dma_resv

struct amdgpu_bo {
	/* Protected by tbo.reserved */
	struct ttm_buffer_object	tbo;
}

struct ttm_buffer_object {
	struct drm_gem_object base;
	...
}

/**
 * struct drm_gem_object - GEM buffer object
 *
 * This structure defines the generic parts for GEM buffer objects, which are
 * mostly around handling mmap and userspace handles.
 *
 * Buffer objects are often abbreviated to BO.
 */
struct drm_gem_object {
	/**
	 * @resv:
	 *
	 * Pointer to reservation object associated with the this GEM object.
	 *
	 * Normally (@resv == &@_resv) except for imported GEM objects.
	 */
	struct dma_resv *resv;

	/**
	 * @_resv:
	 *
	 * A reservation object for this GEM object.
	 *
	 * This is unused for imported GEM objects.
	 */
	struct dma_resv _resv;
}

/**
 * struct dma_resv_list - a list of shared fences
 * @rcu: for internal use
 * @shared_count: table of shared fences
 * @shared_max: for growing shared fence table
 * @shared: shared fence table
 */
struct dma_resv_list {
	struct rcu_head rcu;
	u32 shared_count, shared_max;
	struct dma_fence __rcu *shared[];
};

/**
 * struct dma_resv - a reservation object manages fences for a buffer
 * @lock: update side lock
 * @seq: sequence count for managing RCU read-side synchronization
 * @fence_excl: the exclusive fence, if there is one currently
 * @fence: list of current shared fences
 */
struct dma_resv {
	struct ww_mutex lock;
	seqcount_t seq;

	struct dma_fence __rcu *fence_excl;
	struct dma_resv_list __rcu *fence;
};

• dma_resv的使用场景如下:
  这里使用面向对象的方式建立dma_resv <- drm_gem_object <- ttm_buffer_object <- amdgpu_bo联系.

• 当访问amdgpu_bo的时候:

  • WW-mutex 负责 dma_resv对象本身的线程安全访问，Fence 负责对资源缓冲区amdgpu_bo 的操作时序同步.

  • WW-mutex管能不能进门(是否能操作 dma_resv)，Fence 管 进门后该按什么顺序做事(操作 amdgpu_bo 的时序). 没有 WW-mutex, 多线程修改 dma_resv 会导致数据错乱，交叉请求锁会导致死锁；没有 Fence, 即使拿到了 dma_resv 的锁，GPU/CPU 异步操作也会因为顺序混乱导致数据错误比如 CPU 写一半，GPU 就开始读). 两者联合才实现了 DRM 中 “多线程 / 进程安全、无死锁、时序正确” 的缓冲区并发访问.