As PaulS wrote, that code might behave weridly if an interrupt occurs during the execution of the code block inside the IF statement... For example if the signal falls after print(fall_time) but before println(rise-time-fall_time), then you'd have a fall time which is greater than rise-time! Worst, the interrupt could happen in the middle of the access to the rise_time or fall_time variable by the print function, which would result in print/println reading e.g. one byte as it was before the interrupt and another one belonging to the new time value. A disaster.
All of this won't happen if the signal timing guarantees nothing would happen during the execution of the if block. A better solution would be this (I'm writing in pseudocode what PaulS said before):
unsigned long aux_fall_time;
unsigned long aux_rise_time;
- disable_interrupts()
- aux_fall_time = fall_time
- aux_rise_time = rise_time
- enable interrupts()
- print/ln as before, using aux_ variables instead of the other ones.
Also, I'd put a delay() as the last statement of the loop() function, to bring the ratio between int_disabled_time and int_enabled_time near zero.
HTH
(disclaimer: I'm a bit in a hurry, so errors might have slipped through the keyboard 