Re: [PATCH v8 2/3] uvcvideo: send a control event when a Control Change interrupt arrives

Guennadi Liakhovetski
2018-07-26T08:19:26Z

Hi Laurent,

Thanks for the diff. I'm about to go over it and check your comments, but
I cannot seem to find a git tree with your current stack. Particularly,
you wanted to apply patches 1 and 3 from this series before applying this
one, so, I'd prefer to work on top of the same tree as you to make sure to
avoid rejects.

Thanks
Guennaddi

On Wed, 25 Jul 2018, Laurent Pinchart wrote:

> Hi Guennadi,
>
> On Wednesday, 25 July 2018 20:21:54 EEST Guennadi Liakhovetski wrote:
> > On Wed, 25 Jul 2018, Laurent Pinchart wrote:
> > > On Wednesday, 18 July 2018 09:55:27 EEST Guennadi Liakhovetski wrote:
> > >> On Wed, 18 Jul 2018, Laurent Pinchart wrote:
> > >>> On Wednesday, 18 July 2018 00:30:45 EEST Guennadi Liakhovetski wrote:
> > >>>> On Tue, 17 Jul 2018, Laurent Pinchart wrote:
> > >>>>> On Thursday, 12 July 2018 10:30:46 EEST Guennadi Liakhovetski wrote:
> > >>>>>> On Thu, 12 Jul 2018, Laurent Pinchart wrote:
> > >>>>>>> On Tuesday, 8 May 2018 18:07:43 EEST Guennadi Liakhovetski wrote:
> > >>>>>>>> UVC defines a method of handling asynchronous controls, which
> > >>>>>>>> sends a USB packet over the interrupt pipe. This patch implements
> > >>>>>>>> support for such packets by sending a control event to the user.
> > >>>>>>>> Since this can involve USB traffic and, therefore, scheduling, this
> > >>>>>>>> has to be done in a work queue.
> > >>>>>>>>
> > >>>>>>>> Signed-off-by: Guennadi Liakhovetski
> > >>>>>>>> <guennadi.liakhovetski@intel.com>
> > >>>>>>>> ---
> > >>>>>>>>
> > >>>>>>>> v8:
> > >>>>>>>>
> > >>>>>>>> * avoid losing events by delaying the status URB resubmission
> > >>>>>>>> until after completion of the current event
> >>>>>>>> * extract control value calculation into __uvc_ctrl_get_value()
> > >>>>>>>> * do not proactively return EBUSY if the previous control hasn't
> > >>>>>>>> completed yet, let the camera handle such cases
> > >>>>>>>> * multiple cosmetic changes
> > >>>>>>>>
> > >>>>>>>> drivers/media/usb/uvc/uvc_ctrl.c | 166 +++++++++++++++++++------
> > >>>>>>>> drivers/media/usb/uvc/uvc_status.c | 112 ++++++++++++++++++++++---
> > >>>>>>>> drivers/media/usb/uvc/uvc_v4l2.c | 4 +-
> > >>>>>>>> drivers/media/usb/uvc/uvcvideo.h | 15 +++-
> > >>>>>>>> include/uapi/linux/uvcvideo.h | 2 +
> > >>>>>>>> 5 files changed, 255 insertions(+), 44 deletions(-)
> > >>>>>>>>
> > >>>>>>>> diff --git a/drivers/media/usb/uvc/uvc_ctrl.c
> > >>>>>>>> b/drivers/media/usb/uvc/uvc_ctrl.c index 2a213c8..796f86a 100644
> > >>>>>>>> --- a/drivers/media/usb/uvc/uvc_ctrl.c
> > >>>>>>>> +++ b/drivers/media/usb/uvc/uvc_ctrl.c
> > >>>>>>
> > >>>>>> [snip]
> > >>>>>>
> > >>>>>>>> +static void uvc_ctrl_status_event_work(struct work_struct *work)
> > >>>>>>>> +{
> > >>>>>>>> + struct uvc_device *dev = container_of(work, struct uvc_device,
> > >>>>>>>> + async_ctrl.work);
> > >>>>>>>> + struct uvc_ctrl_work *w = &dev->async_ctrl;
> > >>>>>>>> + struct uvc_control_mapping *mapping;
> > >>>>>>>> + struct uvc_control *ctrl = w->ctrl;
> > >>>>>>>> + unsigned int i;
> > >>>>>>>> + int ret;
> > >>>>>>>> +
> > >>>>>>>> + mutex_lock(&w->chain->ctrl_mutex);
> > >>>>>>>> +
> > >>>>>>>> + list_for_each_entry(mapping, &ctrl->info.mappings, list) {
> > >>>>>>>> + s32 value = __uvc_ctrl_get_value(mapping, w->data);
> > >>>>>>>> +
> > >>>>>>>> + /*
> > >>>>>>>> + * So far none of the auto-update controls in the uvc_ctrls[]
> > >>>>>>>> + * table is mapped to a V4L control with slaves in the
> > >>>>>>>> + * uvc_ctrl_mappings[] list, so slave controls so far never
> > >>>>>>>> have
> > >>>>>>>> + * handle == NULL, but this can change in the future
> > >>>>>>>> + */
> > >>>>>>>> + for (i = 0; i < ARRAY_SIZE(mapping->slave_ids); ++i) {
> > >>>>>>>> + if (!mapping->slave_ids[i])
> > >>>>>>>> + break;
> > >>>>>>>> +
> > >>>>>>>> + __uvc_ctrl_send_slave_event(ctrl->handle, w->chain,
