Fast interrupts

Interrupts flagged with INTR_FAST are fast interrupts. Kernel does not create separate kernel threads to handle fast interrupts. Instead the handler functions are directly executed from primary interrupt handler, intr_execute_handlers().

Fast interrupts are interrupts that happen often, with each occurrences must be handled fast. When Fast interrupts are executed in intr_execute_handlers(), they are protected by a critical section between critical_enter() and critical_exit() that prohibits preemption. So there is no mutex or Giant Lock needed.