 Solution

In an ordinary general-purpose operating system, untrusted user code should not be allowed to disable the system's timer and device interrupts. Otherwise a nonterminating program could prevent scheduling and device service. User mode is not a reason to permit arbitrary writes to protected interrupt controls.

A restricted trusted environment can nevertheless make this combination useful: a kernel may permit a short, bounded user-level real-time or device operation to run without maskable interrupts, or provide a virtualized per-task interrupt mask which delays that task's notifications while leaving system service active. Such permission must not let an untrusted task disable global service indefinitely. On a uniprocessor a short hardware-masked interval can reduce interruption latency, but it does not prevent faults, nonmaskable interrupts or activity on other cores. Thus the combination is meaningful in controlled cases, not a normal synchronization technique for arbitrary user programs.