Re: [PATCH 16/18] KVM: Don't take mmu_lock for range invalidation unless necessary
From: Paolo Bonzini <pbonzini@redhat.com>
Date: 2021-03-31 20:31:01
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kvm, kvmarm, linux-mips, lkml
On 31/03/21 22:15, Sean Christopherson wrote:
On Wed, Mar 31, 2021, Paolo Bonzini wrote:quoted
On 26/03/21 03:19, Sean Christopherson wrote:quoted
+ /* + * 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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel