linux-kernel - Re: [PATCH] usb: cdc-wdm: close race between read and workqueue

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Date: Mon, 15 Apr 2024 11:26:00 +0200
From: Bjørn Mork <bjorn@...k.no>
To: Oliver Neukum <oneukum@...e.com>
Cc: Aleksander Morgado <aleksandermj@...omium.org>, linux-usb@...r.kernel.org,
        gregkh@...uxfoundation.org, linux@...ck-us.net,
        linux-kernel@...r.kernel.org, ejcaruso@...omium.org
Subject: Re: [PATCH] usb: cdc-wdm: close race between read and workqueue

Oliver Neukum <oneukum@...e.com> writes:
> On 15.04.24 08:47, Bjørn Mork wrote:
>
>> urb from service_outstanding_interrupt(). That's why it was added. See
>> the explanation Robert wrote when introducing it:
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/usb/class/cdc-wdm.c?id=c1da59dad0ebd3f9bd238f3fff82b1f7ffda7829
>
> Well, the explanation is correct in that we must read
> data available. However, if the RESPONDING flag is set
> and the URB submitted, we are already doing so.

Sounds reasonable.  Except that the bug proves we didn't.

If you are right that service_outstanding_interrupt can race againts
itself (and I don't doubt that), then I guess this could also happen
between failure to submit the URB and clearing the flag?

>> As for the XMM behaviour: it's been a long time since I tried any of
>> those, but AFAIR one the major differences compared to Qualcomm was the
>> strict queue handling in the firmware.  This caused a number of problems
>> where the cdc-wdm driver wanted to skip a message for some reason.  So
>> I'm not surprised that a bug like this is triggered by one of those
>> modems. That's probably the only thing they are good for :-)
>
> I am not sure where exactly the issue lies here. Suggestions for
> debugging?

Nothing more than the obvious;  Get one of those modems and do some
usbmon snooping.


Bjørn