- Remove the populating of the tree from the revmap function as
   you already do
 - Move it to irq_create_mapping() for the normal case
 - For pre-existing interrupt, have the generic code that initializes
   the radix tree walk through all interrupts and setup the revmap for
   them. If that needs locking vs. concurrent irq_create_mapping, it's
   easy to use one of the available spinlocks for that.

And in fact, you may even be able to avoid GFP_ATOMIC completely here
and switch it to GFP_KERNEL since irq_create_mapping() can sleep afaik,
provided that you avoid the spinlocking.

Ben.