Re: [patch 07/15] clocksource: Provide unbind interface in sysfs

[John Stultz]

On 05/15/2013 02:47 AM, Thomas Gleixner wrote:
> On Mon, 29 Apr 2013, John Stultz wrote:
> > On 04/25/2013 01:31 PM, Thomas Gleixner wrote:
> > > With the module refcount held for the current clocksource there is no
> > > way to unload the module.
> > >
> > > Provide a sysfs interface which allows to unbind the clocksource. One
> > > could argue that the clocksource override could be (ab)used to do so,
> > > but the clocksource override cannot be used from the kernel itself,
> > > while an unbind function can be used to programmatically check whether
> > > a clocksource can be shutdown or not.
> > >
> > > The unbind functionality uses the new skip current feature of
> > > clocksource_select and verifies that a fallback clocksource has been
> > > installed. If the clocksource which should be unbound is the current
> > > clocksource and no fallback can be found, unbind returns -EBUSY.
> > >
> > > This does not support the unbinding of a clocksource which is used as
> > > the watchdog clocksource. No point in fostering crappy hardware.
> > So.. if the clocksource you want to unbind is the highest rated continuous
> > clocksource that doesn't need a watchdog (basically what's likely to be in-use
> > and required to be unbinded), its likely to be selected as the watchdog
> > already.
> >
> > ie: on a system that has only HPET/ACPI_PM, you can't unbind HPET, since its a
> > watchdog.
> No. The thing is that I only prevent unbinding if it is used as the
> watchdog. In the above HPET/PM scenario both are potential watchdogs,
> but w/o a user it's valid to unbind one of them.
>
> What I need to prevent is:
>
> TSC is current clocksource and we only have ACPI_PM as watchdog and
> its used. So now you try to unbind ACPI_PM then the TSC would be left
> w/o a watchdog instance. That's what I'm preventing. Will reword the
> changelog accordingly.

You might double check the logic, because I feel like I actually hit 
this issue where on my vm machine w/ only hpet/acpi_pm I couldn't unbind 
the hpet.

Though its been a few weeks, so maybe I'm confusing things?
Let me know if you want me to try to reproduce it.

thanks
-john

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