On Thu, Apr 26, 2018 at 09:34:36AM +0530, Kohli, Gaurav wrote:
> On 4/26/2018 1:39 AM, Peter Zijlstra wrote:
>
> > On Wed, Apr 25, 2018 at 02:03:19PM +0530, Gaurav Kohli wrote:
> > > diff --git a/kernel/smpboot.c b/kernel/smpboot.c
> > > index 5043e74..c5c5184 100644
> > > --- a/kernel/smpboot.c
> > > +++ b/kernel/smpboot.c
> > > @@ -122,7 +122,45 @@ static int smpboot_thread_fn(void *data)
> > > }
> > > if (kthread_should_park()) {
> > > + /*
> > > + * Serialize against wakeup.
> > *
> > * Prior wakeups must complete and later wakeups
> > * will observe TASK_RUNNING.
> > *
> > * This avoids the case where the TASK_RUNNING
> > * store from ttwu() competes with the
> > * TASK_PARKED store from kthread_parkme().
> > *
> > * If the TASK_PARKED store looses that
> > * competition, kthread_unpark() will go wobbly.
> > > + */
> > > + raw_spin_lock(¤t->pi_lock);
> > > __set_current_state(TASK_RUNNING);
> > > + raw_spin_unlock(¤t->pi_lock);
> > > preempt_enable();
> > > if (ht->park && td->status == HP_THREAD_ACTIVE)
> > > {
> > > BUG_ON(td->cpu != smp_processor_id());
> > Does that work for you?
>
> We have given patch for testing, usually it takes around 2-3 days for
> reproduction(we will update for the same).
I only changed the comment; surely your compiler doesn't generate
different code for that?
I was asking if the proposed comment was good with you; but see my more
recent email, that actually proposes a different fix.
> > /*
> > * A similar race is possible here, but loosing
> > * the TASK_INTERRUPTIBLE store is harmless and
> > * will make us go around the loop once more.
> > */
>
> Actually instead of race, i am seeing wakeup miss problem which is
> very rare, if we take case of hotplug thread
Yes, triggering these issues is tricky, no doubt about that.
> > And of course, I suspect we actually want to use TASK_IDLE, smpboot
> > threads don't want signals do they? But that probably ought to be a
> > separate patch.
>
> Yes I agree, we can control race from here as well, Please suggest
> would below change be any help here:
That is not what I suggested. I said the thing should use TASK_IDLE
instead of TASK_INTERRUPTIBLE. Not change the location of it.
