On Mon, Dec 04, 2017 at 09:48:18PM +0100, Daniel Vetter wrote:
> In
>
> commit 613051dac40da1751ab269572766d3348d45a197
> Author: Daniel Vetter <daniel.vetter@xxxxxxxx>
> 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@xxxxxxxxxxxx>
> Cc: Ben Widawsky <ben@xxxxxxxxxxxx>
> Fixes: 613051dac40d ("drm: locking&new iterators for connector_list")
> Cc: Dave Airlie <airlied@xxxxxxxxx>
> Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Sean Paul <seanpaul@xxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # v4.11+
> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
s/savely/safely/ in the summary and
Reviewed-by: Dave Airlie <airlied@xxxxxxxxx>
from irc. CI is also happy, I'll merge as soon as Ben has approved this
too.
-Daniel
> ---
> 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(-)
>
> diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
> index 25f4b2e9a44f..482014137953 100644
> --- a/drivers/gpu/drm/drm_connector.c
> +++ b/drivers/gpu/drm/drm_connector.c
> @@ -152,6 +152,16 @@ static void drm_connector_free(struct kref *kref)
> 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 *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;
> @@ -529,6 +541,18 @@ void drm_connector_list_iter_begin(struct drm_device *dev,
> }
> 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
> @@ -561,7 +585,7 @@ drm_connector_list_iter_next(struct drm_connector_list_iter *iter)
> 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;
> }
> @@ -580,7 +604,7 @@ void drm_connector_list_iter_end(struct drm_connector_list_iter *iter)
> {
> 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);
> diff --git a/drivers/gpu/drm/drm_mode_config.c b/drivers/gpu/drm/drm_mode_config.c
> index 7623607c0f1e..346c19c6ce01 100644
> --- a/drivers/gpu/drm/drm_mode_config.c
> +++ b/drivers/gpu/drm/drm_mode_config.c
> @@ -431,6 +431,8 @@ void drm_mode_config_cleanup(struct drm_device *dev)
> 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)
> diff --git a/include/drm/drm_connector.h b/include/drm/drm_connector.h
> index 66d6c99d15e5..c5c753a1be85 100644
> --- a/include/drm/drm_connector.h
> +++ b/include/drm/drm_connector.h
> @@ -926,6 +926,14 @@ 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)
> --
> 2.15.0
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch