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

原文网址：http://blog.youkuaiyun.com/skyflying2012/article/details/8265869

今天在写触摸屏驱动时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，于是研究了一下才发现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.