local_bh_disable的简单注释

__local_bh_disable(SOFTIRQ_OFFSET)/__local_bh_enable(SOFTIRQ_OFFSET)
仅仅是增加/减少抢占计数, 这两个函数被__do_softirq()调用，应当不会被kernel线程来调用

static void __local_bh_enable(unsigned int cnt)
{
 WARN_ON_ONCE(in_irq());
 WARN_ON_ONCE(!irqs_disabled());

 if (softirq_count() == cnt)
  trace_softirqs_on((unsigned long)__builtin_return_address(0));
 sub_preempt_count(cnt); /* 减少抢占计数 */
}

static inline void __local_bh_disable(unsigned long ip, unsigned int cnt)
{
 add_preempt_count(cnt); /* 增加抢占计数 */ 
 barrier();
}
--------------------------------------------------------------------
在线程环境下调用local_bh_enable时，需要执行被耽误的softirq
static inline void _local_bh_enable_ip(unsigned long ip)
{
 WARN_ON_ONCE(in_irq() || irqs_disabled());
#ifdef CONFIG_TRACE_IRQFLAGS
 local_irq_disable();
#endif
 /*
  * Are softirqs going to be turned on now:
  */
 if (softirq_count() == SOFTIRQ_DISABLE_OFFSET)
  trace_softirqs_on(ip);
 /*
  * Keep preemption disabled until we are done with
  * softirq processing:
   */
 sub_preempt_count(SOFTIRQ_DISABLE_OFFSET - 1);  /* 此时抢占计数为1，还不能被抢占 */
 /* 但在执行softirq的callback函数时， 可以被中断和softirq打断 */

 if (unlikely(!in_interrupt() && local_softirq_pending()))
  do_softirq(); /*  这里是线程上下文，不是中断上下文，所以需要调用do_softirq（这个函数里面有先关闭中断的操作） */
    /* 因为线程里面禁止softirq，会导致softirq的推迟，所以在使能softirq后，主动执行已经被推迟的softirq */

 dec_preempt_count(); /* 抢占计数再减1，此时应当变为0，开启抢占 */
#ifdef CONFIG_TRACE_IRQFLAGS
 local_irq_enable();
#endif
 preempt_check_resched();
}