drm: rework delayed connector cleanup in connector_iter

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).

v3
- Add init_llist_head (Chris).

Fixes: a703c55004e1 ("drm: safely free connectors from connector_iter")
Fixes: 613051dac40d ("drm: locking&new iterators for connector_list")
Cc: Ben Widawsky <[e-mail]>
Cc: Dave Airlie <[e-mail]>
Cc: Chris Wilson <[e-mail]>
Cc: Sean Paul <[e-mail]>
Cc: <[e-mail]> # v4.11+: 613051dac40d ("drm: locking&new iterators for connector_list"
Cc: <[e-mail]> # v4.11+
Cc: Daniel Vetter <[e-mail]>
Cc: Jani Nikula <[e-mail]>
Cc: Gustavo Padovan <[e-mail]>
Cc: David Airlie <[e-mail]>
Cc: Javier Martinez Canillas <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Cc: Guillaume Tucker <[e-mail]>
Cc: Mark Brown <[e-mail]>
Cc: Kevin Hilman <[e-mail]>
Cc: Matt Hart <[e-mail]>
Cc: Thierry Escande <[e-mail]>
Cc: Tomeu Vizoso <[e-mail]>
Cc: Enric Balletbo i Serra <[e-mail]>
Tested-by: Marek Szyprowski <[e-mail]>
Reviewed-by: Chris Wilson <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_connector.c

···
 	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);
+	}
 }
 
 /**
···
 				    false, drm_connector_free);
 	if (ret)
 		return ret;
-
-	INIT_WORK(&connector->free_work, drm_connector_free_work_fn);
 
 	connector->base.properties = &connector->properties;
 	connector->dev = dev;
···
  * 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);
 }
 
 /**
···
 
 		/* 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;
 }
···
  */
 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);

drivers/gpu/drm/drm_crtc_internal.h

···
 				    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,

drivers/gpu/drm/drm_mode_config.c

···
 	ida_init(&dev->mode_config.connector_ida);
 	spin_lock_init(&dev->mode_config.connector_list_lock);
 
+	init_llist_head(&dev->mode_config.connector_free_list);
+	INIT_WORK(&dev->mode_config.connector_free_work, drm_connector_free_work_fn);
+
 	drm_mode_create_standard_properties(dev);
 
 	/* Just to be sure */
···
 	}
 	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)

include/drm/drm_connector.h

···
 #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>
···
 	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)

include/drm/drm_mode_config.h

···
 #include <linux/types.h>
 #include <linux/idr.h>
 #include <linux/workqueue.h>
+#include <linux/llist.h>
 
 #include <drm/drm_modeset_lock.h>
 
···
 
 	/**
 	 * @connector_list_lock: Protects @num_connector and
-	 * @connector_list.
+	 * @connector_list and @connector_free_list.
 	 */
 	spinlock_t connector_list_lock;
 	/**
···
 	 * &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:
 	 *