From: Roddy, Mark [Mark_Roddy@stratus.com]
Sent: Tuesday, January 09, 2001 9:15 AM
To: NT Developers Interest List
Subject: [ntdev] RE: IRQL question


In the case of a standalone or topmost driver, and assuming that we are not
discussing filesystem drivers here, your dispatch routines will always be
called at PASSIVE_LEVEL. However, not all drivers are at the top of the
stack, and even if they are, somebody else could install a filter driver
above your 'topmost' driver. Once there is another driver above you then it
is not true that you must always be called at PASSIVE_LEVEL, it is instead
the much weaker: you will generally be called at less than DISPATCH_LEVEL.
There are a few examples of driver stacks where some IRP dispatching occurs
at DISPATCH_LEVEL, although in my opinion, despite the terminology
relationship, this is rather rude.

If you have your own standalone monolithic topmost driver, then you can
simply enforce your own rule that IO requests must arrive at PASSIVE_LEVEL.
This may screw up a filter driver that acquires one of the synchronization
primitives that raise to APC_LEVEL, but that is a problem for the filter
driver. How to enforce this? Just complete the incoming irp with an error if
the IRQL is too high.

Another, 'nicer' approach is to test the IRQL level and if it is
inappropriate for your requirements, queue the IRP for background processing
by a worker thread (running at PASSIVE_LEVEL,) mark the IRP pending, setup a
cancel routine (etc. etc. and not in the order stated here,) and return
STATUS_PENDING to the caller.

Mark Roddy
markr@hollistech.com
www.hollistech.com
WindowsNT Windows 2000 Consulting Services



> -----Original Message-----
> From: Barak Mandelovich [mailto:barak@mercury.co.il]
> Sent: Tuesday, January 09, 2001 9:05 AM
> To: NT Developers Interest List
> Subject: [ntdev] IRQL question
> 
> 
> Hi, Guys!
> 
> I did some "research" about IRQ levels etc., and I think I 
> understand it
> better now.
> Just one thing that I still do not understand:
> 
> Let's say that I read from my driver using ReadFile().
> This function is called in IRQ_PASSIVE_LEVEL. 
> 
> Now, why should I check the IRQL in the function that takes 
> care of ReadFile
> inside
> the driver? I mean - is there a chance that this function 
> would be called in
> IRQL > IRQ_PASSIVE_LEVEL ?
> How?
> 
> If it cannot happen - can you please give an example of when 
> it CAN happen
> (A general case, with any kind of IoCtl )?
> 
> Now, let's say that I check IRQL, and it's not the correct 
> level - what
> should I do next?
> After all - I should return an answer to the user, and I 
> don't think that a
> "failure" message is the correct one to return.
> 
> 	thanks in advance,
> 
> 		- Barak
> 
> --------------------------------------------------------------
> ----------
> Barak Mandelovich		barak@mercury.co.il
> Mercury Interactive ltd.
> --------------------------------------------------------------
> ----------
> 
> 
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