Re: [RFC 06/13] v4l2-async: per notifier locking

[Sakari Ailus]

Hi Pavel and Sebastian,

On Tue, Feb 14, 2017 at 02:39:56PM +0100, Pavel Machek wrote:
> From: Sebastian Reichel <s...@kernel.org>
> 
> Without this, camera support breaks boot on N900.
> 
> Signed-off-by: Ivaylo Dimitrov <ivo.g.dimitrov...@gmail.com>
> ---
>  drivers/media/v4l2-core/v4l2-async.c | 54 
> ++++++++++++++++++------------------
>  include/media/v4l2-async.h           |  2 ++
>  2 files changed, 29 insertions(+), 27 deletions(-)
> 
> diff --git a/drivers/media/v4l2-core/v4l2-async.c 
> b/drivers/media/v4l2-core/v4l2-async.c
> index 96cc733..26492a2 100644
> --- a/drivers/media/v4l2-core/v4l2-async.c
> +++ b/drivers/media/v4l2-core/v4l2-async.c
> @@ -57,7 +57,6 @@ static bool match_custom(struct v4l2_subdev *sd, struct 
> v4l2_async_subdev *asd)
>  
>  static LIST_HEAD(subdev_list);
>  static LIST_HEAD(notifier_list);
> -static DEFINE_MUTEX(list_lock);
>  
>  static struct v4l2_async_subdev *v4l2_async_belongs(struct 
> v4l2_async_notifier *notifier,
>                                                   struct v4l2_subdev *sd)
> @@ -102,12 +101,15 @@ static int v4l2_async_test_notify(struct 
> v4l2_async_notifier *notifier,
>  
>       if (notifier->bound) {
>               ret = notifier->bound(notifier, sd, asd);
> -             if (ret < 0)
> +             if (ret < 0) {
> +                     dev_warn(notifier->v4l2_dev->dev, "subdev bound 
> failed\n");
>                       return ret;
> +             }
>       }
>  
>       ret = v4l2_device_register_subdev(notifier->v4l2_dev, sd);
>       if (ret < 0) {
> +             dev_warn(notifier->v4l2_dev->dev, "subdev register failed\n");
>               if (notifier->unbind)
>                       notifier->unbind(notifier, sd, asd);
>               return ret;
> @@ -141,7 +143,7 @@ int v4l2_async_notifier_register(struct v4l2_device 
> *v4l2_dev,
>  {
>       struct v4l2_subdev *sd, *tmp;
>       struct v4l2_async_subdev *asd;
> -     int i;
> +     int ret = 0, i;
>  
>       if (!notifier->num_subdevs || notifier->num_subdevs > V4L2_MAX_SUBDEVS)
>               return -EINVAL;
> @@ -149,6 +151,7 @@ int v4l2_async_notifier_register(struct v4l2_device 
> *v4l2_dev,
>       notifier->v4l2_dev = v4l2_dev;
>       INIT_LIST_HEAD(&notifier->waiting);
>       INIT_LIST_HEAD(&notifier->done);
> +     mutex_init(&notifier->lock);
>  
>       for (i = 0; i < notifier->num_subdevs; i++) {
>               asd = notifier->subdevs[i];
> @@ -168,28 +171,22 @@ int v4l2_async_notifier_register(struct v4l2_device 
> *v4l2_dev,
>               list_add_tail(&asd->list, &notifier->waiting);
>       }
>  
> -     mutex_lock(&list_lock);
> +     /* Keep also completed notifiers on the list */
> +     list_add(&notifier->list, &notifier_list);
> +     mutex_lock(&notifier->lock);
>  
>       list_for_each_entry_safe(sd, tmp, &subdev_list, async_list) {
> -             int ret;
> -
>               asd = v4l2_async_belongs(notifier, sd);
>               if (!asd)
>                       continue;
>  
>               ret = v4l2_async_test_notify(notifier, sd, asd);
> -             if (ret < 0) {
> -                     mutex_unlock(&list_lock);
> -                     return ret;
> -             }
> +             if (ret < 0)
> +                     break;
>       }
> +     mutex_unlock(&notifier->lock);
>  
> -     /* Keep also completed notifiers on the list */
> -     list_add(&notifier->list, &notifier_list);
> -
> -     mutex_unlock(&list_lock);
> -
> -     return 0;
> +     return ret;
>  }
>  EXPORT_SYMBOL(v4l2_async_notifier_register);
>  
> @@ -210,7 +207,7 @@ void v4l2_async_notifier_unregister(struct 
> v4l2_async_notifier *notifier)
>                       "Failed to allocate device cache!\n");
>       }
>  
> -     mutex_lock(&list_lock);
> +     mutex_lock(&notifier->lock);
>  
>       list_del(&notifier->list);
>  
> @@ -237,7 +234,7 @@ void v4l2_async_notifier_unregister(struct 
> v4l2_async_notifier *notifier)
>                       put_device(d);
>       }
>  
> -     mutex_unlock(&list_lock);
> +     mutex_unlock(&notifier->lock);
>  
>       /*
>        * Call device_attach() to reprobe devices
> @@ -262,6 +259,7 @@ void v4l2_async_notifier_unregister(struct 
> v4l2_async_notifier *notifier)
>       }
>       kfree(dev);
>  
> +     mutex_destroy(&notifier->lock);
>       notifier->v4l2_dev = NULL;
>  
>       /*
> @@ -274,6 +272,7 @@ EXPORT_SYMBOL(v4l2_async_notifier_unregister);
>  int v4l2_async_register_subdev(struct v4l2_subdev *sd)
>  {
>       struct v4l2_async_notifier *notifier;
> +     struct v4l2_async_notifier *tmp;
>  
>       /*
>        * No reference taken. The reference is held by the device
> @@ -283,24 +282,25 @@ int v4l2_async_register_subdev(struct v4l2_subdev *sd)
>       if (!sd->of_node && sd->dev)
>               sd->of_node = sd->dev->of_node;
>  
> -     mutex_lock(&list_lock);
> -
>       INIT_LIST_HEAD(&sd->async_list);
>  
> -     list_for_each_entry(notifier, &notifier_list, list) {
> -             struct v4l2_async_subdev *asd = v4l2_async_belongs(notifier, 
> sd);
> +     list_for_each_entry_safe(notifier, tmp, &notifier_list, list) {

