Device Driver IRQL and Thread/Context Switches

Posted by Christian Hoglund on Stack Overflow See other posts from Stack Overflow or by Christian Hoglund
Published on 2010-02-22T13:15:09Z Indexed on 2010/04/03 12:23 UTC
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device-driver

Hi,

I'm new to Windows device driver programming. I know that certain operations can only be performed at IRQL PASSIVE_LEVEL. For example, Microsoft have this sample code of how to write to a file from a kernel driver:

if (KeGetCurrentIrql() != PASSIVE_LEVEL)
    return STATUS_INVALID_DEVICE_STATE; 

Status = ZwCreateFile(...);

My question is this: What is preventing the IRQL from being raised after the KeGetCurrentIrql() check above? Say a context or thread swithch occurs, couldn't the IRQL suddenly be DISPATCH_LEVEL when it gets back to my driver which would then result in a system crash?

If this is NOT possible then why not just check the IRQL in the DriverEntry function and be done with it once for all?

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