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

[Alexandre Courbot <gnurou@gmail.com>]

On Thu, Nov 5, 2020 at 9:36 PM Hans Verkuil <hverkuil-cisco@xs4all.nl> wrote:
>
> On 05/11/2020 13:21, Alexandre Courbot wrote:
> > On Tue, Nov 3, 2020 at 6:48 PM Hans Verkuil <hverkuil-cisco@xs4all.nl> 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@xs4all.nl> 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!