Question regarding Locking in the MMIO Handling Framework on Arm

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

Hello everyone,

I have a question regarding the locking in the MMIO handling framework 
on Arm.
Is it sufficient to have read_lock() only in find_mmio_handler()? If
register_mmio_handler() is executed in parallel with find_mmio_handler() 
(which I assume was the reason for introducing the rwlock), aren't we 
still exposed to a race condition?
find_mmio_handler() returns a pointer to a handler, but the object it 
points to could be changed by a subsequent call to register_mmio_handler().
If register_mmio_handler() runs between find_mmio_handler() and 
handle_{write,read}(), we could end up operating on a different handler 
than the one that was originally found.
In other words, shouldn't we acquire the read_lock() in 
try_handle_mmio() and keep it held for the entire duration of 
try_handle_mmio()?
It seems this is not an issue at the moment because 
register_mmio_handler() is only called during domain creation. If that's 
the case, do we really need the rwlock at all?
One more thing: it looks like
  BUG_ON(vmmio->num_entries >= vmmio->max_num_entries);
in register_mmio_handler() would be better placed inside the 
write_lock() section. As again in the case if register_mmio_handler() 
will start to be called in parallel we will have an issue.
Thanks in advance.

~ Oleksii