在非process context中，不能做什麼

 When you are outside of process context (i.e., in interrupt context), you must observe the
following rules:
  • No access to user space is allowed. Because there is no process context, there is
     no path to the user space associated with any particular process.

     不能訪問任何user space的東東

  • The current pointer is not meaningful in atomic mode and cannot be used since
     the relevant code has no connection with the process that has been interrupted.

     不能用current指標
  • No sleeping or scheduling may be performed. Atomic code may not call sched-
     ule or a form of wait_event, nor may it call any other function that could sleep.
     For example, calling kmalloc(..., GFP_KERNEL) is against the rules. Sema-
     phores also must not be used since they can sleep.
    不能sleep 或者 schedule。

   也不能調用任何會sleep的函數。