iommufd: Make destroy_rwsem use a lock class per object type

The selftest invokes things like replace under the object lock of its
idev which protects the idev in a similar way to a real user.
Unfortunately this triggers lockdep. A lock class per type will solve the
problem.

Link: https://lore.kernel.org/r/15-v8-6659224517ea+532-iommufd_alloc_jgg@nvidia.com
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

Author: jgunthorpe

drivers/iommu/iommufd/iommufd_private.h

@@ -119,6 +119,7 @@ enum iommufd_object_type {
 #ifdef CONFIG_IOMMUFD_TEST
 	IOMMUFD_OBJ_SELFTEST,
 #endif
+	IOMMUFD_OBJ_MAX,
 };
 
 /* Base struct for all objects with a userspace ID handle. */

drivers/iommu/iommufd/main.c

@@ -33,14 +33,23 @@ struct iommufd_object *_iommufd_object_alloc(struct iommufd_ctx *ictx,
 					     size_t size,
 					     enum iommufd_object_type type)
 {
+	static struct lock_class_key obj_keys[IOMMUFD_OBJ_MAX];
 	struct iommufd_object *obj;
 	int rc;
 
 	obj = kzalloc(size, GFP_KERNEL_ACCOUNT);
 	if (!obj)
 		return ERR_PTR(-ENOMEM);
 	obj->type = type;
-	init_rwsem(&obj->destroy_rwsem);
+	/*
+	 * In most cases the destroy_rwsem is obtained with try so it doesn't
+	 * interact with lockdep, however on destroy we have to sleep. This
+	 * means if we have to destroy an object while holding a get on another
+	 * object it triggers lockdep. Using one locking class per object type
+	 * is a simple and reasonable way to avoid this.
+	 */
+	__init_rwsem(&obj->destroy_rwsem, "iommufd_object::destroy_rwsem",
+		     &obj_keys[type]);
 	refcount_set(&obj->users, 1);
 
 	/*