Re: [PATCH] media: v4l2-mem2mem: always call poll_wait() on queues

[Alexandre Courbot]

On Wed, Nov 11, 2020 at 9:47 PM Hans Verkuil <hverkuil-cisco@xxxxxxxxx> wrote:
>
> On 11/11/2020 13:41, Alexandre Courbot wrote:
> > On Thu, Nov 5, 2020 at 11:05 PM Alexandre Courbot <gnurou@xxxxxxxxx> wrote:
> >>
> >> On Thu, Nov 5, 2020 at 10:12 PM Hans Verkuil <hverkuil-cisco@xxxxxxxxx> wrote:
> >>>
> >>> On 05/11/2020 13:52, Alexandre Courbot wrote:
> >>>> On Thu, Nov 5, 2020 at 9:36 PM Hans Verkuil <hverkuil-cisco@xxxxxxxxx> wrote:
> >>>>>
> >>>>> On 05/11/2020 13:21, Alexandre Courbot wrote:
> >>>>>> On Tue, Nov 3, 2020 at 6:48 PM Hans Verkuil <hverkuil-cisco@xxxxxxxxx> wrote:
> >>>>>>>
> >>>>>>> On 03/11/2020 09:51, Alexandre Courbot wrote:
> >>>>>>>> Hi Hans,
> >>>>>>>>
> >>>>>>>> On Sat, Oct 31, 2020 at 12:09 AM Hans Verkuil <hverkuil-cisco@xxxxxxxxx> wrote:
> >>>>>>>>>
> >>>>>>>>> On 22/10/2020 14:24, Alexandre Courbot wrote:
> >>>>>>>>>> do_poll()/do_select() seem to set the _qproc member of poll_table to
> >>>>>>>>>> NULL the first time they are called on a given table, making subsequent
> >>>>>>>>>> calls of poll_wait() on that table no-ops. This is a problem for mem2mem
> >>>>>>>>>> which calls poll_wait() on the V4L2 queues' waitqueues only when a
> >>>>>>>>>> queue-related event is requested, which may not necessarily be the case
> >>>>>>>>>> during the first poll.
> >>>>>>>>>>
> >>>>>>>>>> For instance, a stateful decoder is typically only interested in
> >>>>>>>>>> EPOLLPRI events when it starts, and will switch to listening to both
> >>>>>>>>>> EPOLLPRI and EPOLLIN after receiving the initial resolution change event
> >>>>>>>>>> and configuring the CAPTURE queue. However by the time that switch
> >>>>>>>>>> happens and v4l2_m2m_poll_for_data() is called for the first time,
> >>>>>>>>>> poll_wait() has become a no-op and the V4L2 queues waitqueues thus
> >>>>>>>>>> cannot be registered.
> >>>>>>>>>>
> >>>>>>>>>> Fix this by moving the registration to v4l2_m2m_poll() and do it whether
> >>>>>>>>>> or not one of the queue-related events are requested.
> >>>>>>>>>
> >>>>>>>>> This looks good, but would it be possible to add a test for this to
> >>>>>>>>> v4l2-compliance? (Look for POLL_MODE_EPOLL in v4l2-test-buffers.cpp)
> >>>>>>>>>
> >>>>>>>>> If I understand this right, calling EPOLL_CTL_ADD for EPOLLPRI, then
> >>>>>>>>> calling EPOLL_CTL_ADD for EPOLLIN/OUT would trigger this? Or does there
> >>>>>>>>> have to be an epoll_wait call in between?
> >>>>>>>>
> >>>>>>>> Even without an epoll_wait() in between the behavior is visible.
> >>>>>>>> v4l2_m2m_poll() will be called once during the initial EPOLL_CTL_ADD
> >>>>>>>> and this will trigger the bug.
> >>>>>>>>
> >>>>>>>>> Another reason for adding this test is that I wonder if regular capture
> >>>>>>>>> or output V4L2 devices don't have the same issue.
> >>>>>>>>>
> >>>>>>>>> It's a very subtle bug and so adding a test for this to v4l2-compliance
> >>>>>>>>> would be very useful.
> >>>>>>>>
> >>>>>>>> I fully agree, this is very counter-intuitive since what basically
> >>>>>>>> happens is that the kernel's poll_wait() function becomes a no-op
> >>>>>>>> after the poll() hook of a driver is called for the first time. There
> >>>>>>>> is no way one can expect this behavior just from browsing the code so
> >>>>>>>> this is likely to affect other drivers.
> >>>>>>>>
> >>>>>>>> As for the test itself, we can easily reproduce the conditions for
> >>>>>>>> failure in v4l2-test-buffers.cpp's captureBufs() function, but doing
> >>>>>>>> so will make the streaming tests fail without being specific about the
> >>>>>>>> cause. Or maybe we should add another pollmode to specifically test
> >>>>>>>> epoll in this setup? Can I get your thoughts?
> >>>>>>>
> >>>>>>> No, just keep it as part of the poll test. Just add comments at the place
> >>>>>>> where it fails describing this error.
> >>>>>>>
> >>>>>>> After all, it *is* a poll() bug, so it is only fair that it is tested as
> >>>>>>> part of the epoll test.
> >>>>>>>
> >>>>>>> Can you call EPOLL_CTL_ADD with ev.events set to 0? And then call it again
> >>>>>>> with the actual value that you need? If that triggers this issue as well,
> >>>>>>> then that is a nice test (but perhaps EPOLL_CTL_ADD won't call poll() if
> >>>>>>> ev.events is 0, but perhaps EPOLLERR would work instead of 0).
> >>>>>>
> >>>>>> Yup, actually the following is enough to make v4l2-compliance -s fail
> >>>>>> with vicodec:
> >>>>>
> >>>>> Does it also fail with vivid? I am curious to know whether this issue is
> >>>>> m2m specific or a more general problem.
> >>>>
> >>>> It does fail actually! And that made me notice that vb2_poll() uses
> >>>> the same pattern as v4l2_m2m_poll() (probably because the latter is
> >>>> inspired by the former?) and needs to be fixed similarly. I will send
> >>>> another patch to fix vb2_poll() as well, thanks for pointing it out!
> >>>
> >>> I was afraid of that.
> >>>
> >>> Testing epoll for control events would be interesting as well. The
> >>> vivid radio device is an example of a device that has controls, but
> >>> does not do streaming (so is not using vb2).
> >>>
> >>> But from what I can see v4l2_ctrl_poll() does the right thing, so this
> >>> should be fine.
> >>
> >> Indeed, it unconditionally calls poll_wait() with all the wait queues
> >> that may wake us up (that is, only one), so there is no problem there.
> >
> > Sorry, I noticed that this patch was marked with "Changes Requested"
> > in patchwork, but isn't it valid as-is? We need a similar change to
> > VB2, but that should go as a separate patch IMHO. I'm fine with doing
> > both in one go if you prefer that though.
> >
>
> In at least one reply you mentioned that you wanted to add a comment (reply
> from 23 Oct). That's why I changed it to 'Changes Requested'.

Ah, that's correct. Thanks for clarifying.

> Also, I prefer to fix both m2m and vb2 at the same time (separate patches,
> but part of the same patch series). And together with a separate patch improving
> v4l2-compliance.

Ack - will submit a series and a separate patch for v4l-utils.