Messages in this thread | | | Subject | Re: [PATCH 16/18] KVM: Don't take mmu_lock for range invalidation unless necessary | From | Paolo Bonzini <> | Date | Wed, 31 Mar 2021 22:30:05 +0200 |
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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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