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SubjectRe: CPUFREQ-CPUHOTPLUG: Possible circular locking dependency
On Thu, 30 Nov 2006 12:03:15 +0100
Ingo Molnar <mingo@elte.hu> wrote:

>
> * Gautham R Shenoy <ego@in.ibm.com> wrote:
>
> > a) cpufreq maintain's it's own cpumask in the variable
> > policy->affected_cpus and says : If a frequency change is issued to
> > any one of the cpu's in the affected_cpus mask, you change frequency
> > on all cpus in the mask. So this needs to be consistent with
> > cpu_online map and hence cpu hotplug aware. Furthermore, we don't want
> > cpus in this mask to go down when we are trying to change frequencies
> > on them. The function which drives the frequency change in
> > cpufreq-core is cpufreq_driver_target and it needs cpu-hotplug
> > protection.
>
> couldnt this complexity be radically simplified by having new kernel
> infrastructure that does something like:
>
> " 'gather' all CPUs mentioned in <mask> via scheduling a separate
> helper-kthread on every CPU that <mask> specifies, disable all
> interrupts, and execute function <fn> once all CPUs have been
> 'gathered' - and release all CPUs once <fn> has executed on each of
> them."
>
> ?

How does this differ from stop_machine_run(), which hot-unplug presently uses?

> This would be done totally serialized and while holding the hotplug
> lock, so no CPU could go away or arrive while this operation is going
> on.

You said "the hotplug lock". That is the problem.
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