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

[Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>]

Hi Daniel,

On 2017-12-13 13:49, Daniel Vetter wrote:
> 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.
>
> v2:
> - Correctly free connectors only on last ref. Oops (Chris).
> - use llist_head/node (Chris).
>
> Fixes: a703c55004e1 ("drm: safely free connectors from connector_iter")
> Fixes: 613051dac40d ("drm: locking&new iterators for connector_list")
> Cc: Ben Widawsky <ben@xxxxxxxxxxxx>
> Cc: Dave Airlie <airlied@xxxxxxxxx>
> Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Sean Paul <seanpaul@xxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # v4.11+: 613051dac40d ("drm: locking&new iterators for connector_list"
> Cc: <stable@xxxxxxxxxxxxxxx> # v4.11+
> Cc: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> Cc: Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx>
> Cc: Gustavo Padovan <gustavo@xxxxxxxxxxx>
> Cc: David Airlie <airlied@xxxxxxxx>
> Cc: Javier Martinez Canillas <javier@xxxxxxxxxxxx>
> Cc: Shuah Khan <shuahkh@xxxxxxxxxxxxxxx>
> Cc: Guillaume Tucker <guillaume.tucker@xxxxxxxxxxxxx>
> Cc: Mark Brown <broonie@xxxxxxxxxx>
> Cc: Kevin Hilman <khilman@xxxxxxxxxxxx>
> Cc: Matt Hart <matthew.hart@xxxxxxxxxx>
> Cc: Thierry Escande <thierry.escande@xxxxxxxxxxxxxxx>
> Cc: Tomeu Vizoso <tomeu.vizoso@xxxxxxxxxxxxx>
> Cc: Enric Balletbo i Serra <enric.balletbo@xxxxxxxxxxxxx>
> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>

This one works fine and fixes deadlock in my test environment.

