各位大神，本人照着Windows驱动开发技术详解上的分层驱动的例子，驱动A为普通的驱动，驱动B负责IoAttachDeviceToDeviceStack到驱动A上，按照书上的说法，驱动B处于高层的设备链上，应该先退出，驱动A后退出，结果: 驱动B能退出，驱动A一直不能退出，即DebugView一直无法显示调用DriverUnload例程：
   驱动A为最普通的helloworld驱动，正常情况下能调用DriverUnload,但是有驱动B挂载的时候就无法退出;
  驱动B的源码为:
  DriverEntry：
        NtStatus = CreateDevice(pDriverObject);
        PDEVICE_EXTENSION pDevExt = (PDEVICE_EXTENSION)pDriverObject->pDeviceObject->DeviceExtension;
        if (!NT_SUCCESS(NtStatus))
        {
                DbgPrint("LayerDriverB CreateDevice Device[%wZ] failed \r\n",pDevExt->usDeviceName);
                return NtStatus;
        }
        //----------------------Attach Target Device-------------------------//
        UNICODE_STRING usTarDeviceName;
        RtlInitUnicodeString(&usTarDeviceName, L"\\Device\\DDKLayerDeviceA");
        PFILE_OBJECT pFileObj        = NULL;
        PDEVICE_OBJECT pDevTar  = NULL;
        NtStatus                                = IoGetDeviceObjectPointer(&usTarDeviceName,FILE_ALL_ACCESS,&pFileObj,&pDevTar);
        if (!NT_SUCCESS(NtStatus))
        {
                IoDeleteSymbolicLink(&pDevExt->usDosDeviceName);
                IoDeleteDevice(pDevExt->pDeviceObj);
                DbgPrint("LayerDriverB IoAttachDeviceToDeviceStack Device[%wZ] failed \r\n",&usTarDeviceName);
                return NtStatus;
        }

        PDEVICE_OBJECT pFilterDev = pDevExt->pDeviceObj;
        PDEVICE_OBJECT pTarDevice = IoAttachDeviceToDeviceStack(pFilterDev,pDevTar);
        if (NULL == pTarDevice)
        {
                ObReferenceObject(pFileObj);
                IoDeleteSymbolicLink(&pDevExt->usDosDeviceName);
                IoDeleteDevice(pDevExt->pDeviceObj);
                DbgPrint("LayerDriverB IoAttachDeviceToDeviceStack Device[%wZ] failed \r\n",&usTarDeviceName);
                return STATUS_INSUFFICIENT_RESOURCES;
        }
        pDevExt->pTargetObj = pTarDevice;

        pFilterDev->DeviceType = pTarDevice->DeviceType;
        pFilterDev->Characteristics = pTarDevice->Characteristics;
        pFilterDev->Flags &= ~DO_DEVICE_INITIALIZING;
        pFilterDev->Flags |= pTarDevice->Flags&(DO_BUFFERED_IO | DO_DIRECT_IO);

        ObReferenceObject(pFileObj);
        DbgPrint("LayerDriverB IoAttachDeviceToDeviceStack Device[%wZ] success \r\n",&usTarDeviceName);

     LayerDriverB_Unload:
    PDEVICE_OBJECT        pNextObj;
        DbgPrint(("DriverB:Enter B DriverUnload\n"));
        pNextObj = pDriverObject->pDeviceObject;

        while (pNextObj != NULL)
        {
                DbgPrint("Current Driver[%wZ] \r\n",&pNextObj->pDriverObject->DriverName);
                PDEVICE_EXTENSION pDevExt = (PDEVICE_EXTENSION)pNextObj->DeviceExtension;       
                //从设备栈中弹出
                IoDetachDevice( pDevExt->pTargetObj);
                //删除该设备对象
                IoDeleteDevice( pDevExt->pDeviceObj );
                IoDeleteSymbolicLink(&pDevExt->usDosDeviceName);
                DbgPrint("Device[%wZ] has been deleted \r\n",&pDevExt->usDosDeviceName);
                pNextObj = pNextObj->NextDevice;
        }

  按道理来说，肯定是驱动A的引用计数没到1，所以当驱动A退出(卸载服务)的时候没有调用unload例程，但是驱动B的代码都检查很多遍了，没找到哪些地方的问题，求各位大神帮帮忙～

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