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Date:   Tue, 24 Apr 2018 20:26:28 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Gaurav Kohli <gkohli@codeaurora.org>
Cc:     tglx@linutronix.de, mpe@ellerman.id.au, dzickus@redhat.com,
        mingo@kernel.org, bigeasy@linutronix.de,
        linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org,
        Neeraj Upadhyay <neeraju@codeaurora.org>
Subject: Re: [PATCH] kthread/smpboot:  Serialize kthread parking against
 wakeup
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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@codeaurora.org>
> Co-developed-by: Neeraj Upadhyay <neeraju@codeaurora.org>
> Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
> Signed-off-by: Gaurav Kohli <gkohli@codeaurora.org>
> 
> 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?

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.