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

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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

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