavankumar Kondeti <pkondeti@xxxxxxxxxxxxxx>
> > Co-developed-by: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
> > Signed-off-by: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
> > Signed-off-by: Gaurav Kohli <gkohli@xxxxxxxxxxxxxx>
> >
> > diff --git a/kernel/smpboot.c b/kernel/smpboot.c
> > index 1650578..514b232 100644
> > --- a/kernel/smpboot.c
> > +++ b/kernel/smpboot.c
> > @@ -121,7 +121,9 @@ static int smpboot_thread_fn(void *data)
> >   		}
> >   
> >   		if (kthread_should_park()) {
> > +			raw_spin_lock(&current->pi_lock);
> >   			__set_current_state(TASK_RUNNING);
> > +			raw_spin_unlock(&current->pi_lock);
> >   			preempt_enable();
> >   			if (ht->park && td->status == HP_THREAD_ACTIVE) {
> >   				BUG_ON(td->cpu != smp_processor_id());
> Note how in your scenario above you didn't actually need the
> TASK_RUNNING state; so how is this change going to fix anything?

Hi Peter,

As with help of this , if kthread_should_park run first so wake_up call of controller

get exited as task is already set as running, otherwise if controller runs first

then we will block here and set running and then sets TASK_PARKED .

So no chance of cpuhp set as running during kthread_parkme call.

But as we discussed this can be fix by 2nd patch as well, So once you get time and able to

see , Please let us know or do you want me to try your 2nd patch

for testing first?

> But yes, I suspect it is right, but it definitely needs a comment
> explaining wth we take that lock there.
>
> Like I said earlier, my brain is entirely fried for the day; but I'll
> have a try tomorrow.
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