You still need to serialise access to the global notifier list.

The _safe iterator variants allow deleting the current entry from the list
but they do not help more than that.

One possible approach could be to gather the matching async sub-devices and
call the v4l2_async_test_notify() while not holding the notifier list mutex
anymore.

I presume the same problem is present in notifier registration.

> +             struct v4l2_async_subdev *asd;
> +
> +             /* TODO: FIXME: if this is called by ->bound() we will also 
> iterate over the locked notifier */
> +             mutex_lock_nested(&notifier->lock, SINGLE_DEPTH_NESTING);
> +             asd = v4l2_async_belongs(notifier, sd);
>               if (asd) {
>                       int ret = v4l2_async_test_notify(notifier, sd, asd);
> -                     mutex_unlock(&list_lock);
> +                     mutex_unlock(&notifier->lock);
>                       return ret;
>               }
> +             mutex_unlock(&notifier->lock);
>       }
>  
>       /* None matched, wait for hot-plugging */
>       list_add(&sd->async_list, &subdev_list);
>  
> -     mutex_unlock(&list_lock);
> -
>       return 0;
>  }
>  EXPORT_SYMBOL(v4l2_async_register_subdev);
> @@ -315,7 +315,7 @@ void v4l2_async_unregister_subdev(struct v4l2_subdev *sd)
>               return;
>       }
>  
> -     mutex_lock(&list_lock);
> +     mutex_lock_nested(&notifier->lock, SINGLE_DEPTH_NESTING);
>  
>       list_add(&sd->asd->list, &notifier->waiting);
>  
> @@ -324,6 +324,6 @@ void v4l2_async_unregister_subdev(struct v4l2_subdev *sd)
>       if (notifier->unbind)
>               notifier->unbind(notifier, sd, sd->asd);
>  
> -     mutex_unlock(&list_lock);
> +     mutex_unlock(&notifier->lock);
>  }
>  EXPORT_SYMBOL(v4l2_async_unregister_subdev);
> diff --git a/include/media/v4l2-async.h b/include/media/v4l2-async.h
> index 8e2a236..690a81f 100644
> --- a/include/media/v4l2-async.h
> +++ b/include/media/v4l2-async.h
> @@ -84,6 +84,7 @@ struct v4l2_async_subdev {
>   * @waiting: list of struct v4l2_async_subdev, waiting for their drivers
>   * @done:    list of struct v4l2_subdev, already probed
>   * @list:    member in a global list of notifiers
> + * @lock:       lock hold when the notifier is being processed
>   * @bound:   a subdevice driver has successfully probed one of subdevices
>   * @complete:        all subdevices have been probed successfully
>   * @unbind:  a subdevice is leaving
> @@ -95,6 +96,7 @@ struct v4l2_async_notifier {
>       struct list_head waiting;
>       struct list_head done;
>       struct list_head list;
> +     struct mutex lock;
>       int (*bound)(struct v4l2_async_notifier *notifier,
>                    struct v4l2_subdev *subdev,
>                    struct v4l2_async_subdev *asd);

-- 
Kind regards,

Sakari Ailus
e-mail: sakari.ai...@iki.fi     XMPP: sai...@retiisi.org.uk