Tested-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>

> ---
>   drivers/gpu/drm/drm_connector.c     | 50 ++++++++++++++++++++++++++-----------
>   drivers/gpu/drm/drm_crtc_internal.h |  1 +
>   drivers/gpu/drm/drm_mode_config.c   |  4 ++-
>   include/drm/drm_connector.h         | 10 +++++---
>   include/drm/drm_mode_config.h       | 18 ++++++++++++-
>   5 files changed, 62 insertions(+), 21 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
> index 0b7e0974e6da..3f53f127e1f2 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, *n;
> +	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;
> +	struct llist_node *freed;
>   
> -	drm_mode_object_unregister(dev, &connector->base);
> -	connector->funcs->destroy(connector);
> +	spin_lock_irqsave(&config->connector_list_lock, flags);
> +	freed = llist_del_all(&config->connector_free_list);
> +	spin_unlock_irqrestore(&config->connector_list_lock, flags);
> +
> +	llist_for_each_entry_safe(connector, n, freed, free_node) {
> +		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 +557,17 @@ 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)
>   {
> -	if (refcount_dec_and_test(&conn->base.refcount.refcount))
> -		schedule_work(&conn->free_work);
> +	struct drm_mode_config *config = &conn->dev->mode_config;
> +
> +	lockdep_assert_held(&config->connector_list_lock);
> +
> +	if (!refcount_dec_and_test(&conn->base.refcount.refcount))
> +		return;
> +
> +	llist_add(&conn->free_node, &config->connector_free_list);
> +	schedule_work(&config->connector_free_work);
>   }
>   
>   /**
> @@ -585,10 +599,10 @@ drm_connector_list_iter_next(struct drm_connector_list_iter *iter)
>   
>   		/* loop until it's not a zombie connector */
>   	} while (!kref_get_unless_zero(&iter->conn->base.refcount));
> -	spin_unlock_irqrestore(&config->connector_list_lock, flags);
>   
>   	if (old_conn)
> -		drm_connector_put_safe(old_conn);
> +		__drm_connector_put_safe(old_conn);
> +	spin_unlock_irqrestore(&config->connector_list_lock, flags);
>   
>   	return iter->conn;
>   }
> @@ -605,9 +619,15 @@ EXPORT_SYMBOL(drm_connector_list_iter_next);
>    */
>   void drm_connector_list_iter_end(struct drm_connector_list_iter *iter)
>   {
> +	struct drm_mode_config *config = &iter->dev->mode_config;
> +	unsigned long flags;
> +
>   	iter->dev = NULL;
> -	if (iter->conn)
> -		drm_connector_put_safe(iter->conn);
> +	if (iter->conn) {
> +		spin_lock_irqsave(&config->connector_list_lock, flags);
> +		__drm_connector_put_safe(iter->conn);
> +		spin_unlock_irqrestore(&config->connector_list_lock, flags);
> +	}
>   	lock_release(&connector_list_iter_dep_map, 0, _RET_IP_);
>   }
>   EXPORT_SYMBOL(drm_connector_list_iter_end);
> diff --git a/drivers/gpu/drm/drm_crtc_internal.h b/drivers/gpu/drm/drm_crtc_internal.h
> index 9ebb8841778c..af00f42ba269 100644
> --- a/drivers/gpu/drm/drm_crtc_internal.h
> +++ b/drivers/gpu/drm/drm_crtc_internal.h
> @@ -142,6 +142,7 @@ int drm_mode_connector_set_obj_prop(struct drm_mode_object *obj,
>   				    uint64_t value);
>   int drm_connector_create_standard_properties(struct drm_device *dev);
>   const char *drm_get_connector_force_name(enum drm_connector_force force);
> +void drm_connector_free_work_fn(struct work_struct *work);
>   
>   /* IOCTL */
>   int drm_mode_connector_property_set_ioctl(struct drm_device *dev,
> diff --git a/drivers/gpu/drm/drm_mode_config.c b/drivers/gpu/drm/drm_mode_config.c
> index 6ffe952142e6..7681269abe41 100644
> --- a/drivers/gpu/drm/drm_mode_config.c
> +++ b/drivers/gpu/drm/drm_mode_config.c
> @@ -382,6 +382,8 @@ void drm_mode_config_init(struct drm_device *dev)
>   	ida_init(&dev->mode_config.connector_ida);
>   	spin_lock_init(&dev->mode_config.connector_list_lock);
>   
> +	INIT_WORK(&dev->mode_config.connector_free_work, drm_connector_free_work_fn);
> +
>   	drm_mode_create_standard_properties(dev);
>   
>   	/* Just to be sure */
> @@ -432,7 +434,7 @@ void drm_mode_config_cleanup(struct drm_device *dev)
>   	}
>   	drm_connector_list_iter_end(&conn_iter);
>   	/* connector_iter drops references in a work item. */
> -	flush_scheduled_work();
> +	flush_work(&dev->mode_config.connector_free_work);
>   	if (WARN_ON(!list_empty(&dev->mode_config.connector_list))) {
>   		drm_connector_list_iter_begin(dev, &conn_iter);
>   		drm_for_each_connector_iter(connector, &conn_iter)
> diff --git a/include/drm/drm_connector.h b/include/drm/drm_connector.h
> index ee3155391243..ed38df4ac204 100644
> --- a/include/drm/drm_connector.h
> +++ b/include/drm/drm_connector.h
> @@ -24,6 +24,7 @@
>   #define __DRM_CONNECTOR_H__
>   
>   #include <linux/list.h>
> +#include <linux/llist.h>
>   #include <linux/ctype.h>
>   #include <linux/hdmi.h>
>   #include <drm/drm_mode_object.h>
> @@ -966,12 +967,13 @@ struct drm_connector {
>   	uint16_t tile_h_size, tile_v_size;
>   
>   	/**
> -	 * @free_work:
> +	 * @free_node:
>   	 *
> -	 * Work used only by &drm_connector_iter to be able to clean up a
> -	 * connector from any context.
> +	 * List used only by &drm_connector_iter to be able to clean up a
> +	 * connector from any context, in conjunction with
> +	 * &drm_mode_config.connector_free_work.
>   	 */
> -	struct work_struct free_work;
> +	struct llist_node free_node;
>   };
>   
>   #define obj_to_connector(x) container_of(x, struct drm_connector, base)
> diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
> index e5f3b43014e1..2cb6f02df64a 100644
> --- a/include/drm/drm_mode_config.h
> +++ b/include/drm/drm_mode_config.h
> @@ -27,6 +27,7 @@
>   #include <linux/types.h>
>   #include <linux/idr.h>
>   #include <linux/workqueue.h>
> +#include <linux/llist.h>
>   
>   #include <drm/drm_modeset_lock.h>
>   
> @@ -402,7 +403,7 @@ struct drm_mode_config {
>   
>   	/**
>   	 * @connector_list_lock: Protects @num_connector and
> -	 * @connector_list.
> +	 * @connector_list and @connector_free_list.
>   	 */
>   	spinlock_t connector_list_lock;
>   	/**
> @@ -422,6 +423,21 @@ struct drm_mode_config {
>   	 * &struct drm_connector_list_iter to walk this list.
>   	 */
>   	struct list_head connector_list;
> +	/**
> +	 * @connector_free_list:
> +	 *
> +	 * List of connector objects linked with &drm_connector.free_head.
> +	 * Protected by @connector_list_lock. Used by
> +	 * drm_for_each_connector_iter() and
> +	 * &struct drm_connector_list_iter to savely free connectors using
> +	 * @connector_free_work.
> +	 */
> +	struct llist_head connector_free_list;
> +	/**
> +	 * @connector_free_work: Work to clean up @connector_free_list.
> +	 */
> +	struct work_struct connector_free_work;
> +
>   	/**
>   	 * @num_encoder:
>   	 *

Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland