软中断上下文切换接口

__do_softirq
     -> softirq_handle_begin

static inline void softirq_handle_begin(void)
{
    __local_bh_disable_ip(_RET_IP_, SOFTIRQ_OFFSET);
}

static __always_inline void __local_bh_disable_ip(unsigned long ip, unsigned int cnt)
{
    preempt_count_add(cnt);
    barrier();
}

softirq_handle_begin 通过调用 __local_bh_disable_ip 进入软中断上下文