drm/omap: fix race condition with dev->obj_list

omap_gem_objects are added to dev->obj_list in omap_gem_new, and removed
in omap_gem_free_object. Unfortunately there's no locking for
dev->obj_list, which eventually leads to a crash:

WARNING: CPU: 1 PID: 1123 at lib/list_debug.c:59 __list_del_entry+0xa4/0xe0()
list_del corruption. prev->next should be e9281344, but was ea722b84

Add a spinlock to protect dev->obj_list.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

drivers/gpu/drm/omapdrm/omap_drv.c

@@ -491,6 +491,7 @@ static int dev_load(struct drm_device *dev, unsigned long flags)
 
 	priv->wq = alloc_ordered_workqueue("omapdrm", 0);
 
+	spin_lock_init(&priv->list_lock);
 	INIT_LIST_HEAD(&priv->obj_list);
 
 	omap_gem_init(dev);

drivers/gpu/drm/omapdrm/omap_drv.h

@@ -105,6 +105,9 @@ struct omap_drm_private {
 
 	struct workqueue_struct *wq;
 
+	/* lock for obj_list below */
+	spinlock_t list_lock;
+
 	/* list of GEM objects: */
 	struct list_head obj_list;
 

drivers/gpu/drm/omapdrm/omap_gem.c

@@ -1273,13 +1273,16 @@ int omap_gem_set_sync_object(struct drm_gem_object *obj, void *syncobj)
 void omap_gem_free_object(struct drm_gem_object *obj)
 {
 	struct drm_device *dev = obj->dev;
+	struct omap_drm_private *priv = dev->dev_private;
 	struct omap_gem_object *omap_obj = to_omap_bo(obj);
 
 	evict(obj);
 
 	WARN_ON(!mutex_is_locked(&dev->struct_mutex));
 
+	spin_lock(&priv->list_lock);
 	list_del(&omap_obj->mm_list);
+	spin_unlock(&priv->list_lock);
 
 	drm_gem_free_mmap_offset(obj);
 
@@ -1377,7 +1380,9 @@ struct drm_gem_object *omap_gem_new(struct drm_device *dev,
 	if (!omap_obj)
 		goto fail;
 
+	spin_lock(&priv->list_lock);
 	list_add(&omap_obj->mm_list, &priv->obj_list);
+	spin_unlock(&priv->list_lock);
 
 	obj = &omap_obj->base;