[PATCH 10/15] drm/modes: reduce fb_lock to just protecting lists

[Daniel Vetter]

On Fri, Apr 15, 2016 at 03:10:41PM +1000, Dave Airlie wrote:
> From: Dave Airlie <airlied at redhat.com>
> 
> This reduces the fb_lock to just protecting the num_fb/fb_list.
> 
> I'd like to have some discussion on if this opens up any race
> conditions.

Here's you're discussion ;-)

"Previously fb refcounting, and especially the weak reference
(kref_get_unless_zero) used in fb lookups have been protected by fb_lock.
But with the refactoring to share refcounting in the drm_mode_object base
class that switched to being protected by idr_mutex, which means fb_lock
critical sections can be reduced."

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

> 
> Signed-off-by: Dave Airlie <airlied at redhat.com>
> ---
>  drivers/gpu/drm/drm_crtc.c | 9 +--------
>  1 file changed, 1 insertion(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
> index e47c4a2..46f32f2 100644
> --- a/drivers/gpu/drm/drm_crtc.c
> +++ b/drivers/gpu/drm/drm_crtc.c
> @@ -433,9 +433,7 @@ static void drm_framebuffer_free(struct kref *kref)
>  	 * The lookup idr holds a weak reference, which has not necessarily been
>  	 * removed at this point. Check for that.
>  	 */
> -	mutex_lock(&dev->mode_config.fb_lock);
>  	drm_mode_object_unregister(dev, &fb->base);
> -	mutex_unlock(&dev->mode_config.fb_lock);
>  
>  	fb->funcs->destroy(fb);
>  }
> @@ -475,9 +473,9 @@ int drm_framebuffer_init(struct drm_device *dev, struct drm_framebuffer *fb,
>  	mutex_lock(&dev->mode_config.fb_lock);
>  	dev->mode_config.num_fb++;
>  	list_add(&fb->head, &dev->mode_config.fb_list);
> +	mutex_unlock(&dev->mode_config.fb_lock);
>  
>  	drm_mode_object_register(dev, &fb->base);
> -	mutex_unlock(&dev->mode_config.fb_lock);
>  out:
>  	return ret;
>  }
> @@ -498,12 +496,9 @@ struct drm_framebuffer *drm_framebuffer_lookup(struct drm_device *dev,
>  	struct drm_mode_object *obj;
>  	struct drm_framebuffer *fb = NULL;
>  
> -	mutex_lock(&dev->mode_config.fb_lock);
>  	obj = _object_find(dev, id, DRM_MODE_OBJECT_FB);
>  	if (obj)
>  		fb = obj_to_fb(obj);
> -	mutex_unlock(&dev->mode_config.fb_lock);
> -
>  	return fb;
>  }
>  EXPORT_SYMBOL(drm_framebuffer_lookup);
> @@ -526,10 +521,8 @@ void drm_framebuffer_unregister_private(struct drm_framebuffer *fb)
>  
>  	dev = fb->dev;
>  
> -	mutex_lock(&dev->mode_config.fb_lock);
>  	/* Mark fb as reaped and drop idr ref. */
>  	drm_mode_object_unregister(dev, &fb->base);
> -	mutex_unlock(&dev->mode_config.fb_lock);
>  }
>  EXPORT_SYMBOL(drm_framebuffer_unregister_private);
>  
> -- 
> 2.5.5
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch