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SubjectRe: [PATCH v1] kthread/smpboot: Serialize kthread parking against wakeup
On Tue, Jun 05, 2018 at 05:08:41PM +0200, Oleg Nesterov wrote:

> On 06/05, Kohli, Gaurav wrote:
> >
> > As last mentioned on mail, we are still seeing issue with the latest
> > approach and below is the susceptible race as mentioned earlier..
> > controller Thread CPUHP Thread
> > takedown_cpu
> > kthread_park
> > kthread_parkme
> > Set KTHREAD_SHOULD_PARK
> > smpboot_thread_fn
> > set Task interruptible
> >
> >
> > wake_up_process
> > if (!(p->state & state))
> > goto out;
> >
> > Kthread_parkme
> > SET TASK_PARKED
> > schedule
> > raw_spin_lock(&rq->lock)
> > ttwu_remote
> > waiting for __task_rq_lock
> > context_switch
> >
> > finish_lock_switch
> >
> >
> >
> > Case TASK_PARKED
> > kthread_park_complete
> >
> >
> > SET Running
>
> I think you are right.
>
> And, now that I look at 85f1abe0019fcb3ea10df7029056cf42702283a8
> ("kthread, sched/wait: Fix kthread_parkme() completion issue") I see this note
> int the changelog:
>
> The alternative is to promote TASK_PARKED to a special state, this
> guarantees wait_task_inactive() cannot observe a 'stale' TASK_RUNNING
> and we'll end up doing the right thing, but this preserves the whole
> icky business of potentially migating the still runnable thing.
>
> OK, but __kthread_parkme() can be preempted before it calls schedule(), so the
> caller still can be migrated? Plus kthread_park_complete() can be called twice.

Argh... I forgot TASK_DEAD does the whole thing with preempt_disable().
Let me stare at that a bit.

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