Re: [PATCH-next] kvm: don't try to take mmu_lock while holding the main raw kvm_lock

[Paolo Bonzini <pbonzini@redhat.com>]

Il 27/06/2013 14:32, Gleb Natapov ha scritto:
>>>>> > >>> In commit e935b8372cf8 ("KVM: Convert kvm_lock to raw_spinlock"),
>>>> > >> I am copying Jan, the author of the patch. Commit message says:
>>>> > >> "Code under this lock requires non-preemptibility", but which code
>>>> > >> exactly is this? Is this still true?
>>> > > 
>>> > > hardware_enable_nolock/hardware_disable_nolock does.
>> > 
>> > IIRC, also the loop in kvmclock_cpufreq_notifier needs it because it
>> > reads the processor ID of the caller. That implies the caller cannot be
>> > preempted, but theses days a migration lock should be fine as well.
>> > 
> OK, adding Marcelo to the party. This code is called from cpufreq
> notifier. I would expect that it will be called from the context that
> prevents migration to another cpu.

No, the CPU is in freq->cpu and may not even be the CPU that changed
frequency.

But even then I'm not sure the loop needs to be non-preemptible.  If it
were, the smp_call_function_single just before/after the loop would have
to be non-preemptable as well.  So it is just an optimization and it can
use raw_smp_processor_id() instead.

Paolo