Introduction to Threaded DPCs
Threaded DPCs are available in Windows Vista and later versions of Windows.
A threaded DPC is a DPC that the system executes at IRQL equal to PASSIVE_LEVEL. Threaded DPCs are enabled by default, but you can disable them by setting the HKLM\System\CCS\Control\SessionManager\Kernel\ThreadDpcEnable registry key to zero. When threaded DPCs are disabled, they execute as ordinary DPCs.
An ordinary DPC preempts the execution of all threads, and cannot be preempted by a thread or by another DPC. If the system has a large number of ordinary DPCs queued, or if one of those DPCs runs for a long time, every thread will remain paused for an arbitrarily long time. Thus, each ordinary DPC increases system latency, which can hurt the performance of time-sensitive applications, such as audio or video playback.
Conversely, a threaded DPC can be preempted by an ordinary DPC, but not by other threads. Therefore, you should use threaded DPCs rather than ordinary DPCs—unless a particular DPC must not be preempted, not even by another DPC.
The system represents threaded DPCs (and ordinary DPCs) as KDPC structures. To initialize a KDPC structure for a threaded DPC, call the KeInitializeThreadedDpc routine, and pass it a CustomThreadedDpc routine that performs the action of the DPC.
Because a CustomThreadedDpc routine can execute at either PASSIVE_LEVEL or DISPATCH_LEVEL, you must ensure that your CustomThreadedDpc routine correctly synchronizes at both IRQLs. For more information about how to do so, see Synchronization and Threaded DPCs.
In addition, you must ensure that your CustomThreadedDpc routine obeys all the restrictions for DISPATCH_LEVEL code. If threaded DPCs are enabled, they run at IRQL = PASSIVE_LEVEL but are still subject to the same restrictions as ordinary DPCs. All of the code that executes in a threaded DPC—including all functions that are called by the CustomThreadedDpc routine—must conform to the restrictions of the DPC environment. For example, code must not block on passive-level synchronization objects, such as KEVENT objects. Many existing device stacks, such as networking and USB, do not support threaded DPC processing, and they might try to block if they detect that they are called at PASSIVE_LEVEL. For similar reasons, the Kernel-Mode Driver Framework (KMDF) does not support threaded DPC processing, and KMDF drivers should not try to use threaded DPCs. For more information about the DPC environment, see Writing DPC Routines.