linux-kernel - Re: [PATCH] ACPI/CPUIDLE: prevent setting pm

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Date:	Mon, 28 Jul 2008 21:38:44 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andi Kleen <andi@...stfloor.org>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Venkatesch Pallipadi <venkatesh.pallipadi@...el.com>,
	Len Brown <lenb@...nel.org>
Subject: Re: [PATCH] ACPI/CPUIDLE: prevent setting pm_idle to NULL

On Mon, 28 Jul 2008, Andi Kleen wrote:
> > +	/*
> > +	 * Fall back to the default idle loop, when pm_idle_save had
> > +	 * been initialized.
> > +	 */
> > +	if (pm_idle_save) {
> > +		pm_idle = pm_idle_save;
> > +		/* Relies on interrupts forcing exit from idle. */
> > +		synchronize_sched();
> > +	}
> 
> I think it would be better to fall  back to default_idle (which
> might need to be exported) when the old pointer is NULL.

No, falling back to default_idle is wrong as hell. When pm_idle was
set to mwait_idle or whatever then you override pm_idle to
default_idle for no good reason.

The problem here is that the acpi/cpuidle code can be in a state where
the _save/old variables _ARE_ NULL because they had not been
initialized with the original pm_idle before the module is removed or
the cst state changes. So all we have to do is to prevent pm_idle to
be set to NULL.

> Now with your patch 
> the cpuidle idle code would run with inconsistent state for some time,
> which is probably not good.

Err, this happens when the original pm_idle pointer _is_ restored. But
if the _save/old pointer is NULL we crash the system and that's what
my patch prevents.

So nothing runs with an inconsistent state. If pm_idle_save contains
the original pm_idle value we restore, if it is NULL we do not touch
pm_idle and keep the original value.

Thanks,

	tglx
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