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

[Sylwester Nawrocki]

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.pinch...@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
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