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SubjectRe: [PATCH 16/18] KVM: Don't take mmu_lock for range invalidation unless necessary
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?

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