+ *
+ * cpu_online_map is updated by the _cpu_down()
+ * using stop_machine_run(). Since we're in irqs disabled
+ * section, stop_machine_run() is not exectuting, hence
+ * the cpu_online_map is stable.
+ *
+ * However, a cpu might have been offlined _just_ before
+ * we disabled irqs while entering here.
+ * And rcu subsystem might not yet have handled the CPU_DEAD
+ * notification, leading to the offlined cpu's bit
+ * being set in the rcp->cpumask.
+ *
+ * Hence cpumask = (rcp->cpumask & cpu_online_map) to prevent
+ * sending smp_reschedule() to an offlined CPU.