Hi,
Q: Isn't it so that INT_Handler execution cannot be interrupted (e.g interrupt in interrupt sequence)?
If yes, then debounce filtering is not needed, because first falling edge trigger INT_Handler. Ringing is ignored during interrupt execution time.
Yes, (as long as we're not talking low priority interrupts) but if the switch bounces longer than the time it takes to execute the ISR then it will get triggered again as soon as it leaves the interrupt routine.

The while-wend loop won't solve the problem either. If the switch bounces, the while-loop may check it at a moment when it's high and then exit the ISR, the switch continues to bounce and trips the interrupt again. The switch probably bounces on release as well so even if the while-loop manages to the job "sometimes" it's likely that once the button is released causing the program to leave the ISR, it will bounce and trip the interrupt again.

How about trying a bit of hardware, a single capacitor should do it.

/Henrik.