func: void scsi_req_cancel(SCSIRequest *req)
  
  {
  
      trace_scsi_req_cancel(req->dev->id, req->lun, req->tag);
  
      if (!req->enqueued) {
  
          return;
  
      }
  
  
  
      scsi_req_ref(req);
  
      scsi_req_dequeue(req);
  
      req->io_canceled = true;
  
      if (req->aiocb) {
  
          blk_aio_cancel(req->aiocb);
  
      } else {
  
          scsi_req_cancel_complete(req);
  
      }
  
  }