Date: Tue, 4 Jul 2017 18:07:21 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Vikram Mulukutla <markivx@codeaurora.org>
Cc: rusty@rustcorp.com.au, tj@kernel.org,
        Thomas Gleixner <tglx@linutronix.de>, akpm@linux-foundation.org,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH] kthread: Atomically set completion and perform dequeue
 in __kthread_parkme
Message-ID: <20170704160721.oawhbx3efaw4d4we@hirez.programming.kicks-ass.net>
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On Mon, Jun 26, 2017 at 03:18:03PM -0700, Vikram Mulukutla wrote:
>  kernel/kthread.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index 26db528..7ad3354 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -171,9 +171,20 @@ static void __kthread_parkme(struct kthread *self)
>  {
>  	__set_current_state(TASK_PARKED);
>  	while (test_bit(KTHREAD_SHOULD_PARK, &self->flags)) {
> +		/*
> +		 * Why the preempt_disable?
> +		 * Hotplug needs to ensure that 'self' is off of the runqueue
> +		 * as well, before scheduling the stopper thread that will
> +		 * migrate tasks off of the runqeue that 'self' was running on.
> +		 * This avoids unnecessary migration work and also ensures that
> +		 * kthread_unpark in the cpu_up path doesn't race with
> +		 * __kthread_parkme.
> +		 */
> +		preempt_disable();
>  		if (!test_and_set_bit(KTHREAD_IS_PARKED, &self->flags))
>  			complete(&self->parked);
> +		schedule_preempt_disabled();

This is broken. schedule_preempt_disable() doesn't guarantee no
preemptions, just makes it less likely.

> +		preempt_enable();
>  		__set_current_state(TASK_PARKED);
>  	}
>  	clear_bit(KTHREAD_IS_PARKED, &self->flags);