[PATCH 02/13] vfio/mdev: embedd struct mdev_parent in the parent data structure

[Kirti Wankhede]

On 6/24/2022 6:03 PM, Jason Gunthorpe wrote:
> On Fri, Jun 24, 2022 at 05:59:58PM +0530, Kirti Wankhede wrote:
> 
>>> The reason this is here is because the type->parent is used in a few
>>> places and is put back in release:
>>>
>>> @@ -81,7 +81,7 @@ static void mdev_type_release(struct kobject *kobj)
>>>
>>>           pr_debug("Releasing group %s\n", kobj->name);
>>>           /* Pairs with the get in add_mdev_supported_type() */
>>> -       mdev_put_parent(type->parent);
>>> +       put_device(type->parent->dev);
>>>           kfree(type);
>>>    }
>>>
>>> If this was a simple sysfs kobj with only a show/store we wouldn't
>>> need to do anything as the natural kobj parentage holds a ref up to
>>> the struct device - but this kobj is used internally, ie dependent
>>> from mdev_device_create(), independently of the normal sysfs
>>> life-cycle so that doesn't protect enough either.
>>>
>>
>>
>> Life span of 'type' is from mdev_register_device to mdev_unregister_device.
>> If device/parent is being unregistered then only types are removed, so
>> referencing 'type' from mdev_device_create() is still safe. Therefore,
>> parent device's reference should be held and release from
>> register-unregister call.
> 
> No, I've already explained this.

Its not correct.

kobject_init_and_add(&type->kobj, ...) which called from
mdev_register_parent()
     -> parent_create_sysfs_files() holds reference for type->kobj

This is released from
  mdev_unregister_parent()
      -> parent_remove_sysfs_files()
          -> kset_unregister()

In the next patch [3/13] of this series, these calltraces are changed as
mdev_register_parent()
     -> mdev_type_add()
         -> kobject_init_and_add(&type->kobj, ...) holds reference for 
type->kobj

which is released from

mdev_unregister_parent()
     -> mdev_type_remove()
         -> kobject_put(&type->kobj)

Thanks,
Kirti