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From: =?utf-8?Q?Bj=C3=B8rn_Mork?= <bjorn@mork.no>
To: Oliver Neukum <oneukum@suse.com>
Cc: Aleksander Morgado <aleksandermj@chromium.org>, linux-usb@vger.kernel.org,
        gregkh@linuxfoundation.org, linux@roeck-us.net,
        linux-kernel@vger.kernel.org, ejcaruso@chromium.org
Subject: Re: [PATCH] usb: cdc-wdm: close race between read and workqueue
Organization: m
References: <385a3519-b45d-48c5-a6fd-a3fdb6bec92f@chromium.org>
	<87mspvi0lk.fsf@miraculix.mork.no>
	<a6ad34d3-9cce-4178-8271-0e09ced2b6f4@suse.com>
Date: Mon, 15 Apr 2024 11:26:00 +0200
In-Reply-To: <a6ad34d3-9cce-4178-8271-0e09ced2b6f4@suse.com> (Oliver Neukum's
	message of "Mon, 15 Apr 2024 11:06:33 +0200")
Message-ID: <878r1fht93.fsf@miraculix.mork.no>
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Oliver Neukum <oneukum@suse.com> writes:
> On 15.04.24 08:47, Bj=C3=B8rn 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/commi=
t/drivers/usb/class/cdc-wdm.c?id=3Dc1da59dad0ebd3f9bd238f3fff82b1f7ffda7829
>
> 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.


Bj=C3=B8rn