V4L/DVB: v4l2: add core serialization lock

Drivers can optionally set a pointer to a mutex in struct video_device.
The core will use that to lock before calling open, read, write, unlocked_ioctl,
poll, mmap or release.

Updated the documentation as well and ensure that v4l2-event knows about the
lock: it will unlock it before doing a blocking wait on an event and relock it
afterwards.

Ensure that the 'video_is_registered' check is done when the lock is held:
a typical disconnect will take the lock as well before unregistering the
device nodes, so to prevent race conditions the video_is_registered check
should also be done with the lock held.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/drivers/media/video/v4l2-event.c b/drivers/media/video/v4l2-event.c
index de74ce0..69fd343 100644
--- a/drivers/media/video/v4l2-event.c
+++ b/drivers/media/video/v4l2-event.c
@@ -134,15 +134,22 @@ int v4l2_event_dequeue(struct v4l2_fh *fh, struct v4l2_event *event,
 	if (nonblocking)
 		return __v4l2_event_dequeue(fh, event);
 
+	/* Release the vdev lock while waiting */
+	if (fh->vdev->lock)
+		mutex_unlock(fh->vdev->lock);
+
 	do {
 		ret = wait_event_interruptible(events->wait,
 					       events->navailable != 0);
 		if (ret < 0)
-			return ret;
+			break;
 
 		ret = __v4l2_event_dequeue(fh, event);
 	} while (ret == -ENOENT);
 
+	if (fh->vdev->lock)
+		mutex_lock(fh->vdev->lock);
+
 	return ret;
 }
 EXPORT_SYMBOL_GPL(v4l2_event_dequeue);