Re: [PATCH 15/15] vfio/mdev: remove an extra parent kobject reference

Alex Williamson
2022-07-07T23:01:47Z

On Thu, 7 Jul 2022 15:40:52 +0200
Christoph Hellwig <hch@lst.de> wrote:

> On Wed, Jul 06, 2022 at 11:38:33AM -0300, Jason Gunthorpe wrote:
> > On Wed, Jul 06, 2022 at 09:42:19AM +0200, Christoph Hellwig wrote:
> > > The mdev_type already holds a reference to the parent through
> > > mdev_types_kset, so drop the extra reference.
> >
> > I would drop this patch, but at least the explanation needs tweaking..
>
> I'm fine with that. Alex, any preferences?

Modulo the bogus kobject_put()s, this essentially reverts:

commit 9a302449a58d45d0ef2aab686f64b35919bc604c
Author: Jason Gunthorpe <jgg@ziepe.ca>
Date: Tue Apr 6 16:40:30 2021 -0300

vfio/mdev: Add missing reference counting to mdev_type

struct mdev_type holds a pointer to the kref'd object struct mdev_parent,
but doesn't hold the kref. The lifetime of the parent becomes implicit
because parent_remove_sysfs_files() is supposed to remove all the access
before the parent can be freed, but this is very hard to reason about.

Make it obviously correct by adding the missing get.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Message-Id: <7-v2-d36939638fc6+d54-vfio2_jgg@nvidia.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

Which we all seemed to think was a good thing 15 months ago. It is
still difficult to reason when the mdev_type_ktype.release function
occurs relative to the parent reference held by the kset, but without
an explanation how we're safe, I'm ok with a little paranoia and
explicit references. Thanks,

Alex