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?
