Date:   Tue, 4 May 2021 05:22:04 +0000
From:   Thinh Nguyen <Thinh.Nguyen@...opsys.com>
To:     Wesley Cheng <wcheng@...eaurora.org>,
        Thinh Nguyen <Thinh.Nguyen@...opsys.com>,
        "balbi@...nel.org" <balbi@...nel.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
CC:     "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jackp@...eaurora.org" <jackp@...eaurora.org>
Subject: Re: [PATCH v2] usb: dwc3: gadget: Avoid canceling current request for
 queuing error

Wesley Cheng wrote:
> 
> 
> On 5/3/2021 8:12 PM, Thinh Nguyen wrote:
>> Hi Wesley,
>>
>> Wesley Cheng wrote:
>>>
>>>
>>> On 5/3/2021 7:20 PM, Thinh Nguyen wrote:
>>>> Hi,
>>>>
>>>> Wesley Cheng wrote:
>>>>> If an error is received when issuing a start or update transfer
>>>>> command, the error handler will stop all active requests (including
>>>>> the current USB request), and call dwc3_gadget_giveback() to notify
>>>>> function drivers of the requests which have been stopped.  Avoid
>>>>> having to cancel the current request which is trying to be queued, as
>>>>> the function driver will handle the EP queue error accordingly.
>>>>> Simply unmap the request as it was done before, and allow previously
>>>>> started transfers to be cleaned up.
>>>>>
>>>
>>> Hi Thinh,
>>>
>>>>
>>>> It looks like you're still letting dwc3 stopping and cancelling all the
>>>> active requests instead letting the function driver doing the dequeue.
>>>>
>>>
>>> Yeah, main issue isn't due to the function driver doing dequeue, but
>>> having cleanup (ie USB request free) if there is an error during
>>> usb_ep_queue().
>>>
>>> The function driver in question at the moment is the f_fs driver in AIO
>>> mode.  When async IO is enabled in the FFS driver, every time it queues
>>> a packet, it will allocate a io_data struct beforehand.  If the
>>> usb_ep_queue() fails it will free this io_data memory.  Problem is that,
>>> since the DWC3 gadget calls the completion with -ECONNRESET, the FFS
>>> driver will also schedule a work item (within io_data struct) to handle
>>> the completion.  So you end up with a flow like below
>>>
>>> allocate io_data (ffs)
>>>  --> usb_ep_queue()
>>>    --> __dwc3_gadget_kick_transfer()
>>>    --> dwc3_send_gadget_ep_cmd(EINVAL)
>>>    --> dwc3_gadget_ep_cleanup_cancelled_requests()
>>>    --> dwc3_gadget_giveback(ECONNRESET)
>>> ffs completion callback
>>> queue work item within io_data
>>>  --> usb_ep_queue returns EINVAL
>>> ffs frees io_data
>>> ...
>>>
>>> work scheduled
>>>  --> NULL pointer/memory fault as io_data is freed
> 
> Hi Thinh,
> 
>>
>> sounds like a race issue.
>>
> 
> It'll always happen if usb_ep_queue() fails with an error. Sorry for not
> clarifying, but the "..." represents executing in a different context
> :). Anything above the "..." is in the same context.
>>>
>>>> BTW, what kinds of command and error do you see in your setup and for
>>>> what type endpoint? I'm thinking of letting the function driver to
>>>> dequeue the requests instead of letting dwc3 automatically
>>>> ending/cancelling the queued requests. However, it's a bit tricky to do
>>>> that if the error is -ETIMEDOUT since we're not sure if the controller
>>>> had already cached the TRBs.
>>>>
>>>
>>> Happens on bulk EPs so far, but I think it wouldn't matter as long as
>>> its over the FFS interface. (and using async IO transfers)
>>
>> Do you know which command and error code? It's strange if
>> UPDATE_TRANSFER command failed.
>>
> 
> Sorry for missing that part of the question.  It is a no xfer resource
> error on a start transfer command.  So far this happens on low system
> memory test cases, so there may be some sequences that were missed,
> which led to this particular command error.
> 
> Thanks
> Wesley Cheng

No xfer resource usually means that the driver attempted to send
START_TRANSFER without waiting for END_TRANSFER command to complete.
This may be a dwc3 driver issue. Did you check this?

Thanks,
Thinh


> 
>>>
>>>> This seems to add more complexity and I don't have a good solution to
>>>> it. Since you're already cancelling all the active request anyway, what
>>>> do you think of always letting dwc3_gadget_ep_queue() to go through with
>>>> success, but report failure through request completion?
>>>>
>>>
>>> We do have something similar as well downstream (returning success
>>> always on dwc3_gadget_ep_queue()) and its been working for us also.
>>> Problem is we don't test the ISOC path much, so this is the only type of
>>> EP that might come into question...
>>>
>>
>> It should be similiar with isoc. I can't think of a potential issue yet.
>>
>>> Coming up with a way to address the concerns you brought up was a bit
>>> difficult as there were scenarios we needed to consider.  next_request()
>>> doesn't always have to be the request being queued (even if ep queue
>>> triggered it).  There was no easy way to determine if kick transfer was
>>> due to ep queue, but even if there was, we'd need to remember the
>>> previous point as well.
>>>
>>
>> Yeah, there are a few pitfalls. I don't have a good solution to it if we
>> want to return failure immediately and let the function driver handle
>> the dequeue (if it wants to).
>>
>> Thanks,
>> Thinh
>>
> 

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