Use the following general guidelines when designing and writing SynchCritSection routines that maintain state information:
To access data that an ISR also accesses, a driver routine must call a SynchCritSection routine. 非关键节代码可能会中断。 Remember that it's not sufficient to just acquire a spin lock to protect data that ISRs also access, because ISRs execute at DIRQL and acquiring a spin lock (KeAcquireSpinLock) only raises IRQL to DISPATCH_LEVEL, which allows an interrupt to invoke the ISR on the current processor.
Give each SynchCritSection routine that maintains state information responsibility for a discrete set of state variables. That is, avoid writing SynchCritSection routines that maintain overlapping state information.
This approach prevents contention, and possibly race conditions, between SynchCritSection routines (and the ISR) trying to access the same state concurrently.
This approach also ensures that each SynchCritSection routine returns control as quickly as possible. That is, one SynchCritSection routine never has to wait for another that updates some of the same state information to return control.
Avoid writing a single, large, general-purpose SynchCritSection routine that does more testing of conditions to determine what to do than actually doing useful work. On the other hand, avoid having many SynchCritSection routines that never execute a conditional statement because each updates only a single byte of state information.
Ensure every SynchCritSection routine returns control as quickly as possible, because running any SynchCritSection routine prevents the driver's ISR from executing.
以下示例演示了在设备扩展中维护计时器计数器的技术。 假设驱动程序使用计数器来确定 I/O作是否超时。此外，假设驱动程序不重叠 I/O作。
The driver's StartIo routine initializes the timer counter to some initial value for each I/O request. The driver then adds a second to its device timeout value, in case its IoTimer routine just returned control.
驱动程序的 ISR 将此计时器计数器设置为减一。
The driver's IoTimer routine is called once per second to read the time counter and determine whether the ISR already set it to minus one. If not, the IoTimer routine decrements the counter by using KeSynchronizeExecution to call a SynchCritSection_1 routine.
如果计数器变为零，指示请求超时，则SynchCritSection_1例程调用SynchCritSection_2例程来对设备重置作进行编程。 If the counter is minus one, the IoTimer routine simply returns.
If the driver's DpcForIsr routine must reprogram the device to begin a partial-transfer operation, it must reinitialize the timer counter as the StartIo routine did.
The DpcForIsr routine also must use KeSynchronizeExecution to call the SynchCritSection_2 routine, or possibly a SynchCritSection_3 routine, to program the device for another transfer operation.
In this scenario, the driver has more than one SynchCritSection routine, each with discrete, specific responsibilities: one to maintain its timer counter, and one or more others to program the device. Each SynchCritSection routine returns control quickly because it performs a single, discrete task.
驱动程序具有单个SynchCritSection_1例程，该例程与驱动程序的 ISR 一起维护计时器计数器的状态。 Thus, there's no contention for access to the timer counter among several SynchCritSection routines and the ISR.