Can a DeviceIoControl block?

NTDEV
1

I have multiple threads that use the same HANDLE for a device object,
created by:
CreateFile(_T(…), GENERIC_READ | GENERIC_WRITE,
0, NULL, OPEN_EXISTING,
FILE_ATTRIBUTE_NORMAL, NULL);

Now i noticed if a DeviceIoControl() of a thread blocks and waits for an event, other threads are blocked whenever they use DeviceIoControl, until the first thread finishes and returns to userspace.

Is this possible?

2

If the DRIVER blocks, then pending Requests also wait, right?

So, the answer to your question depends on how the driver is written.

Peter
OSR
@OSRDrivers

3

I’m talking about different ioctls on the same device.
One ioctl blocks, the other ioctl just changes a flag(no syncronization mechanism over there, nothing… no blocking)

Yet, the “not-blocking” ioctl is blocked.

4

>I’m talking about different ioctls on the same device.

The IOCTL function code doesn’t make any difference.

To repeat what I said in my previous reply: “the answer to your question depends on hiw the driver is written.”

Peter
OSR
@OSRDrivers

5

Sure it depends on how the driver is written…
Sure the IOCTL function code doesn’t make any difference…

…what might still make a difference though is the presence or absence of CreateFile parameter FILE_FLAG_OVERLAPPED

Marcel Ruedinger
datronicsoft

6

Marcel nailed it. The ioctl code and the driver architecture do not matter in this case. Without FILE_FLAG_OVERLAPPED in the CreateFile call, regardless of how the driver is written, the operating system will only allow one outstanding I/O operation at a time on that file handle. It doesn’t matter how many threads you have. All other calls will block until the first completes.

7

Good pickup, guys. I bet that?s it.

I foolishly assumed this want a user-mode issue!

Peter
OSR
@OSRDrivers

8

By design. You opened a synchronous/serialized handle. If you want multiple simultaneous io, open it with file flag overlapped.

Bent from my phone

From: xxxxx@lists.osr.com on behalf of xxxxx@gmail.com
Sent: Thursday, March 15, 2018 6:27:49 AM
To: Windows System Software Devs Interest List
Subject: [ntdev] Can a DeviceIoControl block?

I have multiple threads that use the same HANDLE for a device object,
created by:
CreateFile(_T(…), GENERIC_READ | GENERIC_WRITE,
0, NULL, OPEN_EXISTING,
FILE_ATTRIBUTE_NORMAL, NULL);

Now i noticed if a DeviceIoControl() of a thread blocks and waits for an event, other threads are blocked whenever they use DeviceIoControl, until the first thread finishes and returns to userspace.

Is this possible?

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9

This indeed solved the issue. FILE_FLAG_OVERLAPPED was missing, this allows me to simultaneously access the device file. Thanks!

10

FILE_FLAG_OVERLAPPED

https://msdn.microsoft.com/en-us/library/windows/desktop/aa363858(v=vs.85).aspx

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________________________________
From: xxxxx@lists.osr.com on behalf of xxxxx@gmail.com
Sent: Thursday, March 15, 2018 9:46:19 AM
To: Windows System Software Devs Interest List
Subject: RE:[ntdev] Can a DeviceIoControl block?

I’m talking about different ioctls on the same device.
One ioctl blocks, the other ioctl just changes a flag(no syncronization mechanism over there, nothing… no blocking)

Yet, the “not-blocking” ioctl is blocked.

—
NTDEV is sponsored by OSR

Visit the list online at: http:

MONTHLY seminars on crash dump analysis, WDF, Windows internals and software drivers!
Details at http:

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