linux-kernel - Re: [PATCH] sched: RCU-protect __set_task_cpu() in set_task

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Date:	Sun, 5 Jun 2011 21:12:33 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: RCU-protect __set_task_cpu() in set_task_cpu()

On 06/03, Peter Zijlstra wrote:
>
> @@ -2200,6 +2201,16 @@ void set_task_cpu(struct task_struct *p,
>  			!(task_thread_info(p)->preempt_count & PREEMPT_ACTIVE));
>
>  #ifdef CONFIG_LOCKDEP
> +	/*
> +	 * The caller should hold either p->pi_lock or rq->lock, when changing
> +	 * a task's CPU.

Is it literally true? IIRC, we need ->pi_lock if the task is not active,
and rq->lock if p->on_rq = 1. And that is why we do not clear p->on_rq
between deactivate_task() + activate_task(), correct?

> +	 *
> +	 * sched_move_task() holds both and thus holding either pins the cgroup,
> +	 * see set_task_rq().
> +	 *
> +	 * Furthermore, all task_rq users should acquire both locks, see
> +	 * task_rq_lock().
> +	 */
>  	WARN_ON_ONCE(debug_locks && !(lockdep_is_held(&p->pi_lock) ||
>  				      lockdep_is_held(&task_rq(p)->lock)));

IOW, perhaps this should be

	WARN_ON_ONCE(debug_locks && !lockdep_is_held(p->on_rq ?
					&task_rq(p)->lock : &p->pi_lock))

?

Not that I really suggest to change this WARN_ON(), I am just trying
to recall the new rules.

Oleg.

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