ExAcquireFastMutex and Filemon puzzle

NTFSD
1

I’m confused: the NT DDK specifically states that in order to call
ExAcquireFastMutex you must be at IRQL < DISPATCH_LEVEL. However, Filemon.c
uses it in a completion routine (see UpdateStore). Unless I am mistaken, it
is completely possible for a completion routine to be called at Dispatch
level. Can someone clarify this?

Thanks.

Neil Weicher

2

> I’m confused: the NT DDK specifically states that in order to call

ExAcquireFastMutex you must be at IRQL < DISPATCH_LEVEL. However, Filemon.c
uses it in a completion routine (see UpdateStore). Unless I am mistaken, it
is completely possible for a completion routine to be called at Dispatch
level. Can someone clarify this?

Possible, yes, but not certain. Filemon.c doesn’t call it if the IRQL isn’t
Passive, see KeGetCurrentIrql() line.

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3

Are you looking at old filemon code (v2.x or earlier)? The latest code
calls KeGetCurrentIrql and queues to a worker thread if at DISPATCH_LEVEL.
Old code did make unsafe calls to ExAcquireFastMutex.

-----Original Message-----
From: Neil R. Weicher [mailto:xxxxx@netlib.com]
Sent: Wednesday, October 11, 2000 15:06
To: File Systems Developers
Subject: [ntfsd] ExAcquireFastMutex and Filemon puzzle

I’m confused: the NT DDK specifically states that in order to call
ExAcquireFastMutex you must be at IRQL < DISPATCH_LEVEL. However, Filemon.c
uses it in a completion routine (see UpdateStore). Unless I am mistaken, it
is completely possible for a completion routine to be called at Dispatch
level. Can someone clarify this?

Thanks.

Neil Weicher

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4

> is completely possible for a completion routine to be called at Dispatch

level. Can someone clarify this?

I saw many code which assumes that the CREATE completion routine is called
at < DISPATCH_LEVEL.

Max

5

This assumption is, of course, not a safe one.

The driver verifier has an optional setting to ensure that all completion
routines for a driver are called at DISPATCH_LEVEL; this can also be enabled
in the kernel debugger (2250) using the “!patch” command.

Completion code should ALWAYS assume that it might be called at
DISPATCH_LEVEL. To write code that assumes otherwise, while it might work
NOW, has the potential for breaking - either in the field with current
versions (e.g., some unusual code path gets exercised) or in a future
version (when, for whatever reason, some FSD developer chooses to perform
the completion at DISPATCH_LEVEL.)

Regards,

Tony Mason
Consulting Partner
OSR Open Systems Resources, Inc.
http://www.osr.com

-----Original Message-----
From: Maxim S. Shatskih [mailto:xxxxx@storagecraft.com]
Sent: Wednesday, October 11, 2000 11:53 PM
To: File Systems Developers
Subject: [ntfsd] Re: ExAcquireFastMutex and Filemon puzzle

is completely possible for a completion routine to be called at Dispatch
level. Can someone clarify this?

I saw many code which assumes that the CREATE completion routine is called
at < DISPATCH_LEVEL.

Max

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To unsubscribe send a blank email to $subst(‘Email.Unsub’)