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Introduction to Kernel Dispatcher Objects

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The kernel defines a set of object types called kernel dispatcher objects, or just dispatcher objects. Dispatcher objects include timer objects, event objects, semaphore objects, mutex objects, and thread objects.

Drivers can use dispatcher objects as synchronization mechanisms within a nonarbitrary thread context while executing at IRQL equal to PASSIVE_LEVEL.

Dispatcher Object States

Every kernel-defined dispatcher object type has a state that is either set to Signaled or set to Not-Signaled.

A group of threads can synchronize their operations if one or more threads call KeWaitForSingleObject, KeWaitForMutexObject, or KeWaitForMultipleObjects. These functions take dispatcher object pointers as input and wait until another routine or thread sets one or more dispatcher objects to the Signaled state.

When a thread calls the KeWaitForSingleObject to wait for a dispatcher object (or KeWaitForMutexObject for a mutex), the thread is put into a wait state until the dispatcher object is set to the Signaled state. A thread can call KeWaitForMultipleObjects to wait either for any, or for all, of a set of dispatcher objects to be set to Signaled.

Whenever a dispatcher object is set to the Signaled state, the kernel changes the state of any thread waiting for that object to ready. (Synchronization timers and synchronization events are exceptions to this rule; when a synchronization event or timer is signaled, only one waiting thread is set to the ready state. For more information, see Timer Objects and DPCs and Event Objects.) A thread in the ready state will be scheduled to run according to its current run-time thread priority and the current availability of processors for any thread with that priority.

When Can Drivers Wait for Dispatcher Objects?

In general, drivers can wait for dispatcher objects to be set only if at least one of the following circumstances is true:

  • The driver is executing in a nonarbitrary thread context.

    That is, you can identify the thread that will enter a wait state. In practice, the only driver routines that execute in a nonarbitrary thread context are the DriverEntry, AddDevice, Reinitialize, and Unload routines of any driver, plus the dispatch routines of highest-level drivers. All these routines are called directly by the system.

  • The driver is performing a completely synchronous I/O request.

    That is, no driver queues any operations while handling the I/O request, and no driver returns until the driver below it has finished handling the request.

Additionally, a driver cannot enter a wait state if it is executing at or above IRQL equal to DISPATCH_LEVEL.

Based on these limitations, you must use the following rules:

  • The DriverEntry, AddDevice, Reinitialize, and Unload routines of any driver can wait for dispatcher objects.

  • The dispatch routines of a highest-level driver can wait for dispatcher objects.

  • The dispatch routines of lower-level drivers can wait for dispatch objects, if the I/O operation is synchronous, such as create, flush, shutdown, and close operations, some device I/O control operations, and some PnP and power operations.

  • The dispatch routines of lower-level drivers cannot wait for a dispatcher object for the completion of asynchronous I/O operations.

  • A driver routine that is executing at or above IRQL DISPATCH_LEVEL must not wait for a dispatcher object to be set to the Signaled state.

  • A driver must not attempt to wait for a dispatcher object to be set to the Signaled state for the completion of a transfer operation to or from a paging device.

  • Driver dispatch routines servicing read/write requests generally cannot wait for a dispatcher object to be set to the Signaled state.

  • A dispatch routine for a device I/O control request can wait for a dispatcher object to be set to the Signaled state only if the transfer type for the I/O control code is METHOD_BUFFERED.

  • SCSI miniport drivers should not use kernel dispatcher objects. SCSI miniport drivers should call only SCSI Port Library Routines.

Every other standard driver routine executes in an arbitrary thread context: that of whatever thread happens to be current when the driver routine is called to process a queued operation or to handle a device interrupt. Moreover, most standard driver routines are run at a raised IRQL, either at DISPATCH_LEVEL, or for device drivers, at DIRQL.

If necessary, a driver can create a device-dedicated thread, which can wait for the driver's other routines (except an ISR or SynchCritSection routine) to set a dispatcher object to the Signaled state and reset to the Not-Signaled state.

As a general guideline, if you expect that your new device driver will often need to stall for longer than 50 microseconds while it waits for device-state changes during I/O operations, consider implementing a driver with a device-dedicated thread. If the device driver is also a highest-level driver, consider using system worker threads and implementing one or more worker-thread callback routines. See PsCreateSystemThread and Managing Interlocked Queues with a Driver-Created Thread.