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SubjectRe: CPUFREQ-CPUHOTPLUG: Possible circular locking dependency

* 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."

?

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.

Ingo
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