Implementing a Cancel Routine
The I/O manager calls a driver-supplied Cancel routine with an input IRP to be canceled and a DeviceObject pointer that represents the target device for the I/O request.
The IRP could be one that the driver's DispatchReadWrite routine has queued just as the current Win32 application is being closed by the user. The IRP also could be one that a higher-level driver explicitly canceled, depending on the nature of the underlying device.
When the Cancel routine is called, the input IRP might already be the CurrentIrp in the target device object or might already be in the device queue associated with the target device object if the driver has a StartIo routine. If the driver has no StartIo routine, the IRP might be in a driver-managed internal queue of IRPs when its Cancel routine is called. In any case, before the I/O manager calls the Cancel routine for the incoming IRP, the I/O manager sets the Cancel member in this IRP to TRUE and sets the CancelRoutine member in the IRP to NULL.
The Cancel routine for a master IRP that has associated IRPs is responsible for calling IoCancelIrp to cancel those associated IRPs.
All Cancel routines must follow these guidelines:
Call IoReleaseCancelSpinLock to release the system's cancel spin lock.
Set the I/O status block's Status member to STATUS_CANCELLED, and set its Information member to zero.
Complete the specified IRP by calling IoCompleteRequest.
Because a Cancel routine is always called with the system cancel spin lock held, this routine must not call IoAcquireCancelSpinLock unless it calls IoReleaseCancelSpinLock first.
A Cancel routine cannot be holding the system cancel spin lock when it returns control. That is, every Cancel routine must call IoReleaseCancelSpinLock at least once before it returns control.
If it calls IoAcquireCancelSpinLock, a Cancel routine must make the reciprocal call to IoReleaseCancelSpinLock as quickly as possible.
Never call IoCompleteRequest with an IRP while holding a spin lock. Attempting to complete an IRP while holding a spin lock can cause deadlocks.
For more information about implementing a Cancel routine, see the I/O Completion/Cancellation Guidelines white paper on the Microsoft Windows Hardware Developer Central (WHDC) website.