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Date: Mon, 3 Jan 2011 19:05:34 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Chris Mason <chris.mason@oracle.com>,
        Frank Rowand <frank.rowand@am.sony.com>, Ingo Molnar <mingo@elte.hu>,
        Thomas Gleixner <tglx@linutronix.de>, Mike Galbraith <efault@gmx.de>,
        Paul Turner <pjt@google.com>, Jens Axboe <axboe@kernel.dk>,
        Yong Zhang <yong.zhang0@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>, linux-kernel@vger.kernel.org
Subject: Re: [RFC][PATCH 08/17] sched: Drop the rq argument to
	sched_class::select_task_rq()
Message-ID: <20110103180534.GA16502@redhat.com>
References: <20101224122338.172750730@chello.nl> <20101224123742.887559254@chello.nl>
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On 12/24, Peter Zijlstra wrote:
>
> @@ -2680,24 +2681,17 @@ void wake_up_new_task(struct task_struct
>  {
>  	unsigned long flags;
>  	struct rq *rq;
> -	int cpu __maybe_unused = get_cpu();

Wait, I think this not right.

>  #ifdef CONFIG_SMP
>  	rq = task_rq_lock(p, &flags);
> -	p->state = TASK_WAKING;
>
>  	/*
>  	 * Fork balancing, do it here and not earlier because:
>  	 *  - cpus_allowed can change in the fork path
>  	 *  - any previously selected cpu might disappear through hotplug
> -	 *
> -	 * We set TASK_WAKING so that select_task_rq() can drop rq->lock
> -	 * without people poking at ->cpus_allowed.
>  	 */
> -	cpu = select_task_rq(rq, p, SD_BALANCE_FORK, 0);
> -	set_task_cpu(p, cpu);
> +	set_task_cpu(p, select_task_rq(p, SD_BALANCE_FORK, 0));
>
> -	p->state = TASK_RUNNING;
>  	task_rq_unlock(rq, &flags);

We need preempt_disable() to protect us against CPU hotplug. This task
was never activated, it won't be found/migrated if that CPU goes away
before we take task_rq_lock().

Oleg.