PIRP

KeyboardClassDequeueRead(

    _In_ PCHAR DeviceExtension

)

/*++

Routine Description:

    Dequeues the next available read irp regardless of FileObject

Assumptions:

    DeviceExtension->SpinLock is already held (so no further sync is required).

  --*/

{

    ASSERT(NULL != DeviceExtension);

    PIRP nextIrp = NULL;

    LIST_ENTRY* ReadQueue = (LIST_ENTRY*)(DeviceExtension + READ_QUEUE_OFFSET_DE);

    while (!nextIrp && !IsListEmpty(ReadQueue)) {

        PDRIVER_CANCEL oldCancelRoutine;

        PLIST_ENTRY listEntry = RemoveHeadList(ReadQueue);

        //

        // Get the next IRP off the queue and clear the cancel routine

        //

        nextIrp = CONTAINING_RECORD(listEntry, IRP, Tail.Overlay.ListEntry);

        oldCancelRoutine = IoSetCancelRoutine(nextIrp, NULL);

        //

        // IoCancelIrp() could have just been called on this IRP.

        // What we're interested in is not whether IoCancelIrp() was called

        // (ie, nextIrp->Cancel is set), but whether IoCancelIrp() called (or

        // is about to call) our cancel routine. To check that, check the result

        // of the test-and-set macro IoSetCancelRoutine.

        //

        if (oldCancelRoutine) {

            //

                //  Cancel routine not called for this IRP.  Return this IRP.

            //

            /*ASSERT(oldCancelRoutine == KeyboardClassCancel);*/

        }

        else {

            //

                // This IRP was just cancelled and the cancel routine was (or will

            // be) called. The cancel routine will complete this IRP as soon as

            // we drop the spinlock. So don't do anything with the IRP.

            //

                // Also, the cancel routine will try to dequeue the IRP, so make the

            // IRP's listEntry point to itself.

            //

            //ASSERT(nextIrp->Cancel);

            InitializeListHead(&nextIrp->Tail.Overlay.ListEntry);

            nextIrp = NULL;

        }

    }

    return nextIrp;

}