drm/ttm: Reorder sys manager cleanup step

With the current cleanup flow, we could trigger a NULL pointer
dereference if there is a delayed destruction of a BO with a
system resource that gets executed on drain_workqueue() call,
as we attempt to free a resource using an already released
resource manager.

Remove the device from the device list and drain its workqueue
before releasing the system domain manager in ttm_device_fini().
Signed-off-by: Karolina Stolarek <karolina.stolarek@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231016121525.2237838-1-karolina.stolarek@intel.comSigned-off-by: Christian König <christian.koenig@amd.com>

drivers/gpu/drm/ttm/ttm_device.c

@@ -232,10 +232,6 @@ void ttm_device_fini(struct ttm_device *bdev)
 	struct ttm_resource_manager *man;
 	unsigned i;
 
-	man = ttm_manager_type(bdev, TTM_PL_SYSTEM);
-	ttm_resource_manager_set_used(man, false);
-	ttm_set_driver_manager(bdev, TTM_PL_SYSTEM, NULL);
-
 	mutex_lock(&ttm_global_mutex);
 	list_del(&bdev->device_list);
 	mutex_unlock(&ttm_global_mutex);
@@ -243,6 +239,10 @@ void ttm_device_fini(struct ttm_device *bdev)
 	drain_workqueue(bdev->wq);
 	destroy_workqueue(bdev->wq);
 
+	man = ttm_manager_type(bdev, TTM_PL_SYSTEM);
+	ttm_resource_manager_set_used(man, false);
+	ttm_set_driver_manager(bdev, TTM_PL_SYSTEM, NULL);
+
 	spin_lock(&bdev->lru_lock);
 	for (i = 0; i < TTM_MAX_BO_PRIORITY; ++i)
 		if (list_empty(&man->lru[0]))