[Intel-gfx] [PATCH 1/2] drm/i915: fix reference counting in i915_gem_create

[Daniel Vetter]

This function is called without the dev->struct_mutex held, hence we
need to use the _unlocked unreference variants.

As soon as the object is registered userspace can sneak in here with a
gem_close ioctl call, so the object can (and with my new evil tests
actually does) get the final unreference in this place. The lack of
locking then results in hilarity and some good leakage.

To fix this we simply need to revert

Chris Wilson <chris at chris-wilson.co.uk>

v2: We need to make the trace call _before_ we drop our ref - the
object might very well be gone by then already.

v3: Just revert the original patch as suggested by Chris Wilson.

Cc: Chris Wilson <chris at chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
 drivers/gpu/drm/i915/i915_gem.c | 15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 957e65a..dc7e6de 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -219,18 +219,13 @@ i915_gem_create(struct drm_file *file,
 		return -ENOMEM;
 
 	ret = drm_gem_handle_create(file, &obj->base, &handle);
-	if (ret) {
-		drm_gem_object_release(&obj->base);
-		i915_gem_info_remove_obj(dev->dev_private, obj->base.size);
-		i915_gem_object_free(obj);
-		return ret;
-	}
-
 	/* drop reference from allocate - handle holds it now */
-	drm_gem_object_unreference(&obj->base);
-	trace_i915_gem_object_create(obj);
+	drm_gem_object_unreference_unlocked(&obj->base);
+	if (ret)
+		return ret;
 
 	*handle_p = handle;
+
 	return 0;
 }
 
@@ -3935,6 +3930,8 @@ struct drm_i915_gem_object *i915_gem_alloc_object(struct drm_device *dev,
 	} else
 		obj->cache_level = I915_CACHE_NONE;
 
+	trace_i915_gem_object_create(obj);
+
 	return obj;
 }
 
-- 
1.8.1.4