linux-kernel - Re: [PATCH] sched: remove unreliable pointer in mutex_spin_on

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Date:	Wed, 8 Jun 2011 21:28:59 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] sched: remove unreliable pointer in mutex_spin_on_owner()

On Wed, Jun 8, 2011 at 8:58 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Wed, Jun 08, 2011 at 08:49:53PM +0800, Hillf Danton wrote:
>> The dereference of unreliable owner pointer is unnecessary in owner_running(),
>> though under RCU protection, because the true result is only determined by
>> checking the validity of lock owner, as the comment says, due to likely heavy
>> lock contention, which has little to do with whether owner->on_cpu is false.
>>
>> If owner->on_cpu is really false, only the owner_running loop is shortened,
>> but also returns incorrect result, since the lock owner is not changed, though
>> maybe changed soon.
>>
>
> Hillf, have you read anything that I posted before?
>

I have, but

> We don't want to spin if the owner of the lock sleeps. If it sleeps,
> then the owner's on_cpu will be zero. That's the point of checking it.
>
not understand the point of checking owner->on_cpu, and the code's
icy beauty blows me down again.

Please thaw another hard to understand in rt_mutex_setprio(),


	if (running)
		p->sched_class->set_curr_task(rq);
	if (on_rq)
-		enqueue_task(rq, p, oldprio < prio ? ENQUEUE_HEAD : 0);
+		enqueue_task(rq, p, oldprio > prio ? ENQUEUE_HEAD : 0);

	check_class_changed(rq, p, prev_class, oldprio);


thanks
            Hillf

> The patch you just added would devastate the performance of the system.
> Now if we have contention on a lock, and the owner sleeps, we continue
> to spin. If the spinner is an RT task, this could also cause a deadlock,
> especially if the owner is bound to the same CPU that the RT task is on.
>
> -- Steve
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