Re: [PATCH v3 8/9] kernel: export task_work_add

[Daniel Vetter]

On Wed, Aug 18, 2021 at 03:38:23PM +0800, Desmond Cheong Zhi Xi wrote:
> The task_work_add function is needed to prevent userspace races with
> DRM modesetting rights.
> 
> Some DRM ioctls can change modesetting permissions while other
> concurrent users are performing modesetting. To prevent races with
> userspace, such functions should flush readers of old permissions
> before returning to user mode. As the function that changes
> permissions might itself be a reader of the old permissions, we intend
> to schedule this flush using task_work_add.
> 
> However, when DRM is compiled as a loadable kernel module without
> exporting task_work_add, we get the following compilation error:
> 
> ERROR: modpost: "task_work_add" [drivers/gpu/drm/drm.ko] undefined!
> 
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>

Just realized another benefit of pushing the dev->master_rwsem write
locks down into ioctls that need them: We wouldn't need this function here
exported for use in drm. But also I'm not sure that works any better than
the design in your current patch set ...
-Daniel

> ---
>  kernel/task_work.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/task_work.c b/kernel/task_work.c
> index 1698fbe6f0e1..90000404af2b 100644
> --- a/kernel/task_work.c
> +++ b/kernel/task_work.c
> @@ -60,6 +60,7 @@ int task_work_add(struct task_struct *task, struct callback_head *work,
>  
>  	return 0;
>  }
> +EXPORT_SYMBOL(task_work_add);
>  
>  /**
>   * task_work_cancel_match - cancel a pending work added by task_work_add()
> -- 
> 2.25.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch