[racket] Racket FFI lets other threads run?

Hi list,

I understand that (barring use of features like places or futures)
Racket threads normally do not run truly concurrently.

But what about C functions called through the FFI?
I that _fun has an option like #:atomic? ;
does this mean that the default behaviour is that the C function
runs non-atomically, that is, concurrently with other threads?

I am asking because I have some potentially long-running C functions
and it would be nice to exploit parallelism here.

Thanks,

Stephan