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Subject[PATCH 4.14 076/164] drm: safely free connectors from connector_iter
Date
4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Daniel Vetter <daniel.vetter@ffwll.ch>

commit a703c55004e1c5076d57e43771b3e11117796ea0 upstream.

In

commit 613051dac40da1751ab269572766d3348d45a197
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date: Wed Dec 14 00:08:06 2016 +0100

drm: locking&new iterators for connector_list

we've went to extreme lengths to make sure connector iterations works
in any context, without introducing any additional locking context.
This worked, except for a small fumble in the implementation:

When we actually race with a concurrent connector unplug event, and
our temporary connector reference turns out to be the final one, then
everything breaks: We call the connector release function from
whatever context we happen to be in, which can be an irq/atomic
context. And connector freeing grabs all kinds of locks and stuff.

Fix this by creating a specially safe put function for connetor_iter,
which (in this rare case) punts the cleanup to a worker.

Reported-by: Ben Widawsky <ben@bwidawsk.net>
Cc: Ben Widawsky <ben@bwidawsk.net>
Fixes: 613051dac40d ("drm: locking&new iterators for connector_list")
Cc: Dave Airlie <airlied@gmail.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171204204818.24745-1-daniel.vetter@ffwll.ch
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/gpu/drm/drm_connector.c | 28 ++++++++++++++++++++++++++--
drivers/gpu/drm/drm_mode_config.c | 2 ++
include/drm/drm_connector.h | 8 ++++++++
3 files changed, 36 insertions(+), 2 deletions(-)

--- a/drivers/gpu/drm/drm_connector.c
+++ b/drivers/gpu/drm/drm_connector.c
@@ -152,6 +152,16 @@ static void drm_connector_free(struct kr
connector->funcs->destroy(connector);
}

+static 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;
+
+ drm_mode_object_unregister(dev, &connector->base);
+ connector->funcs->destroy(connector);
+}
+
/**
* drm_connector_init - Init a preallocated connector
* @dev: DRM device
@@ -181,6 +191,8 @@ int drm_connector_init(struct drm_device
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;
@@ -525,6 +537,18 @@ void drm_connector_list_iter_begin(struc
}
EXPORT_SYMBOL(drm_connector_list_iter_begin);

+/*
+ * Extra-safe connector put function that works in any context. Should only be
+ * used from the connector_iter functions, where we never really expect to
+ * actually release the connector when dropping our final reference.
+ */
+static void
+drm_connector_put_safe(struct drm_connector *conn)
+{
+ if (refcount_dec_and_test(&conn->base.refcount.refcount))
+ schedule_work(&conn->free_work);
+}
+
/**
* drm_connector_list_iter_next - return next connector
* @iter: connectr_list iterator
@@ -557,7 +581,7 @@ drm_connector_list_iter_next(struct drm_
spin_unlock_irqrestore(&config->connector_list_lock, flags);

if (old_conn)
- drm_connector_put(old_conn);
+ drm_connector_put_safe(old_conn);

return iter->conn;
}
@@ -576,7 +600,7 @@ void drm_connector_list_iter_end(struct
{
iter->dev = NULL;
if (iter->conn)
- drm_connector_put(iter->conn);
+ drm_connector_put_safe(iter->conn);
lock_release(&connector_list_iter_dep_map, 0, _RET_IP_);
}
EXPORT_SYMBOL(drm_connector_list_iter_end);
--- a/drivers/gpu/drm/drm_mode_config.c
+++ b/drivers/gpu/drm/drm_mode_config.c
@@ -428,6 +428,8 @@ void drm_mode_config_cleanup(struct drm_
drm_connector_put(connector);
}
drm_connector_list_iter_end(&conn_iter);
+ /* connector_iter drops references in a work item. */
+ flush_scheduled_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)
--- a/include/drm/drm_connector.h
+++ b/include/drm/drm_connector.h
@@ -905,6 +905,14 @@ struct drm_connector {
uint8_t num_h_tile, num_v_tile;
uint8_t tile_h_loc, tile_v_loc;
uint16_t tile_h_size, tile_v_size;
+
+ /**
+ * @free_work:
+ *
+ * Work used only by &drm_connector_iter to be able to clean up a
+ * connector from any context.
+ */
+ struct work_struct free_work;
};

#define obj_to_connector(x) container_of(x, struct drm_connector, base)

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