drm/i915: Reference objects on the ww object list

Since the ww transaction endpoint easily end up far out-of-scope of
the objects on the ww object list, particularly for contending lock
objects, make sure we reference objects on the list so they don't
disappear under us.

This comes with a performance penalty so it's been debated whether this
is really needed. But I think this is motivated by the fact that locking
is typically difficult to get right, and whatever we can do to make it
simpler for developers moving forward should be done, unless the
performance impact is far too high.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210617063018.92802-2-thomas.hellstrom@linux.intel.com

drivers/gpu/drm/i915/gem/i915_gem_object.h

@@ -169,13 +169,17 @@ static inline int __i915_gem_object_lock(struct drm_i915_gem_object *obj,
 	else
 		ret = dma_resv_lock(obj->base.resv, ww ? &ww->ctx : NULL);
 
-	if (!ret && ww)
+	if (!ret && ww) {
+		i915_gem_object_get(obj);
 		list_add_tail(&obj->obj_link, &ww->obj_list);
+	}
 	if (ret == -EALREADY)
 		ret = 0;
 
-	if (ret == -EDEADLK)
+	if (ret == -EDEADLK) {
+		i915_gem_object_get(obj);
 		ww->contended = obj;
+	}
 
 	return ret;
 }

drivers/gpu/drm/i915/i915_gem.c

@@ -1216,13 +1216,15 @@ static void i915_gem_ww_ctx_unlock_all(struct i915_gem_ww_ctx *ww)
 	while ((obj = list_first_entry_or_null(&ww->obj_list, struct drm_i915_gem_object, obj_link))) {
 		list_del(&obj->obj_link);
 		i915_gem_object_unlock(obj);
+		i915_gem_object_put(obj);
 	}
 }
 
 void i915_gem_ww_unlock_single(struct drm_i915_gem_object *obj)
 {
 	list_del(&obj->obj_link);
 	i915_gem_object_unlock(obj);
+	i915_gem_object_put(obj);
 }
 
 void i915_gem_ww_ctx_fini(struct i915_gem_ww_ctx *ww)
@@ -1247,6 +1249,8 @@ int __must_check i915_gem_ww_ctx_backoff(struct i915_gem_ww_ctx *ww)
 
 	if (!ret)
 		list_add_tail(&ww->contended->obj_link, &ww->obj_list);
+	else
+		i915_gem_object_put(ww->contended);
 
 	ww->contended = NULL;