[Jackaudio] Processing callback thread safety guarantees

Hi all, I'm in the process of building a small application that utilises Jack in order to interface with audio hardware. Specifically, although this detail is only partially relevant, I'm writing it in Rust (using some community FFI bindings). I was wondering what the thread safety guarantees of calling the processing callback are. Specifically, I was wondering if it can be guaranteed that the callback will only be called on a single thread at any given time? This is primarily because it determines whether or not it is required for the callback to be marked as "Sync" in Rust - which would indicate that the callback's data must be thread safe, a restriction I'd prefer to avoid. Thanks in advance.