Inform rcu that current cpu is leaving idle
void rcu_idle_exit(void);
void
no arguments
Exit idle mode, in other words, -enter- the mode in which RCU read-side critical sections can occur.
We crowbar the ->dynticks_nesting field to DYNTICK_TASK_NEST to allow for the possibility of usermode upcalls messing up our count of interrupt nesting level during the busy period that is just now starting.