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

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Message-ID: <BANLkTikvFk7pbcw9PEtPGSK=6WCNLYXCuQ@mail.gmail.com>
Date:	Tue, 7 Jun 2011 22:10:55 +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 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.
	 ***************************
         * .....
         */

>> and if NEED_RESCHED is happenly set on the current task of local
>> CPU, we get incorrect result.
>
> Only if your above assumption holds, which it doesn't. It explicitly
> checks to see if _this_ cpu needs a resched while spinning, if so it
> bails the spinning and calls schedule in the lock slow path.
>
> If the owner cpu reschedules, owner will leave the rq and
> owner_running() will return false, also breaking the loop.
>

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.

thanks
           Hillf
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