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Date:   Wed, 18 Aug 2021 13:06:36 +0200
From:   Daniel Vetter <daniel@ffwll.ch>
To:     Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Cc:     maarten.lankhorst@linux.intel.com, mripard@kernel.org,
        tzimmermann@suse.de, airlied@linux.ie, daniel@ffwll.ch,
        sumit.semwal@linaro.org, christian.koenig@amd.com, axboe@kernel.dk,
        oleg@redhat.com, tglx@linutronix.de, dvyukov@google.com,
        walter-zh.wu@mediatek.com, dri-devel@lists.freedesktop.org,
        linux-kernel@vger.kernel.org, intel-gfx@lists.freedesktop.org,
        linux-media@vger.kernel.org, linaro-mm-sig@lists.linaro.org,
        skhan@linuxfoundation.org, gregkh@linuxfoundation.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        kernel test robot <lkp@intel.com>
Subject: Re: [PATCH v3 8/9] kernel: export task_work_add
Message-ID: <YRzpvHBQuq0kFtzH@phenom.ffwll.local>
Mail-Followup-To: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>,
        maarten.lankhorst@linux.intel.com, mripard@kernel.org,
        tzimmermann@suse.de, airlied@linux.ie, sumit.semwal@linaro.org,
        christian.koenig@amd.com, axboe@kernel.dk, oleg@redhat.com,
        tglx@linutronix.de, dvyukov@google.com, walter-zh.wu@mediatek.com,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        intel-gfx@lists.freedesktop.org, linux-media@vger.kernel.org,
        linaro-mm-sig@lists.linaro.org, skhan@linuxfoundation.org,
        gregkh@linuxfoundation.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        kernel test robot <lkp@intel.com>
References: <20210818073824.1560124-1-desmondcheongzx@gmail.com>
 <20210818073824.1560124-9-desmondcheongzx@gmail.com>
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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@intel.com>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>

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