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

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Date:   Tue, 4 May 2021 03:12:00 +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

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

sounds like a race issue.

> 
>> 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.

> 
>> 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