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

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Jun 16, 2022 at 12:59:47AM +0530, Kirti Wankhede wrote:

> > @@ -110,7 +110,7 @@ static struct mdev_type *add_mdev_supported_type(struct mdev_parent *parent,
> >   	type->kobj.kset = parent->mdev_types_kset;
> >   	type->parent = parent;
> >   	/* Pairs with the put in mdev_type_release() */
> > -	mdev_get_parent(parent);
> > +	get_device(parent->dev);
> >   	type->type_group_id = type_group_id;
> > 
> 
> Here device reference is held and release multiple times for each mdev_type.
> It should be held once from mdev_register_parent() and released from
> mdev_unregister_parent().

It doesn't make any sense to put a paired references in the
register/unregister on the struct device - the caller is already
required to hold a reference until unregister completes.

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.

Perhaps after this series things could be reworked to use the parent
instead of the type in more places and perhaps take a device reference
on the parent not the kobj in mdev_device_create() (which is screwy
already), but at least this patch is correct as is.

Jason