title: spin_lock_irqsave vs spin_lock_irq
body: <p>On an SMP machine we must use  <code>spin_lock_irqsave</code> and not <code>spin_lock_irq</code> from interrupt context. </p>
  
  <p>Why would we want to save the flags (which contain the IF)?</p>
  
  <p>Is there another interrupt routine that could interrupt us?</p>