接前一篇文章：KWin、libdrm、DRM从上到下全过程 —— drmModeAddFBxxx（23）
上一回（上两回）讲解完了i915_gem_object_lookup_rcu函数的两个参数struct drm_file *file和u32 handle，本回开始讲解函数体代码。为了便于理解，再次贴出函数源码，在drivers/gpu/drm/i915/gem/i915_gem_object.h中，代码如下：
/**
* i915_gem_object_lookup_rcu - look up a temporary GEM object from its handle
* @filp: DRM file private date
* @handle: userspace handle
*
* Returns:
*
* A pointer to the object named by the handle if such exists on @filp, NULL
* otherwise. This object is only valid whilst under the RCU read lock, and
* note carefully the object may be in the process of being destroyed.
*/
static inline struct drm_i915_gem_object *
i915_gem_object_lookup_rcu(struct drm_file *file, u32 handle)
{
#ifdef CONFIG_LOCK