Jack audio kit API - how to synchronize streams?

Question

C++ / C#

The Jack audio kit API uses a callback function to process and output audio. It is called from an additional high priority thread.

jack_set_process_callback (client, process, 0);
...

int process(jack_nframes_t nframes, void *arg)
{
    ...
}

How can I pass variables to this callback thread-safely and without blocking? I'm looking for an event-based solution, tk. in addition to passing the values ​​of variables, it is sometimes necessary to reinitialize the filter object that is used in the callback.