From 283862516e031a8f5e3dd59390e7df7391041b7b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Thomas=20Hellstr=C3=B6m?= <thomas.hellstrom@linux.intel.com>
Date: Thu, 17 Jun 2021 08:30:07 +0200
Subject: drm/i915: Reference objects on the ww object list
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Git-commit: 1c4dbe056dab0b7c2a2f42f4d393cc7b9bdb98ad
Patch-mainline: v5.15-rc1
References: jsc#PED-1166 jsc#PED-1168 jsc#PED-1170 jsc#PED-1218 jsc#PED-1220 jsc#PED-1222 jsc#PED-1223 jsc#PED-1225

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
Acked-by: Patrik Jakobsson <pjakobsson@suse.de>
---
 drivers/gpu/drm/i915/gem/i915_gem_object.h | 8 ++++++--
 drivers/gpu/drm/i915/i915_gem.c            | 4 ++++
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_object.h b/drivers/gpu/drm/i915/gem/i915_gem_object.h
index d66aa00d023a..241666931945 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_object.h
+++ b/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;
 }
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 07aa80773a02..0a815e15ffc7 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1216,6 +1216,7 @@ 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);
 	}
 }
 
@@ -1223,6 +1224,7 @@ 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;
 
-- 
2.38.1