关于取消IRP

这是DS的帮助文件里对KIrp的类函数TestAndSetCancelRoutine的一段描述
KIrp::TestAndSetCancelRoutine
BOOLEAN TestAndSetCancelRoutine(
   PDRIVER_CANCEL OriginalCancelRoutine,
   PDRIVER_CANCEL NewCancelRoutine,
   PIRP& CurrentIrp
);

If the IRP was previously in a non-cancelable state (OriginalCancelRoutine==NULL), and the IRP\'s cancel bit is set, then this function returns FALSE, and the caller is responsible for completing the IRP with STATUS_CANCELLED.

这句话是说，IRP以前是处于不可被取消的状态（原始取消例程是空），现在设置了IRP的取消位，往下，我理解不了，怎么就返回状态排队呢？

谁能详细说明这个??