KVM: Fix possible circular locking in kvm_vm_ioctl_assign_device()

One possible order is:

KVM_CREATE_IRQCHIP ioctl(took kvm->lock) -> kvm_iobus_register_dev() ->
down_write(kvm->slots_lock).

The other one is in kvm_vm_ioctl_assign_device(), which take kvm->slots_lock
first, then kvm->lock.

Update the comment of lock order as well.

Observe it due to kernel locking debug warnings.

Cc: stable@kernel.org
Signed-off-by: Sheng Yang <sheng@linux.intel.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

virt/kvm/assigned-dev.c

@@ -508,8 +508,8 @@ static int kvm_vm_ioctl_assign_device(struct kvm *kvm,
 	struct kvm_assigned_dev_kernel *match;
 	struct pci_dev *dev;
 
-	down_read(&kvm->slots_lock);
 	mutex_lock(&kvm->lock);
+	down_read(&kvm->slots_lock);
 
 	match = kvm_find_assigned_dev(&kvm->arch.assigned_dev_head,
 				      assigned_dev->assigned_dev_id);
@@ -573,8 +573,8 @@ static int kvm_vm_ioctl_assign_device(struct kvm *kvm,
 	}
 
 out:
-	mutex_unlock(&kvm->lock);
 	up_read(&kvm->slots_lock);
+	mutex_unlock(&kvm->lock);
 	return r;
 out_list_del:
 	list_del(&match->list);
@@ -585,8 +585,8 @@ static int kvm_vm_ioctl_assign_device(struct kvm *kvm,
 	pci_dev_put(dev);
 out_free:
 	kfree(match);
-	mutex_unlock(&kvm->lock);
 	up_read(&kvm->slots_lock);
+	mutex_unlock(&kvm->lock);
 	return r;
 }
 

virt/kvm/kvm_main.c

@@ -64,7 +64,7 @@ MODULE_LICENSE("GPL");
 /*
  * Ordering of locks:
  *
- * 		kvm->slots_lock --> kvm->lock --> kvm->irq_lock
+ * 		kvm->lock --> kvm->slots_lock --> kvm->irq_lock
  */
 
 DEFINE_SPINLOCK(kvm_lock);