[PATCH] drm/client: Fix: drm_client_new: Don't require DRM to be registered

[Daniel Vetter]

On Wed, Jul 11, 2018 at 05:56:32PM +0200, Noralf Trønnes wrote:
> Commit 894a677f4b3e ("drm/cma-helper: Use the generic fbdev emulation")
> broke almost all drivers that use the CMA helper.
> 
> The reason is that drm_client_new() requires that the DRM device has
> been registered, but the drivers register fbdev before registering DRM.
> 
> Remove the requirement that DRM should be registered when creating a
> new client.
> 
> This creates a theoretical race condition where drm_client_new() races
> with drm_dev_unregister() resulting in the .unregister hook not being
> called.
> 
> Case:
> User loads module which calls drm_client_new().
> Device is unplugged and drm_dev_unregister() is called.
> 
> If drm_dev_unregister() gets the clientlist_mutex first, the new client
> will never get it's unregister hook called.

As long as we assume that drm_client_new() is serialized against
drm_dev_unregister (which it has to, or all the current fbdev code would
be busted) I'm not seeing a race here.

We can't allow drm_client_new concurrently with drm_dev_register, but that
feature got canned. So looks all good to me, and no races anywhere to be
seen. Assuming you agree and update the commit message:

Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>

> 
> Fixes: c76f0f7cb546 ("drm: Begin an API for in-kernel clients")
> Cc: Maxime Ripard <maxime.ripard at bootlin.com>
> Cc: Icenowy Zheng <icenowy at aosc.io>
> Cc: Chen-Yu Tsai <wens at csie.org>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> Signed-off-by: Noralf Trønnes <noralf at tronnes.org>
> ---
>  drivers/gpu/drm/drm_client.c | 11 +----------
>  1 file changed, 1 insertion(+), 10 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_client.c b/drivers/gpu/drm/drm_client.c
> index 4039a4d103a8..9b142f58d489 100644
> --- a/drivers/gpu/drm/drm_client.c
> +++ b/drivers/gpu/drm/drm_client.c
> @@ -78,7 +78,6 @@ EXPORT_SYMBOL(drm_client_close);
>  int drm_client_new(struct drm_device *dev, struct drm_client_dev *client,
>  		   const char *name, const struct drm_client_funcs *funcs)
>  {
> -	bool registered;
>  	int ret;
>  
>  	if (!drm_core_check_feature(dev, DRIVER_MODESET) ||
> @@ -97,21 +96,13 @@ int drm_client_new(struct drm_device *dev, struct drm_client_dev *client,
>  		goto err_put_module;
>  
>  	mutex_lock(&dev->clientlist_mutex);
> -	registered = dev->registered;
> -	if (registered)
> -		list_add(&client->list, &dev->clientlist);
> +	list_add(&client->list, &dev->clientlist);
>  	mutex_unlock(&dev->clientlist_mutex);
> -	if (!registered) {
> -		ret = -ENODEV;
> -		goto err_close;
> -	}
>  
>  	drm_dev_get(dev);
>  
>  	return 0;
>  
> -err_close:
> -	drm_client_close(client);
>  err_put_module:
>  	if (funcs)
>  		module_put(funcs->owner);
> -- 
> 2.15.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch