Re: [PATCH v2 4/9] iommu/ioasid: Add reference couting functions

[Jean-Philippe Brucker]

On Mon, Aug 24, 2020 at 10:26:55AM +0800, Lu Baolu wrote:
> Hi Jacob,
> 
> On 8/22/20 12:35 PM, Jacob Pan wrote:
> > There can be multiple users of an IOASID, each user could have hardware
> > contexts associated with the IOASID. In order to align lifecycles,
> > reference counting is introduced in this patch. It is expected that when
> > an IOASID is being freed, each user will drop a reference only after its
> > context is cleared.
> > 
> > Signed-off-by: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
[...]
> > +/**
> >    * ioasid_find - Find IOASID data
> >    * @set: the IOASID set
> >    * @ioasid: the IOASID to find
> 
> Do you need to increase the refcount of the found ioasid and ask the
> caller to drop it after use? Otherwise, the ioasid might be freed
> elsewhere.

ioasid_find() takes a getter function as parameter, which ensures that the
returned data is valid. It fetches the IOASID data under rcu_read_lock()
and calls the getter on the private data (for example mmget_not_zero() for
bare-metal SVA). Given that, I don't think returning with a reference to
the IOASID is necessary. The IOASID may be freed once ioasid_find()
returns but not the returned data.

Thanks,
Jean