Architecture neither relies on exception_enter()/exception_exit()
nor on schedule_user(). Also preempt_schedule_notrace() and
preempt_schedule_irq() can't be called in a preemptible section
while context tracking is CT_STATE_USER. This feature reflects a sane
entry implementation where the following requirements are met on
critical entry code, ie: before user_exit() or after user_enter():

- Critical entry code isn't preemptible (or better yet:
not interruptible).
- No use of RCU read side critical sections, unless ct_nmi_enter()
got called.
- No use of instrumentation, unless instrumentation_begin() got
called.