iommufd/selftest: Add refcount to mock_iommu_device

For an iommu_dev that can unplug (so far only this selftest does so), the
viommu->iommu_dev pointer has no guarantee of its life cycle after it is
copied from the idev->dev->iommu->iommu_dev.

Track the user count of the iommu_dev. Postpone the exit routine using a
completion, if refcount is unbalanced. The refcount inc/dec will be added
in the following patch.

Link: https://patch.msgid.link/r/33f28d64841b497eebef11b49a571e03103c5d24.1730836219.git.nicolinc@nvidia.com
Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

Author: nicolinc

drivers/iommu/iommufd/selftest.c

@@ -533,14 +533,17 @@ static bool mock_domain_capable(struct device *dev, enum iommu_cap cap)
 
 static struct iopf_queue *mock_iommu_iopf_queue;
 
-static struct iommu_device mock_iommu_device = {
-};
+static struct mock_iommu_device {
+	struct iommu_device iommu_dev;
+	struct completion complete;
+	refcount_t users;
+} mock_iommu;
 
 static struct iommu_device *mock_probe_device(struct device *dev)
 {
 	if (dev->bus != &iommufd_mock_bus_type.bus)
 		return ERR_PTR(-ENODEV);
-	return &mock_iommu_device;
+	return &mock_iommu.iommu_dev;
 }
 
 static void mock_domain_page_response(struct device *dev, struct iopf_fault *evt,
@@ -1556,24 +1559,27 @@ int __init iommufd_test_init(void)
 	if (rc)
 		goto err_platform;
 
-	rc = iommu_device_sysfs_add(&mock_iommu_device,
+	rc = iommu_device_sysfs_add(&mock_iommu.iommu_dev,
 				    &selftest_iommu_dev->dev, NULL, "%s",
 				    dev_name(&selftest_iommu_dev->dev));
 	if (rc)
 		goto err_bus;
 
-	rc = iommu_device_register_bus(&mock_iommu_device, &mock_ops,
+	rc = iommu_device_register_bus(&mock_iommu.iommu_dev, &mock_ops,
 				  &iommufd_mock_bus_type.bus,
 				  &iommufd_mock_bus_type.nb);
 	if (rc)
 		goto err_sysfs;
 
+	refcount_set(&mock_iommu.users, 1);
+	init_completion(&mock_iommu.complete);
+
 	mock_iommu_iopf_queue = iopf_queue_alloc("mock-iopfq");
 
 	return 0;
 
 err_sysfs:
-	iommu_device_sysfs_remove(&mock_iommu_device);
+	iommu_device_sysfs_remove(&mock_iommu.iommu_dev);
 err_bus:
 	bus_unregister(&iommufd_mock_bus_type.bus);
 err_platform:
@@ -1583,15 +1589,32 @@ int __init iommufd_test_init(void)
 	return rc;
 }
 
+static void iommufd_test_wait_for_users(void)
+{
+	if (refcount_dec_and_test(&mock_iommu.users))
+		return;
+	/*
+	 * Time out waiting for iommu device user count to become 0.
+	 *
+	 * Note that this is just making an example here, since the selftest is
+	 * built into the iommufd module, i.e. it only unplugs the iommu device
+	 * when unloading the module. So, it is expected that this WARN_ON will
+	 * not trigger, as long as any iommufd FDs are open.
+	 */
+	WARN_ON(!wait_for_completion_timeout(&mock_iommu.complete,
+					     msecs_to_jiffies(10000)));
+}
+
 void iommufd_test_exit(void)
 {
 	if (mock_iommu_iopf_queue) {
 		iopf_queue_free(mock_iommu_iopf_queue);
 		mock_iommu_iopf_queue = NULL;
 	}
 
-	iommu_device_sysfs_remove(&mock_iommu_device);
-	iommu_device_unregister_bus(&mock_iommu_device,
+	iommufd_test_wait_for_users();
+	iommu_device_sysfs_remove(&mock_iommu.iommu_dev);
+	iommu_device_unregister_bus(&mock_iommu.iommu_dev,
 				    &iommufd_mock_bus_type.bus,
 				    &iommufd_mock_bus_type.nb);
 	bus_unregister(&iommufd_mock_bus_type.bus);