Does io_callback(..., ordered=True) ensure sequential execution, or is locking still required?

[HeavyCrab]

Hi all,

I'm trying to clarify the behavior of jax.experimental.io_callback when used with the ordered=True argument.

When ordered=True is set, does JAX guarantee that multiple io_callback invocations will be executed sequentially on the host, i.e., the next callback only starts after the previous one finishes?

Or, does JAX still launch multiple threads for the callbacks, and it is still necessary to manually add locking inside the callback function to prevent race conditions, e.g., when accessing global variables, writing to files, or using libraries like TensorBoardX?