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

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Date: Mon, 15 Apr 2024 12:53:31 +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 11:26, Bjørn Mork wrote:
>> 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.
>
> Why? I am afraid I do not get that part.

I don't get how it happens either.  But that's the only thing changed by
the patch.

>  > 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?
>
> Yes, it can. In fact in this case the behavior should not change.
> I am afraid we have a misunderstanding. It seems to me that in the
> unchanged driver the result of service_outstanding_interrupt()
> is undefined.
> Please explain.

Sorry, I am so lost here that I am probably only confusing things.  I do
not understand why we unlock &desc->iuspin around the usb_submit_urb
call.  And git tells me I wrote that.


Bjørn