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How Completion Processing Is Performed

Note

For optimal reliability and performance, use file system minifilter drivers with Filter Manager support instead of legacy file system filter drivers. To port your legacy driver to a minifilter driver, see Guidelines for Porting Legacy Filter Drivers.

Completion processing is performed in two stages. The first stage is performed in an arbitrary thread context, at IRQL <= DISPATCH_LEVEL. In this stage, the following tasks are performed:

  • Each completion routine registered for the IRP is called in turn, beginning with the lowest IRP stack location. If a completion routine returns STATUS_MORE_PROCESSING_REQUIRED, completion processing is halted.

  • If the IRP contains a memory descriptor list (MDL), any physical pages mapped by the MDL are unlocked.

  • The second phase of I/O completion is queued to the target (requesting) thread as a special kernel APC.

The second stage is performed in the context of the thread that originated the I/O request. It's executed as a special kernel APC and therefore runs at IRQL APC_LEVEL. In this stage, the following tasks are performed:

  • If the IRP represents a buffered operation, the contents of Irp->AssociatedIrp.SystemBuffer are copied to Irp->UserBuffer.

  • If the IRP contains an MDL, the MDL is freed.

  • The contents of Irp->IoStatus are copied to Irp->UserIosb so that the originator of the I/O request can see the final status of the operation.

  • If an event has been supplied in Irp->UserEvent, it's signaled. Otherwise, if there's a file object for this IRP, its event is signaled.

  • If the IRP was created by calling IoBuildDeviceIoControlRequest or IoBuildSynchronousFsdRequest, it's dequeued from the thread's pending I/O request list.

  • A user APC is queued, if the caller requested one.

  • The IRP is freed.

If completion processing for an IRP is halted because a completion routine returned STATUS_MORE_PROCESSING_REQUIRED, it can be resumed by calling IoCompleteRequest on the same IRP. When this situation happens, first-stage processing resumes, beginning with the completion routine for the driver immediately above the one whose completion routine returned STATUS_MORE_PROCESSING_REQUIRED.