//中断退出时
1.void irq_exit(void)
{
       ...
       //递减中断嵌套深度
       sub_preempt_count(IRQ_EXIT_OFFSET);
       //如果已经退出中断，并且软中断标志被置位
       if (!in_interrupt() && local_softirq_pending())
       			//执行软中断
           		do_softirq();           

       ...
}
//APIC时钟中断中
2.void __irq_entry smp_apic_timer_interrupt(struct pt_regs *regs)
{
        ...
        irq_exit();
        ...
}

//重新激活下半部时
3.void local_bh_enable(void)
{
        _local_bh_enable_ip((unsigned long)__builtin_return_address(0));
}
static inline void _local_bh_enable_ip(unsigned long ip)
{
		...
        if (unlikely(!in_interrupt() && local_softirq_pending()))
                do_softirq();
        ...
}