Re: [PATCH 01/10] vfio: Simplify the lifetime logic for vfio_device

[Christoph Hellwig <hch@lst.de>]

On Tue, Mar 09, 2021 at 05:38:43PM -0400, Jason Gunthorpe wrote:
> The vfio_device is using a 'sleep until all refs go to zero' pattern for
> its lifetime, but it is indirectly coded by repeatedly scanning the group
> list waiting for the device to be removed on its own.
> 
> Switch this around to be a direct representation, use a refcount to count
> the number of places that are blocking destruction and sleep directly on a
> completion until that counter goes to zero. kfree the device after other
> accesses have been excluded in vfio_del_group_dev(). This is a fairly
> common Linux idiom.
> 
> This simplifies a couple of things:
> 
> - kref_put_mutex() is very rarely used in the kernel. Here it is being
>   used to prevent a zero ref device from being seen in the group
>   list. Instead allow the zero ref device to continue to exist in the
>   device_list and use refcount_inc_not_zero() to exclude it once refs go
>   to zero.
> 
> - get/putting the group while get/putting the device. The device already
>   holds a reference to the group, set during vfio_group_create_device(),
>   there is no need for extra reference traffic. Cleanly have the balancing
>   group put in vfio_del_group_dev() before the kfree().

Might it be worth to split this further up into separate patches for
each of the changes?

The actual changes do look good, though:

Reviewed-by: Christoph Hellwig <hch@lst.de>