> > >>>>>>>> + ctrl, mapping->slave_ids[i]);
> > >>>>>>>> + }
> > >>>>>>>> +
> > >>>>>>>> + uvc_ctrl_send_event(ctrl->handle, ctrl, mapping, value,
> > >>>>>>>> + V4L2_EVENT_CTRL_CH_VALUE);
> > >>>>>>>> + }
> > >>>>>>>> +
> > >>>>>>>> + mutex_unlock(&w->chain->ctrl_mutex);
> > >>>>>>>> +
> > >>>>>>>> + ctrl->handle = NULL;
> > >>>>>>>
> > >>>>>>> Can't this race with a uvc_ctrl_set() call, resulting in
> > >>>>>>> ctrl->handle being NULL after the control gets set ?
> > >>>>>>
> > >>>>>> Right, it's better to set .handle to NULL before sending events.
> > >>>>>> Something like
> > >>>>>>
> > >>>>>> mutex_lock();
> > >>>>>>
> > >>>>>> handle = ctrl->handle;
> > >>>>>> ctrl->handle = NULL;
> > >>>>>>
> > >>>>>> list_for_each_entry() {
> > >>>>>>
> > >>>>>> ...
> > >>>>>> uvc_ctrl_send_event(handle,...);
> > >>>>>>
> > >>>>>> }
> > >>>>>>
> > >>>>>> mutex_unlock();
> > >>>>>>
> > >>>>>> ?
> > >>>>>
> > >>>>> I think you also have to take the same lock in the uvc_ctrl_set()
> > >>>>> function to fix the problem, otherwise the ctrl->handle = NULL line
> > >>>>> could still be executed after the ctrl->handle assignment in
> > >>>>> uvc_ctrl_set(), resulting in ctrl->handle being NULL while the
> > >>>>> control is being set.
> > >>>>
> > >>>> Doesn't this mean, that you're attempting to send a new instance of
> > >>>> the same control before the previous has completed? In which case also
> > >>>> taking the lock in uvc_ctrl_set() wouldn't help either, because you
> > >>>> can anyway do that immediately after the first instance, before the
> > >>>> completion even has fired.
> > >>>
> > >>> You're right that it won't solve the race completely, but wouldn't it
> > >>> at least prevent ctrl->handle from being NULL ? We can't guarantee
> > >>> which of the old and new handle will be used for events when multiple
> > >>> control set operations are invoked, but we should try to guarantee
> > >>> that the handle won't be NULL.
> > >>
> > >> Sorry, I'm probably misunderstanding something. What exactly are you
> > >> proposing to lock and what and how is it supposed to protect? Wouldn't
> > >> the following flow still be possible, if you protect setting .handle =
> > >> NULL in uvc_set_ctrl():
> > >>
> > >> CPU 1 CPU 2
> > >>
> > >> control completion interrupt
> > >> (.handle = HANDLE_1)
> > >> work scheduled
> > >>
> > >> uvc_set_ctrl()
> > >> .handle = HANDLE_2
> > >>
> > >> uvc_ctrl_status_event_work()
> > >> .handle = NULL
> > >> usb_submit_urb()
> > >>
> > >> control completion interrupt
> > >> (.handle = NULL)
> > >>
> > >> ?
> > >
> > > You're absolutely right, there's no easy way to guard against this with a
> > > mere lock. I think we can ignore the issue for now and address it later
> > > if really needed, as the only adverse effect would be a spurious control
> > > change event sent to a file handle that hasn't set the
> > > V4L2_EVENT_SUB_FL_ALLOW_FEEDBACK flag.
> >
> > Ok, but I still think the above change - setting .handle to NULL before
> > sending the event - should be useful?
>
> You're right, it's useful. It will help in case userspace calls S_CTRL on CPU
> 2 in response to the event. I forgot to include this change in the patch I've
> just sent. Maybe something like
>
> mutex_lock(&chain->ctrl_mutex);
>
> /*
> * Set ctrl->handle to NULL before sending events, to avoid a race with
> * userspace setting the control in response to the event.
> */
> handle = ctrl->handle;
> ctrl->handle = NULL;
>
> list_for_each_entry(mapping, &ctrl->info.mappings, list) {
> ...
>
> > >>>>>>>> + /* Resubmit the URB. */
> > >>>>>>>> + w->urb->interval = dev->int_ep->desc.bInterval;
> > >>>>>>>> + ret = usb_submit_urb(w->urb, GFP_KERNEL);
> > >>>>>>>> + if (ret < 0)
> > >>>>>>>> + uvc_printk(KERN_ERR, "Failed to resubmit status URB (%d).\n",
> > >>>>>>>> + ret);
> > >>>>>>>> +}
> > >>>
> > >>> [snip]
>
> --
> Regards,
>
> Laurent Pinchart
>
>
>