• If a dispatch routine returns STATUS_PENDING, the IRP passed into the
  dispatch routine must be marked pending with IoMarkIrpPending().
• If an IRP is marked pending with IoMarkIrpPending() in a dispatch routine,
  that dispatch routine must return STATUS_PENDING.
• If an IRP is to be marked pending, IoMarkIrpPending must be called before
  the IRP is accessible from any context in which it might be completed.
• If you complete the IRP in the dispatch routine, you must:
• Fill Irp->IoStatus.Status with the IRP's completion status (this may NOT be STATUS_PENDING)
• Fill Irp->IoStatus.Information with the number of bytes read or written by the request if the request is being completed with a success status (for data transfer operations and IOCTLs), and
• Call IoCompleteRequest().

On return from the dispatch routine, you must return the same status that you filled into Irp->IoStatus.Status. Note that in some cases Irp-> IoStatus.Information can hold a pointer.

• Once you call IoCompleteRequest(), you no longer "own" the IRP, and must not refer to any of its contents.
• Irp->IoStatus.Status must be filled before calling IoCompleteRequest. In some cases, you can use Irp->IoStatus as a temporary storage before this, assuming you do not pass the IRP to another driver. However, this does not apply to IRPs for IRP_MJ_PNP, because these IRPs arrive from the PnP Manager with a default status already filled into the IoStatus.Status field.
• Because of special logic in the I/O Manager's completion handling code, drivers can also pass an IRP to another by "return IoCallDriver(...)" without setting a completion routine in the IRP. In this case, IoMarkIrpPending must not be called.
• The following sequence is also valid, though slower. It is necessary as a
  workaround for filtering some buggy drivers like CDFS in NT V4.0):
  
  IoMarkIrpPending(Irp);
  (VOID)IoCallDriver(BottomDeviceObject, Irp);
  return STATUS_PENDING;

I/O Completion Routines

• There are only three valid return codes from an I/O Completion Routine:
  STATUS_MORE_PROCESSING_REQUIRED, STATUS_SUCCESS, and
  STATUS_CONTINUE_COMPLETION (which is identical to STATUS_SUCCESS, see NTDDK.H)
•  STATUS_MORE_PROCESSING_REQUIRED means that IoCompleteRequest returns to its caller immediately. No further completion routines are called, and IRP is not returned to the I/O Manager.
• If and only if the completion routine returns STATUS_SUCCESS (or
  STATUS_CONTINUE_COMPLETION) , then it must do the following:
  
  if( Irp->PendingReturned ) {
  IoMarkIrpPending(Irp);
  }
• If you set a completion routine into an IRP that's passed into your driver, and if that completion routine returns STATUS_MORE_PROCESSING_REQUIRED, your dispatch routine for that IRP must either return STATUS_PENDING or it must block until the completion routine has run (for example, waiting on an event
  that is set from within the completion routine).