linux-kernel - Re: [BUG] "sched: Remove rq->lock from the first half of ttwu()" locks up on ARM

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Date:	Thu, 26 May 2011 19:49:03 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Marc Zyngier <Marc.Zyngier@....com>,
	Yong Zhang <yong.zhang0@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Frank Rowand <frank.rowand@...sony.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [BUG] "sched: Remove rq->lock from the first half of ttwu()"
	locks up on ARM

On 05/26, Peter Zijlstra wrote:
>
> It has the extra cpu == smp_processor_id() check, but I'm not sure this
> whole case is worth the trouble.

Agreed, this case is very unlikely. Perhaps it makes the code more clear
though, up to you.

But, if we keep this check,

> @@ -2636,9 +2636,17 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
>  		 * to spin on ->on_cpu if p is current, since that would
>  		 * deadlock.
>  		 */
> -		if (p == current) {
> -			ttwu_queue(p, cpu);
> -			goto stat;
> +		if (cpu == smp_processor_id()) {
> +			struct rq *rq;
> +
> +			rq = __task_rq_lock(p);
> +			if (p->on_cpu) {
> +				ttwu_activate(rq, p, ENQUEUE_WAKEUP);
> +				ttwu_do_wakeup(rq, p, wake_flags);
> +				__task_rq_unlock(rq);

then why we re-check ->on_cpu? Just curious.

Oleg.

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