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

[Alexandre Courbot]

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:

diff --git a/utils/v4l2-compliance/v4l2-test-buffers.cpp
b/utils/v4l2-compliance/v4l2-test-buffers.cpp
index 8000db23..b63326cd 100644
--- a/utils/v4l2-compliance/v4l2-test-buffers.cpp
+++ b/utils/v4l2-compliance/v4l2-test-buffers.cpp
@@ -903,6 +903,10 @@ static int captureBufs(struct node *node, struct
node *node_m2m_cap, const cv4l_
                epollfd = epoll_create1(0);

                fail_on_test(epollfd < 0);
+
+               ev.events = 0;
+               fail_on_test(epoll_ctl(epollfd, EPOLL_CTL_ADD,
node->g_fd(), &ev));
+
                if (node->is_m2m)
                        ev.events = EPOLLIN | EPOLLOUT | EPOLLPRI;
                else if (v4l_type_is_output(q.g_type()))
@@ -910,7 +914,7 @@ static int captureBufs(struct node *node, struct
node *node_m2m_cap, const cv4l_
                else
                        ev.events = EPOLLIN;
                ev.data.fd = node->g_fd();
-               fail_on_test(epoll_ctl(epollfd, EPOLL_CTL_ADD,
node->g_fd(), &ev));
+               fail_on_test(epoll_ctl(epollfd, EPOLL_CTL_MOD,
node->g_fd(), &ev));
        }

        if (pollmode)

>
> The epoll_wait() will fail when this issue hits, so that's a good place
> to add comments explaining this problem.
>
> There is one other place where this needs to be tested: testEvents() in
> v4l2-test-controls.cpp: currently this only tests select(), but there
> should be a second epoll test here as well that just tests EPOLLPRI.
>
> This would catch drivers that do not stream (i.e. no EPOLLIN/OUT) but
> that do have controls (so support EPOLLPRI).

I'll take a look there as well, and think about a proper comment
before sending a patch towards you.

Cheers,
Alex.