callout handler re-entrancy

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

If I install the same function on two different callouts, i.e.
        callout_reset(&c1, n1, f, &x1);
and
        callout_reset(&c2, n2, f, &x2);
does anything prevent f being called twice concurrently?
I.e., am I correct that f has to be re-entrant in this case?