中断处理函数中不用disable_irq而用disable_irq_nosync原因

在中断处理函数中使用disable_irq关中断发现在进入中断处理后内核就挂掉了，发现disable_irq关闭中断并等待中断处理完后返回, 而disable_irq_nosync立即返回. 在中断处理程序中应该使用disable_irq_nosync来关闭中断

先看一下disable_irq_nosync,内核代码中是这样解释的:
/**
 *    disable_irq_nosync - disable an irq without waiting
 *    @irq: Interrupt to disable
 *
 *    Disable the selected interrupt line. Disablesand Enables are
 *    nested.
 *    Unlike disable_irq(),this function doesnot ensure existing
 *    instances of the IRQ handler have completed before returning.
 *
 *    This function may be called from IRQ context.
 */
void disable_irq_nosync(unsigned int irq)
{
    struct irq_desc *desc = irq_to_desc(irq);
    unsigned long flags;

    if (!desc)
        return;

    chip_bus_lock(irq, desc);
    spin_lock_irqsave(&desc->lock, flags);
    __disable_irq(desc, irq, false);
    spin_unlock_irqrestore(&desc->lock, flags);
    chip_bus_sync_unlock(irq, desc);
}

关闭中断后程序返回, 如果在中断处理程序中, 那么会继续将中断处理程序执行完.

/**
 * disable_irq - disable an irq and wait for completion
 * @irq: Interrupt to disable
 *
 * Disable the selected interrupt line. Enables and Disables are
 * nested.
 * This function waits for any pending IRQ handlers for this interrupt
 * to complete before returning. If you use this function while
 * holding a resource the IRQ handler may need you will deadlock.
 *
 * This function may be called - with care - from IRQ context.
 */
void disable_irq(unsignedint irq)
{
        struct irq_desc *desc = irq_desc + irq;
        if (irq>= NR_IRQS)
                return;
        disable_irq_nosync(irq);
        if (desc->action)
                synchronize_irq(irq);
}

关闭中断并等待中断处理完后返回.从代码中可以看到, disable_irq先是调用了disable_irq_nosync, 然后检测desc->action是否为1. 在中断处理程序中, action是置1的, 所以进入synchronize_irq函数中.

/**
 * synchronize_irq - wait for pending IRQ handlers (on other CPUs)
 * @irq: interrupt number to wait for
 *
 * This function waits for any pending IRQ handlers for this interrupt
 * to complete before returning. If you use this function while
 * holding a resource the IRQ handler may need you will deadlock.
 *
 * This function may be called - with care - from IRQ context.
 */
void synchronize_irq(unsignedint irq)
{
 struct irq_desc *desc= irq_to_desc(irq);
 unsigned int status;
 if (!desc)
  return;
 do {
  unsigned long flags;
  /*
   * Wait until we're out of the critical section. This might
   * give the wrong answer due to the lack of memory barriers.
   */
  while (desc->status& IRQ_INPROGRESS)
   cpu_relax();
  /* Ok, that indicated we're done: double-check carefully. */
  spin_lock_irqsave(&desc->lock, flags);
  status = desc->status;
  spin_unlock_irqrestore(&desc->lock, flags);
  /* Oops, that failed? */
 } while (status & IRQ_INPROGRESS);
 /*
  * We made sure that no hardirq handler is running. Now verify
  * that no threaded handlers are active.
  */
 wait_event(desc->wait_for_threads,!atomic_read(&desc->threads_active));
}

注释中说明该函数是在等待中断处理程序的结束, 这也是disable_irq与disable_irq_nosync不同的主要所在. 但是在中断处理函数中调用会发生什么情况呢? 进入中断处理函数前IRQ_INPROGRESS会被__setup_irq设置, 所以程序会一直陷在while循环中, 而此时内核以经被独占, 这就导致系统死掉.
 
总结:
由于在disable_irq中会调用synchronize_irq函数等待中断返回, 所以在中断处理程序中不能使用disable_irq, 否则会导致cpu被synchronize_irq独占而发生系统崩溃.

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作者：kerneler_ 
来源：CSDN 
原文：https://blog.csdn.net/skyflying2012/article/details/8265869 
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