代码就是驱动详解的11章test1吧，我特意重新编译运行了下，好像没你说的问题

DriverA:Enter A HelloDDKRead
DriverA:Leave A HelloDDKRead

DriverA:complete the Driver A IRP_MJ_READ irp!ontimedpc调用了iocompleterequest()
此时已经返回到DriverB，才能继续调用ZwClose(hDevice);下面两行就是证据
DriverA:Enter A HelloDDKClose
DriverA:Leave A HelloDDKClose

DriverB:Leave B HelloDDKRead
DriverB:Enter B HelloDDKClose
DriverB:Leave B HelloDDKClose
watchdog!WdUpdateRecoveryState: Recovery enabled.

谢谢， 我也调出来了， 呵呵， 加了一句“IoMarkIrpPending(pIrp);”
NTSTATUS HelloDDKRead(IN PDEVICE_OBJECT pDevObj, IN PIRP pIrp)
{

        PDEVICE_EXTENSION pDevExt = NULL;
        ULONG ulMicroSecond = 0;
        LARGE_INTEGER timeout ;
        _asm int 3
        KdPrint(("DriverA: Enter A HelloDDKRead\n"));
       
        //获取设备扩展
        pDevExt = (PDEVICE_EXTENSION) pDevObj->DeviceExtension;
       
        //将IRP设置为挂起
        IoMarkIrpPending(pIrp);
        //将挂起的IRP记录下来
        pDevExt->currentPendingIRP = pIrp;

        //将定时器的时间设置为3s
        ulMicroSecond = 3000000;

        //将32位整数转化成64位整数
        timeout = RtlConvertLongToLargeInteger(-10 * ulMicroSecond);

        //开启定时器
        KeSetTimer(&pDevExt->pollingTimer, timeout, &pDevExt->pollingDPC);
        KdPrint(("DriverA: leave A HelloDDKRead\n"));

        //返回pending状态
        return STATUS_PENDING;
}

原来楼主少了一句，额，我把那句去掉后真是没法返回到dirverB。
没搞懂为什么必须有这一句IoMarkIrpPending。
查资料查了1，2个小时，终于搞明白了，
爽！

DriverB->ZwReadfile->NtReadFile()->IopPerformSynchronousRequest()
......
[COLOR="Red"]//在此情景中,DeviceObject对应与DriverA的设备对象.IoCallDriver即是调用DriverA的Read派遣函数。
由上面的代码可以看出，DriverA的read派遣函数没做什么操作，直接返回STATUS_PENDING了[/COLOR]
  Status = IoCallDriver(DeviceObject, Irp);

...
    if (SynchIo)
    {
        /* Make sure the IRP was completed, but returned pending */
        if (Status == STATUS_PENDING)
        [COLOR="red"]{/*于是，NtReadFile就在这里一直等着，直到事件被设置。
此事件是DriverA的ontimedpc函数中的IoCompleteRequest来设置的，转到下面的IoCompleteRequest源代码[/COLOR]
            /* Wait for the IRP */
            Status = KeWaitForSingleObject(&FileObject->Event,
                                           Executive,
                                           PreviousMode,
                                           (FileObject->Flags &
                                            FO_ALERTABLE_IO),
                                           NULL);
     ................
    }

 DriverA->OnTimeDpc->IoCompleteRequest->IofCompleteRequest
.......
 do
    {
        /* Set Pending Returned */
  [COLOR="red"]     //只有在DriverA中调用IoMarkIrpPending，才能在此处使Irp->PendingRetured = 1,
从而才能调用后面蓝色部分，使DriverB等待的事件得以设置。这样就能从DriverB中返回了。
正因为如此，如果不调用IoMarkIrpPending，DriverB就只有一直等待了，终于搞明白了[/COLOR]
        Irp->PendingReturned = StackPtr->Control & SL_PENDING_RETURNED;
........
        /*
         * Either we didn't return from the request, or we did return but this
         * request was synchronous.
         */
        if (([COLOR="blue"]Irp->PendingReturned) [/COLOR]&& (FileObject))
        {
            /* So we did return with a synch operation, was it the IRP? */
            if (Irp->Flags & IRP_SYNCHRONOUS_API)
            {
                /* Yes, this IRP was synchronous, so return the I/O Status */
                *Irp->UserIosb = Irp->IoStatus;

                /* Now check if the user gave an event */
                if (Irp->UserEvent)
                {
                    /* Signal it */
                   [COLOR="blue"] KeSetEvent(Irp->UserEvent, 0, FALSE);[/COLOR]                }
                else
                {
                    /* No event was given, so signal the FO instead */
                    KeSetEvent(&FileObject->Event, 0, FALSE);
                }
            }
            else
            {
                /*
                 * It's not the IRP that was synchronous, it was the FO
                 * that was opened this way. Signal its event.
                 */
                FileObject->FinalStatus = Irp->IoStatus.Status;
                KeSetEvent(&FileObject->Event, 0, FALSE);
            }
        }

#define IoMarkIrpPending(_Irp) \
  (IoGetCurrentIrpStackLocation(_Irp)->Control |= SL_PENDING_RETURNED)

哎，还解决了一个困扰我很久的问题，在完成例程里为什么必须传播pending位
多谢楼主了好问题，呵呵

虽然知道要传播pending位， 但我也不知道为什么， 学习了

不错不错！