linux-kernel - Re: [PATCH] kthread: Atomically set completion and perform dequeue in __kthread

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Message-ID: <20170704160721.oawhbx3efaw4d4we@hirez.programming.kicks-ass.net>
Date:   Tue, 4 Jul 2017 18:07:21 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vikram Mulukutla <markivx@...eaurora.org>
Cc:     rusty@...tcorp.com.au, tj@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH] kthread: Atomically set completion and perform dequeue
 in __kthread_parkme

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);