Interrupt handlers and mutex

[Frank Zerangue <frank.zerangue%gmail.com@localhost>]

Help request -- Mutex(9) indicates that mutex replaces the spl(9) system.

(1) When writing an interrupt handler, should the handler acquire a spin mutex 
before modifying some IO that may be accessed also by a LWP?

(2) What happens when the interrupt handler cannot acquire the mutex? Will the 
LWP that holds it ever be able to run again?

(3) Will a LWP that holds a spin mutex be pre-empted by the scheduler?

Thanks to anyone who might offer some help to this poor soul --
Frank Zerangue