[PATCH 3/13] vfio/mdev: simplify mdev_type handling

[Jason Gunthorpe]

On Tue, Jun 14, 2022 at 06:54:18AM +0200, Christoph Hellwig wrote:

> diff --git a/drivers/vfio/mdev/mdev_sysfs.c b/drivers/vfio/mdev/mdev_sysfs.c
> index b71ffc5594870..09745e8e120f9 100644
> --- a/drivers/vfio/mdev/mdev_sysfs.c
> +++ b/drivers/vfio/mdev/mdev_sysfs.c
> @@ -80,8 +80,6 @@ static void mdev_type_release(struct kobject *kobj)
>  	struct mdev_type *type = to_mdev_type(kobj);
>  
>  	pr_debug("Releasing group %s\n", kobj->name);
> -	/* Pairs with the get in add_mdev_supported_type() */
> -	put_device(type->parent->dev);

I couldn't figure out why is it now OK to delete this? 

It still looks required because mdev_core.c continues to use
mdev-type->parent in various places and that pointer was being
protected by the

   kobject_get(&type->kobj);

in mdev_device_create() through the above kref..

eg after the whole series is applied this looks troubled:

	/* Pairs with the get in mdev_device_create() */
	kobject_put(&mdev->type->kobj);

	mutex_lock(&mdev_list_lock);
	list_del(&mdev->next);
	mdev->type->parent->available_instances++;
        ^^^^^^^^^^^^^^^^^^^^^

As there is now nothing keeping parent or type alive?

I think what would be good here is to directly
get_device(type->parent->dev) in mdev_device_create() and put in
mdev_device_release() then it is really clear how parent and
parent->dev are kept alive.

It looks like we also have to keep the type around and ref'd too for
the sysfs manipulation.

But overall I think this is fine

Jason