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

[Matthew Rosato <mjrosato@linux.ibm.com>]

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--;