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

[Hans Verkuil]

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).

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).

Congratulation, you found a really nasty corner case! :-)

Regards,

	Hans