Re: [PATCH v2] usb: dwc3: gadget: Avoid canceling current request for queuing error

[Felipe Balbi <balbi@kernel.org>]

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Hi,

Thinh Nguyen <Thinh.Nguyen@synopsys.com> writes:
>>>>>> 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
>> 
>> Hi Thinh,
>> 
>>>
>>> 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
>>>
>>>
>> 
>> Yes, we know the reason why this happens, and its due to one of the
>> downstream changes we had that led to the scenario above.  Although,
>> that has been fixed, I still believe the error path is a potential
>> scenario we'd still want to address.
>> 
>> I think the returning success always on dwc3_gadget_ep_queue(), and
>> allowing the error in the completion handler/giveback at the function
>> driver level to do the cleanup is a feasible solution.  Doesn't change
>> the flow of the DWC3 gadget, and so far all function drivers we've used
>> handle this in the correct manner.
>> 
>> Thanks
>> Wesley Cheng
>
> Right. I think for now we should do that (return success always except
> for cases of disconnect or already in-flight etc). This helps keeping it

no, let's not lie to our users ;-)

> simple and avoid some pitfalls dealing with giving back the request.
> Currently we return the error status to dwc3_gadget_ep_queue if we
> failed to send a command that may not even related to the same request
> being queued.

I think the fix should be simple, but we're trying to patch it in the
wrong way. Can y'all comment on my suggestion on the other subthread?

-- 
balbi

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