Let's start assuming that something in the idle loop posts a callback,
and scheduling-clock interrupt occurs:

1. The system is idle and stays that way, no runnable tasks.

2. Scheduling-clock interrupt occurs, rcu_check_callbacks() is called
   as result, which in turn calls rcu_is_cpu_rrupt_from_idle().

3. rcu_is_cpu_rrupt_from_idle() reports the CPU was interrupted from
   idle, which results in rcu_sched_qs() call, which does a
   raise_softirq(RCU_SOFTIRQ).

4. Upon return from interrupt, rcu_irq_exit() is invoked, which calls
   rcu_idle_enter_common(), which in turn calls rcu_sched_qs() again,
   which does another raise_softirq(RCU_SOFTIRQ).

5. The softirq happens shortly and invokes rcu_process_callbacks(),
   which invokes __rcu_process_callbacks().

6. So now callbacks can be invoked. At least they can be if
   ->donetail has been updated. Which it will have been because
   rcu_sched_qs() invokes rcu_qsctr_help().

In the described scenario rcu_sched_qs() and raise_softirq(RCU_SOFTIRQ)
get called twice in steps 3 and 4. This redundancy could be eliminated
by removing rcu_is_cpu_rrupt_from_idle() function.
Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>