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SubjectRe: [PATCH 16/18] KVM: Don't take mmu_lock for range invalidation unless necessary
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On 31/03/21 22:15, Sean Christopherson wrote:
> On Wed, Mar 31, 2021, Paolo Bonzini wrote:
>> On 26/03/21 03:19, Sean Christopherson wrote:
>>> + /*
>>> + * Reset the lock used to prevent memslot updates between MMU notifier
>>> + * range_start and range_end. At this point no more MMU notifiers will
>>> + * run, but the lock could still be held if KVM's notifier was removed
>>> + * between range_start and range_end. No threads can be waiting on the
>>> + * lock as the last reference on KVM has been dropped. If the lock is
>>> + * still held, freeing memslots will deadlock.
>>> + */
>>> + init_rwsem(&kvm->mmu_notifier_slots_lock);
>>
>> I was going to say that this is nasty, then I noticed that
>> mmu_notifier_unregister uses SRCU to ensure completion of concurrent calls
>> to the MMU notifier. So I guess it's fine, but it's better to point it out:
>>
>> /*
>> * At this point no more MMU notifiers will run and pending
>> * calls to range_start have completed, but the lock would
>> * still be held and never released if the MMU notifier was
>> * removed between range_start and range_end. Since the last
>> * reference to the struct kvm has been dropped, no threads can
>> * be waiting on the lock, but we might still end up taking it
>> * when freeing memslots in kvm_arch_destroy_vm. Reset the lock
>> * to avoid deadlocks.
>> */
>
> An alternative would be to not take the lock in install_new_memslots() if
> kvm->users_count == 0. It'd be weirder to document, and the conditional locking
> would still be quite ugly. Not sure if that's better than blasting a lock
> during destruction?

No, that's worse...

Paolo

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