关于信号(signal)处理机制的疑问

发布于 2020-08-07 14:21:39 浏览：1856 订阅该版

RT-Thread版本：4.0.3
```c
static void _signal_deliver(rt_thread_t tid)
{
    rt_ubase_t level;

level = rt_hw_interrupt_disable();

/* thread is not interested in pended signals */
    if (!(tid->sig_pending & tid->sig_mask))
    {
        rt_hw_interrupt_enable(level);
        return;
    }

if ((tid->stat & RT_THREAD_STAT_MASK) == RT_THREAD_SUSPEND)
    {
        /* resume thread to handle signal */
        rt_thread_resume(tid);
        /* add signal state */
        tid->stat |= (RT_THREAD_STAT_SIGNAL | RT_THREAD_STAT_SIGNAL_PENDING);

rt_hw_interrupt_enable(level);

/* re-schedule */
        rt_schedule();
    }
    else
    {
        if (tid == rt_thread_self())
        {
            /* add signal state */
            tid->stat |= RT_THREAD_STAT_SIGNAL;

rt_hw_interrupt_enable(level);

/* do signal action in self thread context */
            if (rt_interrupt_get_nest() == 0)
            {
                rt_thread_handle_sig(RT_TRUE);
            }
        }
        else if (!((tid->stat & RT_THREAD_STAT_SIGNAL_MASK) & RT_THREAD_STAT_SIGNAL))
        {
            /* add signal state */
            tid->stat |= (RT_THREAD_STAT_SIGNAL | RT_THREAD_STAT_SIGNAL_PENDING);

#ifdef RT_USING_SMP
            {
                int cpu_id;

cpu_id = tid->oncpu;
                if ((cpu_id != RT_CPU_DETACHED) && (cpu_id != rt_hw_cpu_id()))
                {
                    rt_uint32_t cpu_mask;

cpu_mask = RT_CPU_MASK ^ (1 << cpu_id);
                    rt_hw_ipi_send(RT_SCHEDULE_IPI, cpu_mask);
                }
            }
#else
            /* point to the signal handle entry */
            tid->stat &= ~RT_THREAD_STAT_SIGNAL_PENDING;
            tid->sig_ret = tid->sp;
            tid->sp = rt_hw_stack_init((void *)_signal_entry, RT_NULL,
                                       (void *)((char *)tid->sig_ret - 32), RT_NULL);
#endif

rt_hw_interrupt_enable(level);
            LOG_D("signal stack pointer @ 0x%08x", tid->sp);

/* re-schedule */
            rt_schedule();
        }
        else
        {
            rt_hw_interrupt_enable(level);
        }
    }
}
```
在src/signal.c的_signal_deliver()函数中，根据目标线程的状态分为三种情况：
1. 目标线程是挂起状态时，恢复成就绪状态，设置信号标志，然后rt_shedule()
2. 目标线程不是挂起状态，如果目标线程就是当前线程，则直接处理
3. 目标线程不是挂起状态，如果目标线程不是当前线程，则在目标线程的栈上构建新的context，且通过rt_hw_stack_init把线程entry改为_signal_entry，然后rt_schedule()  
我的疑问是：
上述3的情况，为什么要采用构建新的context的方式来处理信号，为什么不采用与情况1相同的处理，也就是通过设置信号标志，然后在目标线程被唤醒的rt_schedule()中来处理？
另外，这里为什么要主动调用一次rt_schedule()？