这个完成函数小弟我看不太明白

这个完成函数我看不太明白
这个完成函数我看不太明白，首先我不明白在什么时候会出现irp->CurrentLocation <= 的情况，我发现将完成函数设置到了本层上。通过IoSkipCurrentIrpStackLocation + IoSetCompletionRoutine完成。在这个完成函数中做的一系列操作我看不明白，请大牛指点，谢谢

/*
* completion routine for case if we use IoSkipCurrentIrpStackLocation way
*/
NTSTATUS
tdi_skip_complete(IN PDEVICE_OBJECT DeviceObject, IN PIRP Irp, IN PVOID Context)
{
  TDI_SKIP_CTX *ctx = (TDI_SKIP_CTX *)Context;
  NTSTATUS status;
  PIO_STACK_LOCATION irps;

  if (Irp->IoStatus.Status != STATUS_SUCCESS)
    FilterDebugPrint(("%s: status 0x%x\n", __FUNCTION__, Irp->IoStatus.Status));

  // restore IRP for using in our completion

  Irp->CurrentLocation--;
  Irp->Tail.Overlay.CurrentStackLocation--;

  irps = IoGetCurrentIrpStackLocation(Irp);

  DeviceObject = irps->DeviceObject;

  if (ctx->new_cr != NULL) {
    // restore fileobject (it's NULL)
    irps->FileObject = ctx->fileobj;
    // set new device object in irps
    irps->DeviceObject = ctx->new_devobj;

    // call new completion
    status = ctx->new_cr(ctx->new_devobj, Irp, ctx->new_context);//(C)ctx->new_context = context;(B)completion->context = query_irp;(A) query_irp = TdiBuildInternalDeviceControlIrp(TDI_QUERY_INFORMATION,//生成一个空的请求

  }
  else
  {
    //i need to release the counter at here.((:(
    status = STATUS_SUCCESS;
  }
  /* patch IRP back */

  // restore routine and context (and even control!)
  irps->CompletionRoutine = ctx->old_cr;
  irps->Context = ctx->old_context;
  irps->Control = ctx->old_control;

  // restore device object
  irps->DeviceObject = DeviceObject;

  Irp->CurrentLocation++;
  Irp->Tail.Overlay.CurrentStackLocation++;

  if (ctx->old_cr != NULL) {

    if (status != STATUS_MORE_PROCESSING_REQUIRED) {
      // call old completion (see the old control)
      BOOLEAN b_call = FALSE;

      if (Irp->Cancel) {
        // cancel
        if (ctx->old_control & SL_INVOKE_ON_CANCEL)
          b_call = TRUE;
      } else {
        if (Irp->IoStatus.Status >= STATUS_SUCCESS) {
          // success
          if (ctx->old_control & SL_INVOKE_ON_SUCCESS)
            b_call = TRUE;
        } else {
          // error
          if (ctx->old_control & SL_INVOKE_ON_ERROR)
            b_call = TRUE;
        }
      }

      if (b_call)
        status = ctx->old_cr(DeviceObject, Irp, ctx->old_context);

    } else {

      /*
       * patch IRP to set IoManager to call completion next time
       */

      // restore Control
      irps->Control = ctx->old_control;

    }
  }

  ExFreePool(ctx);

  return status;
}

引用它的地方：

if (cr == NULL || irp->CurrentLocation <= 1/*前提建立在cr!=0时*/) // WINDBG时：irp->CurrentLocation = 2  一个是过滤设备，下一个是Tcp&Udp原设备
    {
      /*
       * we use _THIS_ way of sending IRP to old driver
       * a) to avoid NO_MORE_STACK_LOCATIONS

这个完成函数小弟我看不太明白

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