Re: [PATCH] V4L: async: Make sure subdevs are stored in a list before being moved

Sylwester Nawrocki
2013-08-08T15:51:49Z

Hi Laurent,

On 08/07/2013 12:41 PM, Laurent Pinchart wrote:
> Subdevices have an async_list field used to store them in the global
> list of subdevices or in the notifier done lists. List entries are moved
> from the former to the latter in v4l2_async_test_notify() using
> list_move(). However, v4l2_async_test_notify() can be called right away
> when the subdev is registered with v4l2_async_register_subdev(), in
> which case the entry is not stored in any list.
>
> Although this behaviour is not correct, the code doesn't crash at the
> moment as the async_list field is initialized as a list head, despite
> being a list entry.
>
> Add the subdev to the global subdevs list a registration time before
> matching them with the notifiers to make sure the list_move() call will
> get a subdev that is stored in a list, and remove the list head
> initialization for the subdev async_list field.
>
> Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

Shouldn't we initialize the async_list field in v4l2_subdev_init() ?

diff --git a/drivers/media/v4l2-core/v4l2-subdev.c
b/drivers/media/v4l2-core/v4l2-subdev.c
index 996c248..31b2375 100644
--- a/drivers/media/v4l2-core/v4l2-subdev.c
+++ b/drivers/media/v4l2-core/v4l2-subdev.c
@@ -460,6 +460,7 @@ EXPORT_SYMBOL_GPL(v4l2_subdev_link_validate);
void v4l2_subdev_init(struct v4l2_subdev *sd, const struct v4l2_subdev_ops *ops)
{
INIT_LIST_HEAD(&sd->list);
+ INIT_LIST_HEAD(&sd->async_list);
BUG_ON(!ops);
sd->ops = ops;
sd->v4l2_dev = NULL;

> ---
> drivers/media/v4l2-core/v4l2-async.c | 11 ++++++-----
> 1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/media/v4l2-core/v4l2-async.c b/drivers/media/v4l2-core/v4l2-async.c
> index b350ab9..4485dfe 100644
> --- a/drivers/media/v4l2-core/v4l2-async.c
> +++ b/drivers/media/v4l2-core/v4l2-async.c
> @@ -122,7 +122,7 @@ static void v4l2_async_cleanup(struct v4l2_subdev *sd)
> {
> v4l2_device_unregister_subdev(sd);
> /* Subdevice driver will reprobe and put the subdev back onto the list */
> - list_del_init(&sd->async_list);
> + list_del(&sd->async_list);

It is not safe to do so, since v4l2_async_cleanup() can be called multiple
times and list_del() leaves async_list in an undefined state. I'm actually
observing a crash with this change.

> sd->asd = NULL;
> sd->dev = NULL;
> }
> @@ -238,7 +238,11 @@ int v4l2_async_register_subdev(struct v4l2_subdev *sd)
>
> mutex_lock(&list_lock);
>
> - INIT_LIST_HEAD(&sd->async_list);
> + /*
> + * Add the subdev to the global subdevs list. It will be moved to the
> + * notifier done list by v4l2_async_test_notify().
> + */
> + list_add(&sd->async_list, &subdev_list);
>
> list_for_each_entry(notifier, &notifier_list, list) {
> struct v4l2_async_subdev *asd = v4l2_async_belongs(notifier, sd);
> @@ -249,9 +253,6 @@ int v4l2_async_register_subdev(struct v4l2_subdev *sd)
> }
> }
>
> - /* None matched, wait for hot-plugging */
> - list_add(&sd->async_list, &subdev_list);
> -
> mutex_unlock(&list_lock);
>
> return 0;
>

Regards,
Sylwester