linux-kernel - Re: [PATCH] sched: correct testing need_resched in mutex_spin_on

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Date:	Tue, 7 Jun 2011 22:36:13 +0800
From:	Hillf Danton <dhillf@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] sched: correct testing need_resched in mutex_spin_on_owner()

On Tue, Jun 7, 2011 at 10:24 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, 2011-06-07 at 22:10 +0800, Hillf Danton wrote:
>> On Tue, Jun 7, 2011 at 9:47 PM, Peter Zijlstra <peterz@...radead.org> wrote:
>> > On Tue, 2011-06-07 at 21:41 +0800, Hillf Danton wrote:
>> >> It is suppose to check the owner task that is not absolutly running on the
>> >> local CPU,
>> >
>> > Oh, why do you think so?
>> >
>> as the comment in __mutex_lock_common() says,
>>
>> #ifdef CONFIG_MUTEX_SPIN_ON_OWNER
>>       /*
>>        * Optimistic spinning.
>>        *
>>        * We try to spin for acquisition when we find that there are no
>>        * pending waiters and the lock owner is currently running on a
>>        ***************************
>>          * (different) CPU.
>>        ***************************
>>          * .....
>>          */
>
> Which is exactly what owner_running() does, clearly it cannot run on the
> current cpu, since then we wouldn't be running to check things, so for
> as long as owner is on_cpu we spin.
>
> However,
>
>> need_resched is checked after true is returned by owner_running(),
>> in other words, owner is still on its CPU, so owner should be check
>> here. Even ower's CPU == this CPU, checking owner also gives
>> correct result.
>
> no, after rcu_read_unlock() in owner_running() -- read that comment
> again -- the owner pointer can be free or reused memory.
>
> Also, since we already check owner_running() this need_resched() is
> clearly something different. By your argument it would be superfluous
> not wrong.
>
> Now since it appears superfluous, ask yourself why it would exist. The
> answer is simple, we should not be greedy and consume our cpu when
> there's someone else that wants to run, we should yield the cpu on
> need_resched(), which is exactly what happens.
>
If you are right, the following comment also in __mutex_lock_common()

	for (;;) {
		struct task_struct *owner;

		/*
		 * If there's an owner, wait for it to either
		 * release the lock or go to sleep.
		 */
		owner = ACCESS_ONCE(lock->owner);
		if (owner && !mutex_spin_on_owner(lock, owner))
			break;

looks misleading too, but if owner is on this CPU, for what does we wait?

thanks
           Hillf
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