xarray reserve/release?

[Jason Gunthorpe]

Hey Matt,

Did you intend that xa_release doesn't work on allocating arrays:

        xa_init_flags(&xa, XA_FLAGS_ALLOC);
        xa_reserve(&xa, 0, GFP_KERNEL);
        WARN_ON(xa_empty(&xa));
        xa_release(&xa, 0);
        WARN_ON(!xa_empty(&xa));

Triggers the second WARN_ON. 

If FLAGS_ALLOC is removed this passes

This creates kind of a confusing / super unexpected situation where 

   xa_for_each (xa, id)
       xa_erase(xa, id);
   WARN_ON(!xa_empty(&xa))

Is not safe with XA_FLAGS_ALLOC??

Seems like at least deserves a documentation note.. Or maybe ALLOC
xa_empty should do something different?

Thanks,
Jason