[PATCH 2/2] drm/vmwgfx: Fix false lockdep warning

[Daniel Vetter]

On Fri, Nov 15, 2013 at 12:24:32AM -0800, Thomas Hellstrom wrote:
> A lockdep warning is hit when evicting surfaces and reserving the backup
> buffer. Since this buffer can only be reserved by the process holding the
> surface reservation or by the buffer eviction processes that use tryreserve,
> there is no real deadlock here, but there's no other way to silence lockdep
> than to use a tryreserve. This means the reservation might fail if the buffer
> is about to be evicted or swapped out, but we now have code in place to
> handle that reasonably well.
> 
> Signed-off-by: Thomas Hellstrom <thellstrom at vmware.com>

Hm, for similar cases where there's an additional hirarchy imposed onto
the locking order lockdep supports subclases. Block devices use that to
nest partitions within the overall block device.

Have you looked into wiring this up for ww_mutexes, i.e. fix lockdep
instaed of working around it in the code? I'm thinking of a
ww_mutex_lock_nested similar to mutex_lock_nested.

Cheers, Daniel
> ---
>  drivers/gpu/drm/vmwgfx/vmwgfx_resource.c |   16 +++++++---------
>  1 file changed, 7 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_resource.c b/drivers/gpu/drm/vmwgfx/vmwgfx_resource.c
> index 0e67cf4..4ea0be2 100644
> --- a/drivers/gpu/drm/vmwgfx/vmwgfx_resource.c
> +++ b/drivers/gpu/drm/vmwgfx/vmwgfx_resource.c
> @@ -992,7 +992,6 @@ void vmw_resource_unreserve(struct vmw_resource *res,
>   */
>  static int
>  vmw_resource_check_buffer(struct vmw_resource *res,
> -			  struct ww_acquire_ctx *ticket,
>  			  bool interruptible,
>  			  struct ttm_validate_buffer *val_buf)
>  {
> @@ -1009,7 +1008,7 @@ vmw_resource_check_buffer(struct vmw_resource *res,
>  	INIT_LIST_HEAD(&val_list);
>  	val_buf->bo = ttm_bo_reference(&res->backup->base);
>  	list_add_tail(&val_buf->head, &val_list);
> -	ret = ttm_eu_reserve_buffers(ticket, &val_list);
> +	ret = ttm_eu_reserve_buffers(NULL, &val_list);
>  	if (unlikely(ret != 0))
>  		goto out_no_reserve;
>  
> @@ -1027,12 +1026,13 @@ vmw_resource_check_buffer(struct vmw_resource *res,
>  	return 0;
>  
>  out_no_validate:
> -	ttm_eu_backoff_reservation(ticket, &val_list);
> +	ttm_eu_backoff_reservation(NULL, &val_list);
>  out_no_reserve:
>  	ttm_bo_unref(&val_buf->bo);
>  	if (backup_dirty)
>  		vmw_dmabuf_unreference(&res->backup);
>  
> +	DRM_INFO("Check buffer ret %d\n", ret);
>  	return ret;
>  }
>  
> @@ -1072,8 +1072,7 @@ int vmw_resource_reserve(struct vmw_resource *res, bool no_backup)
>   * @val_buf:        Backup buffer information.
>   */
>  static void
> -vmw_resource_backoff_reservation(struct ww_acquire_ctx *ticket,
> -				 struct ttm_validate_buffer *val_buf)
> +vmw_resource_backoff_reservation(struct ttm_validate_buffer *val_buf)
>  {
>  	struct list_head val_list;
>  
> @@ -1082,7 +1081,7 @@ vmw_resource_backoff_reservation(struct ww_acquire_ctx *ticket,
>  
>  	INIT_LIST_HEAD(&val_list);
>  	list_add_tail(&val_buf->head, &val_list);
> -	ttm_eu_backoff_reservation(ticket, &val_list);
> +	ttm_eu_backoff_reservation(NULL, &val_list);
>  	ttm_bo_unref(&val_buf->bo);
>  }
>  
> @@ -1096,13 +1095,12 @@ int vmw_resource_do_evict(struct vmw_resource *res)
>  {
>  	struct ttm_validate_buffer val_buf;
>  	const struct vmw_res_func *func = res->func;
> -	struct ww_acquire_ctx ticket;
>  	int ret;
>  
>  	BUG_ON(!func->may_evict);
>  
>  	val_buf.bo = NULL;
> -	ret = vmw_resource_check_buffer(res, &ticket, true, &val_buf);
> +	ret = vmw_resource_check_buffer(res, true, &val_buf);
>  	if (unlikely(ret != 0))
>  		return ret;
>  
> @@ -1117,7 +1115,7 @@ int vmw_resource_do_evict(struct vmw_resource *res)
>  	res->backup_dirty = true;
>  	res->res_dirty = false;
>  out_no_unbind:
> -	vmw_resource_backoff_reservation(&ticket, &val_buf);
> +	vmw_resource_backoff_reservation(&val_buf);
>  
>  	return ret;
>  }
> -- 
> 1.7.10.4
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch