Disabeling interrupts to solve critical section issue on multiprocessor system
We are generally concerned about controlling concurrent access to kernel data structures.  User-level processes don't have access to kernel data structures, so they can't do any harm from user space.  The only risks are (1) if a user process does a syscall or causes a context switch to kernel mode, (2) if an interrupt arrives.  (1) could be guarded by explicitly checking for a lock, or by other methods.  (2) could be guarded by disabling interrupts, if you had a way to disable them on all cores.