RE: [PATCH 9/9] vfio: Remove calls to vfio_group_add_container_user()

["Tian, Kevin" <kevin.tian@intel.com>]

> From: Jason Gunthorpe <jgg@nvidia.com>
> Sent: Thursday, April 14, 2022 10:22 PM
> 
> On Thu, Apr 14, 2022 at 09:51:49AM -0400, Matthew Rosato wrote:
> > On 4/12/22 11:53 AM, Jason Gunthorpe wrote:
> > > When the open_device() op is called the container_users is incremented
> and
> > > held incremented until close_device(). Thus, so long as drivers call
> > > functions within their open_device()/close_device() region they do not
> > > need to worry about the container_users.
> > >
> > > These functions can all only be called between
> > > open_device()/close_device():
> > >
> > >    vfio_pin_pages()
> > >    vfio_unpin_pages()
> > >    vfio_dma_rw()
> > >    vfio_register_notifier()
> > >    vfio_unregister_notifier()
> > >
> > > So eliminate the calls to vfio_group_add_container_user() and add a
> simple
> > > WARN_ON to detect mis-use by drivers.
> > >
> >
> > vfio_device_fops_release decrements dev->open_count immediately
> before
> > calling dev->ops->close_device, which means we could enter close_device
> with
> > a dev_count of 0.
> >
> > Maybe vfio_device_fops_release should handle the same way as
> > vfio_group_get_device_fd?
> >
> > 	if (device->open_count == 1 && device->ops->close_device)
> > 		device->ops->close_device(device);
> > 	device->open_count--;
> 
> Yes, thanks alot! I have nothing to test these flows on...
> 
> It matches the ordering in the only other place to call close_device.
> 
> I folded this into the patch:

While it's a welcomed fix is it actually related to this series? The point
of this patch is that those functions are called when container_users
is non-zero. This is true even without this fix given container_users
is decremented after calling device->ops->close_device().

iiuc this might be better sent out as a separate fix out of this series?
Or at least add a comment in the commit msg about taking chance
to fix an unrelated issue to not cause confusion...

Thanks
Kevin