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
