The documentation (online MSDN library) says: __drv_requiresIRQL(APC_LEVEL) VOID ExReleaseFastMutex( __inout __drv_in(__drv_restoresIRQL __drv_releasesResource(FastMutex)) PFAST_MUTEX FastMutex ); The correct annotations for ExReleaseFastMutex from the model file are: __drv_requiresIRQL(APC_LEVEL) __drv_restoresIRQLGlobal(FastMutex, FastMutex) VOID ExReleaseFastMutex( __inout __drv_in( __drv_deref(__drv_releasesResource(FastMutex)))) PFAST_MUTEX FastMutex );