Re: [PATCH] usb: dwc3: ep0: Fix the possible missed request for handling delay STATUS phase

[Baolin Wang]

Hi,

On 17 January 2017 at 18:39, Felipe Balbi <balbi@xxxxxxxxxx> wrote:
>
> Hi,
>
> Baolin Wang <baolin.wang@xxxxxxxxxx> writes:
>>>>>>> Baolin Wang <baolin.wang@xxxxxxxxxx> writes:
>>>>>>>> When handing the SETUP packet by composite_setup(), we will release the
>>>>>>>> dwc->lock. If we get the 'USB_GADGET_DELAYED_STATUS' result from setup
>>>>>>>> function, which means we need to delay handling the STATUS phase.
>>>>>>>
>>>>>>> this sentence needs a little work. Seems like it's missing some
>>>>>>> information.
>>>>>>>
>>>>>>> anyway, I get that we release the lock but...
>>>>>>>
>>>>>>>> But during the lock release period, maybe the request for handling delay
>>>>>>>
>>>>>>> execution of ->setup() itself should be locked. I can see that it's only
>>>>>>> locked for set_config() which is rather peculiar.
>>>>>>>
>>>>>>> What exact request you had when you triggered this? (Hint: dwc3
>>>>>>> tracepoints print out ctrl request bytes). IIRC, only set_config() or
>>>>>>> f->set_alt() can actually return USB_GADGET_DELAYED_STATUS.
>>>>>>
>>>>>> Yes, when host set configuration for mass storage driver, it can
>>>>>> produce this issue.
>>>>>>
>>>>>>>
>>>>>>> Which gadget driver were you using when you triggered this?
>>>>>>
>>>>>> mass storage driver. When host issues the setting config request, we
>>>>>> will get USB_GADGET_DELAYED_STATUS result from
>>>>>> set_config()--->fsg_set_alt(). Then the mass storage driver will issue
>>>>>> one thread to complete the status stage by ep0_queue() (this thread
>>>>>> may be running on another core), then if the thread issues ep0_queue()
>>>>>> too fast before we get the dwc->lock in dwc3_ep0_delegate_req() or
>>>>>> before we get into the STATUS stage, then we can not handle this
>>>>>> request for the delay STATUS stage in dwc3_gadget_ep0_queue().
>>>>>>
>>>>>>>
>>>>>>> Another point here is that the really robust way of fixing this is to
>>>>>>> get rid of USB_GADGET_DELAYED_STATUS altogether and just make sure
>>>>>>> gadget drivers know how to queue requests for all three phases of a
>>>>>>> Control Transfer.
>>>>>>>
>>>>>>> A lot of code will be removed from all gadget drivers and UDC drivers
>>>>>>> while combining all of it in a single place in composite.c.
>>>>>>>
>>>>>>> The reason I'm saying this is that other UDC drivers might have similar
>>>>>>> races already but they just haven't triggered.
>>>>>>
>>>>>> Yes, maybe.
>>>>>>
>>>>>>>
>>>>>>>> STATUS phase has been queued into list before we set 'dwc->delayed_status'
>>>>>>>> flag or entering 'EP0_STATUS_PHASE' phase, then we will miss the chance
>>>>>>>> to handle the STATUS phase. Thus we should check if the request for delay
>>>>>>>> STATUS phase has been enqueued when entering 'EP0_STATUS_PHASE' phase in
>>>>>>>> dwc3_ep0_xfernotready(), if so, we should handle it.
>>>>>>>>
>>>>>>>> Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxx>
>>>>>>>> ---
>>>>>>>>  drivers/usb/dwc3/ep0.c |   14 ++++++++++++++
>>>>>>>>  1 file changed, 14 insertions(+)
>>>>>>>>
>>>>>>>> diff --git a/drivers/usb/dwc3/ep0.c b/drivers/usb/dwc3/ep0.c
>>>>>>>> index 9bb1f85..e689ced 100644
>>>>>>>> --- a/drivers/usb/dwc3/ep0.c
>>>>>>>> +++ b/drivers/usb/dwc3/ep0.c
>>>>>>>> @@ -1123,7 +1123,21 @@ static void dwc3_ep0_xfernotready(struct dwc3 *dwc,
>>>>>>>>               dwc->ep0state = EP0_STATUS_PHASE;
>>>>>>>>
>>>>>>>>               if (dwc->delayed_status) {
