linux-kernel - [PATCH v2 0/2] media: always call poll

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Date:   Tue, 24 Nov 2020 00:18:41 +0900
From:   Alexandre Courbot <gnurou@...il.com>
To:     Hans Verkuil <hverkuil-cisco@...all.nl>,
        Tomasz Figa <tfiga@...omium.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>
Cc:     linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alexandre Courbot <gnurou@...il.com>
Subject: [PATCH v2 0/2] media: always call poll_wait() on queues

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 behavior causes a bug
with the current poll implementations of vb2 and mem2mem, which only
call poll_wait() if a queue-related (EPOLLIN or EPOLLOUT) event if
present: if there is none during the first call (e.g. because userspace
only wanted to listen to EPOLLPRI), then EPOLLIN and EPOLLOUT will never
be signaled, event if they are requested later.

This can be fixed by making the call to poll_wait() unconditional, thus
making sure it will also be invoked during the first call.

The issue has been discussed in more detail on
https://www.spinics.net/lists/linux-media/msg179618.html.

Alexandre Courbot (2):
  media: videobuf2: always call poll_wait() on queues
  media: v4l2-mem2mem: always call poll_wait() on queues

 drivers/media/common/videobuf2/videobuf2-core.c | 11 +++++++++--
 drivers/media/v4l2-core/v4l2-mem2mem.c          | 15 ++++++++++++---
 2 files changed, 21 insertions(+), 5 deletions(-)

--
2.29.2