Implementing an IoCompletion Routine

On entry, an IoCompletion routine receives a Context pointer. When a dispatch routine calls IoSetCompletionRoutine, it can supply a Context pointer. This pointer can reference whatever driver-determined context information the IoCompletion routine requires to process an IRP. Note that the context area cannot be pageable because the IoCompletion routine can be called at IRQL = DISPATCH_LEVEL.

Consider the following implementation guidelines for IoCompletion routines:

  • An IoCompletion routine can check the IRP's I/O status block to determine the result of the I/O operation.

  • If the input IRP was allocated by the dispatch routine using IoAllocateIrp or IoBuildAsynchronousFsdRequest, the IoCompletion routine must call IoFreeIrp to release that IRP, preferably before it completes the original IRP.

    • The IoCompletion routine must release any per-IRP resources the dispatch routine allocated for the driver-allocated IRP, preferably before it frees the corresponding IRP.

      For example, if the dispatch routine allocates an MDL with IoAllocateMdl and calls IoBuildPartialMdl for a partial-transfer IRP it allocates, the IoCompletion routine must release the MDL with IoFreeMdl. If it allocates resources to maintain state about the original IRP, it must free those resources, preferably before it calls IoCompleteRequest with the original IRP and definitely before it returns control.

      In general, before freeing or completing an IRP, the IoCompletion routine should free any per-IRP resources allocated by the Dispatch routine. Otherwise, the driver must maintain state about the resources to be freed before its IoCompletion routine returns control from completing the original request.

    • If the IoCompletion routine cannot complete the original IRP with STATUS_SUCCESS, it must set the I/O status block in the original IRP to the value returned in the driver-allocated IRP that caused the IoCompletion routine to fail the original request.

    • If the IoCompletion routine will complete the original request with STATUS_PENDING, it must call IoMarkIrpPending with the original IRP before it calls IoCompleteRequest.

    • If the IoCompletion routine must fail the original IRP with an error STATUS_XXX, it can log an error. However, it is the responsibility of the underlying device driver to log any device I/O errors that occur, so IoCompletion routines usually do not log errors.

    • When the IoCompletion routine has processed and freed the driver-allocated IRP, the routine must return control with STATUS_MORE_PROCESSING_REQUIRED.

      Returning STATUS_MORE_PROCESSING_REQUIRED from the IoCompletion routine forestalls the I/O manager's completion processing for a driver-allocated and freed IRP. A second call to IoCompleteRequest causes the I/O manager to resume calling the IRP's completion routines, starting with the completion routine immediately above the routine that returned STATUS_MORE_PROCESSING_REQUIRED.

  • If the IoCompletion routine reuses an incoming IRP to send one or more requests to lower drivers, or if the routine retries failed operations, it should update whatever context the IoCompletion routine maintains about each reuse or retry of the IRP. Then it can set up the next-lower driver's I/O stack location again, call IoSetCompletionRoutine with its own entry point, and call IoCallDriver for the IRP.

    • The IoCompletion routine should not call IoMarkIrpPending at each reuse or retry of the IRP.

      The dispatch routine already marked the original IRP as pending. Until all drivers in the chain complete the original IRP with IoCompleteRequest, it remains pending.

    • Before retrying a request, the IoCompletion routine should reset the I/O status block with STATUS_SUCCESS for Status and zero for Information, possibly after saving the returned error information.

      For each retry, the IoCompletion routine usually decrements a retry count set up by the Dispatch routine. Typically, the IoCompletion routine must call IoCompleteRequest to fail the IRP when some limited number of retries have failed.

    • The IoCompletion routine must return STATUS_MORE_PROCESSING_REQUIRED after it calls IoSetCompletionRoutine and IoCallDriver with an IRP that is being reused or retried.

      Returning STATUS_MORE_PROCESSING_REQUIRED from the IoCompletion routine forestalls the I/O manager's completion processing of a reused or retried IRP.

    • If the IoCompletion routine cannot complete the original IRP with STATUS_SUCCESS, it must leave the I/O status block as returned by lower drivers for the reuse or retry operation that causes the IoCompletion routine to fail the IRP.

    • If the IoCompletion routine will complete the original request with STATUS_PENDING, it must call IoMarkIrpPending with the original IRP before it calls IoCompleteRequest.

    • If the IoCompletion routine must fail the original IRP with an error STATUS_XXX, it can log an error. However, it is the responsibility of the underlying device driver to log any device I/O errors that occur, so IoCompletion routines usually do not log errors.
  • Any driver that sets an IoCompletion routine in an IRP and then passes the IRP down to a lower driver should check the IRP->PendingReturned flag in the IoCompletion routine. If the flag is set, the IoCompletion routine must call IoMarkIrpPending with the IRP. Note, however, that a driver that passes down the IRP and then waits on an event should not mark the IRP pending. Instead, its IoCompletion routine should signal the event and return STATUS_MORE_PROCESSING_REQUIRED.

  • The IoCompletion routine must release any resources the dispatch routine allocated for processing the original IRP, preferably before the IoCompletion routine calls IoCompleteRequest with the original IRP and definitely before the IoCompletion routine returns control from completing the original IRP.

If any higher-level driver has set its IoCompletion routine in the original IRP, that driver's IoCompletion routine is not called until the IoCompletion routines of all lower-level drivers have been called.

Supplying a Priority Boost in Calls to IoCompleteRequest

If a lowest-level device driver can complete an IRP in its dispatch routine, it calls IoCompleteRequest with a PriorityBoost of IO_NO_INCREMENT. No run-time priority increase is needed because the driver can assume that the original requester did not wait for its I/O operation to be completed.

Otherwise, the lowest-level driver supplies a system-defined and device-type-specific value that boosts the requester's run-time priority to compensate for the time the requester waited on its device I/O request. See Wdm.h or Ntddk.h for the boost values.

Higher-level drivers apply the same PriorityBoost as their respective underlying device drivers when they call IoCompleteRequest.

Effect of Calling IoCompleteRequest

When a driver calls IoCompleteRequest, the I/O manager fills that driver's I/O stack location with zeros before calling the next higher-level driver, if any, that has set up an IoCompletion routine to be called for the IRP.

A higher-level driver's IoCompletion routine can check only the IRP's I/O status block to determine how all lower drivers handled the request.

The caller of IoCompleteRequest must not attempt to access the just-completed IRP. Such an attempt is a programming error that causes a system crash.