linux-kernel - Re: [PATCH 2/6] move_task_off_dead_cpu: take rq->lock around select_fallback

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Message-ID: <20100324163356.GA6380@redhat.com>
Date:	Wed, 24 Mar 2010 17:33:56 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Ben Blum <bblum@...gle.com>,
	Jiri Slaby <jirislaby@...il.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Miao Xie <miaox@...fujitsu.com>,
	Paul Menage <menage@...gle.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/6] move_task_off_dead_cpu: take rq->lock around
	select_fallback_rq()

On 03/24, Peter Zijlstra wrote:
>
> Yeah, you made a few good points in 0/6, am now staring at the code on
> how to close those holes, hope to post something sensible soon.

Yes, great.

Speaking of 0/6, I forgot to ask a couple more question...

try_to_wake_up() does task_rq_lock() which checks TASK_WAKING. Perhaps
it shouldn't ? I mean, perhaps try_to_wake_up() can take rq->lock without
checking task->state. It can never race with the owner of TASK_WAKING,
before anything else we check "p->state & state".

And. Without the change above, any owner of TASK_WAKING must disable
preemption and clear irqs.

What do you think?

And a stupid question. While doing these changes I was really, really
puzzled by task_rq_lock() which does

	local_irq_save(*flags);
	rq = task_rq(p);
	raw_spin_lock(&rq->lock);

to the point, I even tried to read the comment which says:

	Note the ordering: we can safely lookup the task_rq without
	explicitly disabling preemption.

Could you please explain what does this mean? IOW, why can't we do

	rq = task_rq(p);
	raw_spin_lock_irqsave(&rq->lock, flags);

instead?

Of course, this doesn't really matter, but I'd like to understand
what I have missed here.

Thanks,

Oleg.

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