Re: [PATCH 1/3] iommu/amd/iommu_v2: Fix pasid_state refcount dec hit 0 warning on pasid unbind

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Jan 24, 2023 at 10:43:53AM +0000, Vasant Hegde wrote:
> From: Daniel Marcovitch <dmarcovitch@nvidia.com>
> 
> When unbinding pasid - a race condition exists vs outstanding page faults.
> 
> To prevent this, the pasid_state object contains a refcount.
>     * set to 1 on pasid bind
>     * incremented on each ppr notification start
>     * decremented on each ppr notification done
>     * decremented on pasid unbind

I've no objection to this quick fix

But the issue here is using a refcount for something that is not a
refcount. This usage model is should just be a normal atomic if it is
only counting the number of notifications in progress.

To be a refcount to have to follow one of the standard refcount
patterns, ie get rid of the event queue and consistently do:

if (refcount_dec_and_test())
    free_pasid_state()

Which implies the ppr notification done can do a defered free of the
pasid_state.

Jason