linux-kernel - Re: [PATCH]: kthread: Fix memory ordering in __kthread

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Date:	Fri, 7 Nov 2014 19:41:03 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Subbaraman Narayanamurthy <subbaram@...eaurora.org>,
	daniel@...ascale.com, yuyang.du@...el.com,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH]: kthread: Fix memory ordering in __kthread_parkme

On 11/07, Peter Zijlstra wrote:
>
>  static void __kthread_parkme(struct kthread *self)
>  {
> -	__set_current_state(TASK_PARKED);
> +	set_current_state(TASK_PARKED);
>  	while (test_bit(KTHREAD_SHOULD_PARK, &self->flags)) {
>  		if (!test_and_set_bit(KTHREAD_IS_PARKED, &self->flags))
>  			complete(&self->parked);
>  		schedule();
> -		__set_current_state(TASK_PARKED);
> +		set_current_state(TASK_PARKED);
>  	}

Perhaps it makses sense to do set_current_state(PARKED) once at the start
of "for (;;)" loop, but this is cosmetic.

What if kthread_unpark() is called right after test_bit(KTHREAD_SHOULD_PARK)
and KTHREAD_IS_PARKED is not set? It seems that __kthread_unpark() should
call wake_up_state() unconditionally ?

Oleg.

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