Re: [PATCH 19/51] drm: Cleanups after drmm_add_final_kfree rollout

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

Hi Daniel,

(On a side note, git-format-patch accepts a -v argument to specify the
version, I didn't realize you were not aware of it :-))

On Mon, Mar 23, 2020 at 03:49:18PM +0100, Daniel Vetter wrote:
> A few things:
> - Update the example driver in the documentation.
> - We can drop the old kfree in drm_dev_release.
> - Add a WARN_ON check in drm_dev_register to make sure everyone calls
>   drmm_add_final_kfree and there's no leaks.
> 
> v2: Restore the full cleanup, I accidentally left some moved code
> behind when fixing the bisectability of the series.
> 
> Acked-by: Sam Ravnborg <sam@ravnborg.org>
> Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
> Cc: Sam Ravnborg <sam@ravnborg.org>
> Cc: Dan Carpenter <dan.carpenter@oracle.com>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> ---
>  drivers/gpu/drm/drm_drv.c | 12 +++++-------
>  1 file changed, 5 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_drv.c b/drivers/gpu/drm/drm_drv.c
> index 877ded348b6e..7f9d7ea543a0 100644
> --- a/drivers/gpu/drm/drm_drv.c
> +++ b/drivers/gpu/drm/drm_drv.c
> @@ -297,8 +297,6 @@ void drm_minor_release(struct drm_minor *minor)
>   *
>   *		drm_mode_config_cleanup(drm);
>   *		drm_dev_fini(drm);
> - *		kfree(priv->userspace_facing);
> - *		kfree(priv);
>   *	}
>   *
>   *	static struct drm_driver driver_drm_driver = {
> @@ -326,10 +324,11 @@ void drm_minor_release(struct drm_minor *minor)
>   *			kfree(drm);
>   *			return ret;
>   *		}
> + *		drmm_add_final_kfree(drm, priv);
>   *
>   *		drm_mode_config_init(drm);
>   *
> - *		priv->userspace_facing = kzalloc(..., GFP_KERNEL);
> + *		priv->userspace_facing = drmm_kzalloc(..., GFP_KERNEL);
>   *		if (!priv->userspace_facing)
>   *			return -ENOMEM;
>   *
> @@ -837,10 +836,7 @@ static void drm_dev_release(struct kref *ref)
>  
>  	drm_managed_release(dev);
>  
> -	if (!dev->driver->release && !dev->managed.final_kfree) {
> -		WARN_ON(!list_empty(&dev->managed.resources));
> -		kfree(dev);
> -	} else if (dev->managed.final_kfree)
> +	if (dev->managed.final_kfree)
>  		kfree(dev->managed.final_kfree);
>  }
>  
> @@ -961,6 +957,8 @@ int drm_dev_register(struct drm_device *dev, unsigned long flags)
>  	if (!driver->load)
>  		drm_mode_config_validate(dev);
>  
> +	WARN_ON(!dev->managed.final_kfree);

That's too aggressive. Driver freeing their private object in .release()
isn't wrong. I'd even go as far as saying that it should be the norm,
until we manage to find a better way to handle that (which this series
doesn't achieve). Many drivers need to allocate resources at probe time
before they get a chance to init the drm device. Those resources must be
released in the error handling paths of probe. By requiring
drmm_add_final_kfree(), you're making that much more complex. I can't
release those resources in the error path anymore after calling
drmm_add_final_kfree(), or they will be released twice. And I can't rely
on drmm_* to release them in all cases, as the error path may be hit
before touching anything drm-related.

Until we figure out a good way forward and test it on a significant
number of drivers, let's not add WARN_ON() that will be hit with the
majority of drivers, forcing them to be converted to something that is
clearly half-baked.

> +
>  	if (drm_dev_needs_global_mutex(dev))
>  		mutex_lock(&drm_global_mutex);
>  

-- 
Regards,

Laurent Pinchart
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