td->td_critnest manipulations do not use atomic_add_int ?

as in the subject... i see that td->td_critnest (used to determine
whether a thread can be preempted or not) is manipulated using
plain ++ or -- instruction instead of the atomic_add_int().

I wonder if declaring it as volatile and possibly its
usage patterns are enough to make the two things equivalent
on all architectures.

cheers
luigi