>>>>>>>> +                     struct dwc3_ep *dep = dwc->eps[0];
>>>>>>>> +
>>>>>>>>                       WARN_ON_ONCE(event->endpoint_number != 1);
>>>>>>>> +                     /*
>>>>>>>> +                      * We should handle the delay STATUS phase here if the
>>>>>>>> +                      * request for handling delay STATUS has been queued
>>>>>>>> +                      * into the list.
>>>>>>>> +                      */
>>>>>>>> +                     if (!list_empty(&dep->pending_list)) {
>>>>>>>> +                             dwc->delayed_status = false;
>>>>>>>> +                             usb_gadget_set_state(&dwc->gadget,
>>>>>>>> +                                                  USB_STATE_CONFIGURED);
>>>>>>>
>>>>>>> Isn't this patch also changing the normal case when usb_ep_queue() comes
>>>>>>> later? I guess list_empty() protects against that...
>>>>>>
>>>>>> I think it will not change other cases, we only handle the delayed
>>>>>> status and I've tested it for a while and I did not find any problem.
>>>>>
>>>>> Alright, it's important enough to fix this bug. Can you also have a look
>>>>> into dropping USB_GADGET_DELAYED_STATUS altogether? If you're too busy,
>>>>> no issues. It'll stay in my queue.
>>>>
>>>> Okay, I will have a look at f_mass_storage driver to see if we can
>>>> drop USB_GADGET_DELAYED_STATUS. Thanks.
>>>
>>> not only mass storage. It needs to be done for all drivers. The way to
>>> do that is to teach functions that control transfers are composed of two
>>> or three phases. If you look at UDC drivers today, they all have
>>> peculiarities about control transfers to handle stuff that *maybe*
>>> gadget drivers won't handle.
>>>
>>> What we should do here is make sure that *all* 3 phases always have a
>>> matching usb_ep_queue() coming from the upper layers. Whether
>>> composite.c or f_*.c handles it, that's an implementation detail. But
>>> just to illustrate the problem, we should be able to get rid of
>>> dwc3_ep0_out_start() and assume that the upper layer will call
>>> usb_ep_queue() when it wants to receive a new SETUP packet.
>>>
>>> Likewise, we should be able to assume that STATUS phase will only start
>>> based on a usb_ep_queue() call. That way we can remove
>>> USB_GADGET_DELAYED_STATUS altogether, because that will *always* be the
>>> case. There will be no races to handle apart from the normal case where
>>> XferNotReady can come before or after usb_ep_queue(), but we already
>>> have proper handling for that too.
>>
>> Thanks to explain, but seems it need lots of work to convert these
>> drivers, and I saw Alan had some concern about that. So I am not sure
>> how to convert these drivers which can meet your requirements, maybe
>> from adding one "wants_explicit_ctrl_phases"  flag in struct
>> usb_gadget as you suggested to start?
>
> yeah. Something like this:
>
> patch 1: add the new flag and document it
> patch 2: implement usb_ep_queue() for data and status phases conditional
>         to that flag being set
> patch 3: (e.g.) change dwc3 to rely on that behavior and pass the flag
>         to usb_gadget.
>
> this will be enough to actually test that the idea and implementation
> works out. After that, just for_each_UDC_driver() port_patch_3(); Then
> you add:
>
> patch N - 1: remove legacy code and force behavior of
>         wants_explicit_ctrl_phases
> patch N: remove wants_explicit_ctrl_phases
>
> something along these lines.

Okay, I will try to do that. Thanks.

-- 
Baolin.wang
Best Regards