Synchronizing Access to Device Data

Typically, a driver's InterruptService or InterruptMessageService routines (ISRs) must share access to driver data and hardware resources with other driver routines. Since ISRs execute in an interrupt context at an elevated IRQL, and since a system might have multiple processors, it is important to synchronize access to shared data and resources so that each routine can be guaranteed to temporarily have exclusive access to this shared information, without interruption.

The system supports this synchronization by executing the ISR within an interrupt critical section. An interrupt has an assigned spin lock, the interrupt spin lock, and IRQL, the interrupt synchronization IRQL. The system guarantees this code executing within the critical section exclusive access to shared information, by raising the processor's IRQL to the interrupt synchronization IRQL and acquiring the interrupt spin lock before executing the code. The system always enters the interrupt's critical section before executing its ISR. Different interrupts can share the same critical section by sharing their interrupt spin lock and synchronization IRQL.

Drivers can implement code that runs in the interrupt's critical section by supplying a SynchCritSection routine. When the driver uses KeSynchronizeExecution to call the SynchCritSection routine, the system automatically enters the critical section for the interrupt specified by the Interrupt parameter.

Raising the processor's IRQL to the interrupt's synchronization IRQL prevents the current processor from being interrupted, except by an interrupt with a higher synchronization IRQL. Acquiring a spin lock prevents other processors from executing any critical section code associated with that spin lock.

The system assigns the interrupt spin lock and synchronization IRQL for the interrupt when the driver calls IoConnectInterruptEx. In most instances, the driver can allow the system to determine both values:

• If the driver uses the CONNECT_LINE_BASED version of IoConnectInterruptEx, and specifies a NULL spin lock, the system will allocate a spin lock for the interrupt line. The system also determines the value for the synchronization IRQL (drivers can optionally specify a higher value).

• If the driver uses the CONNECT_MESSAGE_BASED version of IoConnectInterruptEx, and specifies a NULL spin lock, the system will allocate a spin lock for each interrupt message. The system also determines the value of the synchronization IRQL for each message (drivers can optionally specify a higher value that will be common to all messages).

A driver must allocate its own spin lock only when using the CONNECT_FULLY_SPECIFIED version of IoConnectInterruptEx and when it has multiple interrupt vectors that must share the same critical section. A driver can specify its own spin lock and synchronization IRQL by using the SpinLock and SynchronizeIrql members of IO_CONNECT_INTERRUPT_PARAMETERS. For more information, see IO_CONNECT_INTERRUPT_PARAMETERS.

For information about writing and entering critical sections, see Using Critical Sections.