If a minifilter driver's preoperation callback routine synchronizes an I/O operation by returning FLT_PREOP_SYNCHRONIZE, the filter manager calls the minifilter driver's postoperation callback routine during I/O completion.
The filter manager calls the minifilter driver's postoperation callback routine in the same thread context as the preoperation callback, at IRQL <= APC_LEVEL. (Note that this thread context is not necessarily the context of the originating thread.)
If the minifilter driver's preoperation callback routine returns FLT_PREOP_SYNCHRONIZE, it can return a non-NULL value in its CompletionContext output parameter. This parameter is an optional context pointer that is passed to the corresponding postoperation callback routine. The postoperation callback routine receives this pointer in its CompletionContext input parameter.
A minifilter driver's preoperation callback routine should return FLT_PREOP_SYNCHRONIZE only for IRP-based I/O operations. However, this status value can be returned for other operation types. If it is returned for an I/O operation that is not an IRP-based I/O operation, the filter manager treats this return value as if it were FLT_PREOP_SUCCESS_WITH_CALLBACK. To determine whether an operation is an IRP-based I/O operation, use the FLT_IS_IRP_OPERATION macro.
Minifilter drivers should not return FLT_PREOP_SYNCHRONIZE for create operations, because these operations are already synchronized by the filter manager. If a minifilter driver has registered preoperation and postoperation callback routines for IRP_MJ_CREATE operations, the post-create callback routine is called at IRQL = PASSIVE_LEVEL, in the same thread context as the pre-create callback routine.
Minifilter drivers must never return FLT_PREOP_SYNCHRONIZE for asynchronous read or write operations. Doing so can severely degrade both minifilter driver and system performance and can even cause deadlocks if, for example, the modified page writer thread is blocked. Before returning FLT_PREOP_SYNCHRONIZE for an IRP-based read or write operation, a minifilter driver should verify that the operation is synchronous by calling FltIsOperationSynchronous.
The following types of I/O operations cannot be synchronized:
Oplock file system control (FSCTL) operations (MajorFunction is IRP_MJ_FILE_SYSTEM_CONTROL; FsControlCode is FSCTL_REQUEST_FILTER_OPLOCK, FSCTL_REQUEST_BATCH_OPLOCK, FSCTL_REQUEST_OPLOCK_LEVEL_1, or FSCTL_REQUEST_OPLOCK_LEVEL_2.)
Notify change directory operations (MajorFunction is IRP_MJ_DIRECTORY_CONTROL; MinorFunction is IRP_MN_NOTIFY_CHANGE_DIRECTORY.)
Byte-range lock requests (MajorFunction is IRP_MJ_LOCK_CONTROL; MinorFunction is IRP_MN_LOCK.)
FLT_PREOP_SYNCHRONIZE cannot be returned for any of these operations.