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.
*/
That said, the easiest way to avoid this would be to always update
mmu_notifier_count. I don't mind the rwsem, but at least I suggest that
you split the patch in two---the first one keeping the
mmu_notifier_count update unconditional, and the second one introducing
the rwsem and the on_lock function kvm_inc_notifier_count. Please
document the new lock in Documentation/virt/kvm/locking.rst too.
Also, related to the first part of the series, perhaps you could
structure the series in a slightly different way:
1) introduce the HVA walking API in common code, complete with on_lock
and patch 15, so that you can use on_lock to increase mmu_notifier_seq
2) then migrate all architectures including x86 to the new API
IOW, first half of patch 10 and all of patch 15; then the second half of
patch 10; then patches 11-14.
+#if defined(CONFIG_MMU_NOTIFIER) && defined(KVM_ARCH_WANT_MMU_NOTIFIER)
+ down_write(&kvm->mmu_notifier_slots_lock);
+#endif
rcu_assign_pointer(kvm->memslots[as_id], slots);
+#if defined(CONFIG_MMU_NOTIFIER) && defined(KVM_ARCH_WANT_MMU_NOTIFIER)
+ up_write(&kvm->mmu_notifier_slots_lock);
+#endif
Please do this unconditionally, the cost is minimal if the rwsem is not
contended (as is the case if the architecture doesn't use MMU notifiers
at all).
Paolo
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