Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation APIs

Jacob Pan
2021-05-04T22:09:11Z

Hi Jason,

On Tue, 4 May 2021 15:00:50 -0300, Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Tue, May 04, 2021 at 08:41:48AM -0700, Jacob Pan wrote:
> > > >
> > > > (also looking at ioasid.c, why do we need such a thin and odd
> > > > wrapper around xarray?)
> > > >
> > >
> > > I'll leave it to Jean and Jacob.
>
> > Could you elaborate?
>
> I mean stuff like this:
>
> int ioasid_set_data(ioasid_t ioasid, void *data)
> {
> struct ioasid_data *ioasid_data;
> int ret = 0;
>
> spin_lock(&ioasid_allocator_lock);
> ioasid_data = xa_load(&active_allocator->xa, ioasid);
> if (ioasid_data)
> rcu_assign_pointer(ioasid_data->private, data);
> else
> ret = -ENOENT;
> spin_unlock(&ioasid_allocator_lock);
>
> /*
> * Wait for readers to stop accessing the old private data, so the
> * caller can free it.
> */
> if (!ret)
> synchronize_rcu();
>
> return ret;
> }
> EXPORT_SYMBOL_GPL(ioasid_set_data);
>
> It is a weird way to use xarray to have a structure which
> itself is just a wrapper around another RCU protected structure.
>
> Make the caller supply the ioasid_data memory, embedded in its own
> element, get rid of the void * and rely on XA_ZERO_ENTRY to hold
> allocated but not active entries.
>
Let me try to paraphrase to make sure I understand. Currently
struct ioasid_data is private to the iasid core, its memory is allocated by
the ioasid core.

You are suggesting the following:
1. make struct ioasid_data public
2. caller allocates memory for ioasid_data, initialize it then pass it to
ioasid_alloc to store in the xarray
3. caller will be responsible for setting private data inside ioasid_data
and do call_rcu after update if needed.

Correct?

> Make the synchronize_rcu() the caller responsiblity, and callers
> should really be able to use call_rcu()
>
> Jason

Thanks,

Jacob