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

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Date: Mon, 15 Apr 2024 11:06:33 +0200
From: Oliver Neukum <oneukum@...e.com>
To: Bjørn Mork <bjorn@...k.no>,
 Aleksander Morgado <aleksandermj@...omium.org>
Cc: oneukum@...e.com, 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

On 15.04.24 08:47, Bjørn Mork wrote:

> I'm not sure I understand what problem that patch is supposed to fix.
> Which means that everything I write could be completely wrong...

wdm_in_callback() can schedule service_outs_intr(), which can call
service_outstanding_interrupt(), which sets WDM_RESPONDING and submits
desc->response.
That is not problematic in itself, but wdm_read() also calls
service_outstanding_interrupt(), which can lead to teh same URB
being submitted twice (which caused me to write the patch)
or, apparently, in this case, it leads to discarding a buffer
by resubmitting and completing an URB.

> But to me it looks like the described issue is exactly what you should
> expect if that change ever triggers.  I believe we must resubmit the

Yes, it does.

> 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.

> 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?

	Regards
		Oliver