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Message-ID: <c35f98be-23a3-41c3-bee5-f394ce504545@suse.com>
Date: Mon, 15 Apr 2024 12:14:48 +0200
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Subject: Re: [PATCH] usb: cdc-wdm: close race between read and workqueue
To: =?UTF-8?Q?Bj=C3=B8rn_Mork?= <bjorn@mork.no>,
 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
References: <385a3519-b45d-48c5-a6fd-a3fdb6bec92f@chromium.org>
 <87mspvi0lk.fsf@miraculix.mork.no>
 <a6ad34d3-9cce-4178-8271-0e09ced2b6f4@suse.com>
 <878r1fht93.fsf@miraculix.mork.no>
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From: Oliver Neukum <oneukum@suse.com>
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On 15.04.24 11:26, Bjørn Mork wrote:
> Oliver Neukum <oneukum@suse.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.
  
> 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.

	Regards
		Oliver