The following table summarizes the IRQL and thread context requirements for file system filter driver dispatch routines.

DriverEntry routines are called in the context of a system thread at IRQL = PASSIVE_LEVEL.

  Most drivers' dispatch routines are called in an arbitrary thread context at IRQL = PASSIVE_LEVEL, with the following exceptions:

1. Any highest-level driver's dispatch routines are called in the context of the thread that originated the I/O request, which is commonly a user-mode application thread.

    In other words, the dispatch routines of file system drivers and other highest-level drivers are called in a   nonarbitrary thread context at IRQL = PASSIVE_LEVEL.

2.The DispatchReadDispatchWrite, and DispatchDeviceControl routines of lowest-level device drivers, and of intermediate drivers layered above them in the system paging path, can be called at IRQL = APC_LEVEL and in an arbitrary thread context.

   The DispatchRead and/or DispatchWrite routines, and any other routine that also processes read and/or write requests in such a lowest-level device or intermediate driver, must be resident at all timesThese driver routines can neither be pageable nor be part of a driver's pageable-p_w_picpath section; they must not access any pageable memory. Furthermore, they should not be dependent on any blocking calls (such as KeWaitForSingleObject with a nonzero time-out).

3. The DispatchPower routine of drivers in the hibernation and/or paging paths can be called at IRQL = DISPATCH_LEVEL. TheDispatchPnP routines of such drivers must be prepared to handle PnP IRP_MN_DEVICE_USAGE_NOTIFICATION requests.

4. The DispatchPower routine of drivers that require inrush power at start-up can be called at IRQL = DISPATCH_LEVEL. 

Dispatch Routine Caller's IRQL: Caller's Thread Context:
Cleanup PASSIVE_LEVEL   Nonarbitrary
Close APC_LEVEL Arbitrary
Create PASSIVE_LEVEL Nonarbitrary
DeviceControl (except paging I/O) PASSIVE_LEVEL Nonarbitrary
DeviceControl (paging I/O path) APC_LEVEL Arbitrary
DirectoryControl APC_LEVEL Arbitrary
FlushBuffers PASSIVE_LEVEL Nonarbitrary
FsControl (except paging I/O) PASSIVE_LEVEL Nonarbitrary
FsControl (paging I/O path) APC_LEVEL Arbitrary
LockControl PASSIVE_LEVEL Nonarbitrary
QueryEa PASSIVE_LEVEL Nonarbitrary
QueryInformation PASSIVE_LEVEL Nonarbitrary
QueryQuota PASSIVE_LEVEL Nonarbitrary
QuerySecurity PASSIVE_LEVEL Nonarbitrary
QueryVolumeInfo PASSIVE_LEVEL Nonarbitrary
Read (except paging I/O) PASSIVE_LEVEL Nonarbitrary
Read (paging I/O path) APC_LEVEL Arbitrary
SetEa PASSIVE_LEVEL Nonarbitrary
SetInformation PASSIVE_LEVEL Nonarbitrary
SetQuota PASSIVE_LEVEL Nonarbitrary
SetSecurity PASSIVE_LEVEL Nonarbitrary
SetVolumeInfo PASSIVE_LEVEL Nonarbitrary
Shutdown APC_LEVEL Arbitrary
Write (except paging I/O) PASSIVE_LEVEL Nonarbitrary
Write (paging I/O path) APC_LEVEL