Consider the following interrupt protection levels in Linux, and arrange them…

Answer (in increasing order of priority): User-Mode Programs (preemptible) → Kernel System Service Routines (preemptible) → Bottom Half Interrupt Handlers → Top Half Interrupt Handlers

Explanation:

• User-Mode Programs (preemptible): these run in user space and are the most easily preemptible, so they have the lowest priority.

• Kernel System Service Routines (preemptible): run in kernel mode and preempt user processes, but they can still be interrupted by interrupt contexts, so they are higher than user-mode but lower than interrupt-related handlers.

• Bottom Half Interrupt Handlers: include softirqs/tasklets that execute in an interrupt context after the top half; they have higher priority than preemptible kernel code because they run as part of interrupt handling.

• Top Half Interrupt Handlers: are hardware interrupt handlers executed immediately in interrupt context and have the highest priority.

Key idea: priority increases from ordinary user processes up through preemptible kernel routines to bottom-half softirqs and finally to top-half hardware interrupt handlers.