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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