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    SubjectRe: [PATCH] sched: Support current clocksource handling in fallback sched_clock().
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    On Fri, 2009-05-29 at 01:40 +0900, Paul Mundt wrote:
    > On Thu, May 28, 2009 at 06:32:09PM +0200, Peter Zijlstra wrote:
    > > On Thu, 2009-05-28 at 09:13 -0700, Daniel Walker wrote:
    > > > On Thu, 2009-05-28 at 14:59 +0200, Peter Zijlstra wrote:
    > > > > CPU0 CPU1
    > > > >
    > > > > clock = ACCESS_ONCE(sched_clocksource);
    > > > >
    > > > > unload module
    > > > > clocksource_unregister()
    > > > > sched_clocksource = jiffies
    > > > > unmap data/text
    > > > >
    > > > > cyc2ns(clock, clocksource_read(clock)) <--- fireworks
    > > > >
    > > > >
    > > >
    > > > Do any module based clocksources even exist right now?
    > > > clocksource_unregister only seems to be used 3 times..
    > >
    > > Good point, it appears its not even exported.
    > >
    > > Thomas mentioned modules, I assumed.
    > >
    > The drivers/clocksource/ drivers in theory could be modular anyways.
    > Handling this transition properly is at least one less barrier to modular
    > clocksources, so I think it's progress regardless. I don't remember what
    > all of the other issues were though, John probably remembers.

    Yea. Its always been something I'd want to have as a capability, but we
    haven't actually had a user of. I still think its something we should
    allow, even if there aren't upstream users, as if you only have to
    backport a module to an old distro kernel to get some oddball bit of
    hardware working, that can be really useful.

    So I think the locking fix is the way to go.

    thanks
    -john





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