Re: [PATCH] drm: rework delayed connector cleanup in connector_iter

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Daniel Vetter (2017-12-13 10:45:53)
> PROBE_DEFER also uses system_wq to reprobe drivers, which means when
> that again fails, and we try to flush the overall system_wq (to get
> all the delayed connectore cleanup work_struct completed), we
> deadlock.
> 
> Fix this by using just a single cleanup work, so that we can only
> flush that one and don't block on anything else. That means a free
> list plus locking, a standard pattern.
> 
> Fixes: a703c55004e1 ("drm: safely free connectors from connector_iter")
> Fixes: 613051dac40d ("drm: locking&new iterators for connector_list")
> Cc: Ben Widawsky <ben@bwidawsk.net>
> Cc: Dave Airlie <airlied@gmail.com>
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Sean Paul <seanpaul@chromium.org>
> Cc: <stable@vger.kernel.org> # v4.11+: 613051dac40d ("drm: locking&new iterators for connector_list"
> Cc: <stable@vger.kernel.org> # v4.11+
> Cc: Daniel Vetter <daniel.vetter@intel.com>
> Cc: Jani Nikula <jani.nikula@linux.intel.com>
> Cc: Gustavo Padovan <gustavo@padovan.org>
> Cc: David Airlie <airlied@linux.ie>
> Cc: Javier Martinez Canillas <javier@dowhile0.org>
> Cc: Shuah Khan <shuahkh@osg.samsung.com>
> Cc: Guillaume Tucker <guillaume.tucker@collabora.com>
> Cc: Mark Brown <broonie@kernel.org>
> Cc: Kevin Hilman <khilman@baylibre.com>
> Cc: Matt Hart <matthew.hart@linaro.org>
> Cc: Thierry Escande <thierry.escande@collabora.co.uk>
> Cc: Tomeu Vizoso <tomeu.vizoso@collabora.com>
> Cc: Enric Balletbo i Serra <enric.balletbo@collabora.com>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> ---
>  drivers/gpu/drm/drm_connector.c     | 46 ++++++++++++++++++++++++++-----------
>  drivers/gpu/drm/drm_crtc_internal.h |  1 +
>  drivers/gpu/drm/drm_mode_config.c   |  5 +++-
>  include/drm/drm_connector.h         |  4 ++--
>  include/drm/drm_mode_config.h       | 17 +++++++++++++-
>  5 files changed, 55 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
> index 0b7e0974e6da..2f43dc217936 100644
> --- a/drivers/gpu/drm/drm_connector.c
> +++ b/drivers/gpu/drm/drm_connector.c
> @@ -153,14 +153,23 @@ static void drm_connector_free(struct kref *kref)
>         connector->funcs->destroy(connector);
>  }
>  
> -static void drm_connector_free_work_fn(struct work_struct *work)
> +void drm_connector_free_work_fn(struct work_struct *work)
>  {
> -       struct drm_connector *connector =
> -               container_of(work, struct drm_connector, free_work);
> -       struct drm_device *dev = connector->dev;
> +       struct drm_connector *connector;
> +       struct drm_device *dev =
> +               container_of(work, struct drm_device, mode_config.connector_free_work);
> +       struct drm_mode_config *config = &dev->mode_config;
> +       unsigned long flags;
> +       LIST_HEAD(list_head);
>  
> -       drm_mode_object_unregister(dev, &connector->base);
> -       connector->funcs->destroy(connector);
> +       spin_lock_irqsave(&config->connector_list_lock, flags);
> +       list_splice(&config->connector_free_list, &list_head);
> +       spin_unlock_irqrestore(&config->connector_list_lock, flags);
> +
> +       list_for_each_entry(connector, &list_head, free_head) {
> +               drm_mode_object_unregister(dev, &connector->base);
> +               connector->funcs->destroy(connector);
> +       }
>  }
>  
>  /**
> @@ -192,8 +201,6 @@ int drm_connector_init(struct drm_device *dev,
>         if (ret)
>                 return ret;
>  
> -       INIT_WORK(&connector->free_work, drm_connector_free_work_fn);
> -
>         connector->base.properties = &connector->properties;
>         connector->dev = dev;
>         connector->funcs = funcs;
> @@ -550,10 +557,15 @@ EXPORT_SYMBOL(drm_connector_list_iter_begin);
>   * actually release the connector when dropping our final reference.
>   */
>  static void
> -drm_connector_put_safe(struct drm_connector *conn)
> +__drm_connector_put_safe(struct drm_connector *conn)
>  {
> +       struct drm_mode_config *config = &conn->dev->mode_config;
> +
> +       lockdep_assert_held(&config->connector_list_lock);
> +       list_add(&conn->free_head, &config->connector_free_list);

Adding to the free_list before refcount==0?

May I suggest,

	if (!refcount_dec_and_test())
		return;

	if (llist_add(&conn->free_link, &config->connector_free_list))
		schedule_work(&config->connector_free_work);

llist being much more friendly for freelists:

void drm_connector_free_work_fn(struct work_struct *work)
{
	struct drm_device *dev =
		container_of(work, struct drm_device, mode_config.connector_free_work);
	struct drm_mode_config *config = &dev->mode_config;
	struct drm_connector *connector, *n;
	struct llist_node *freed;

	freed = llist_del_all(config->connector_free_list);
	llist_for_each_entry_safe(connector, n, freed, free_link) {
		drm_mode_object_unregister(dev, &connector->base);
		connector->funcs->destroy(connector);
	}
}
-Chris