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

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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



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