The problem in 'C' (or other high level languages) with re-entrant functions is that if the function uses local variables, the compiler has to be smart enough to know not to use the same memory location for each instance of the function.

If you are using assembler, you have to take care of memory management yourself. So as long as you are sure that each call to the function isn't overwriting a memory location or register that another instance of the function is using, then your only real concern is, as Rob mentioned, the stack.

Regards,
Craig Steiner