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

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Date:   Tue, 5 Jun 2018 17:08:41 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     "Kohli, Gaurav" <gkohli@...eaurora.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, tglx@...utronix.de,
        mpe@...erman.id.au, mingo@...nel.org, bigeasy@...utronix.de,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against
 wakeup

I have to admit that I didn't try to follow this discussion, somehow I thought
that the plan was to use set_special_state(PARKED)...

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.

No?

Oleg.