Re: [PATCH] rpmsg: mtk_rpmsg: Fix circular locking dependency

AngeloGioacchino Del Regno
2022-02-16T16:06:35Z

Il 14/01/22 15:47, AngeloGioacchino Del Regno ha scritto:
> During execution of the worker that's used to register rpmsg devices
> we are safely locking the channels mutex but, when creating a new
> endpoint for such devices, we are registering a IPI on the SCP, which
> then makes the SCP to trigger an interrupt, lock its own mutex and in
> turn register more subdevices.
> This creates a circular locking dependency situation, as the mtk_rpmsg
> channels_lock will then depend on the SCP IPI lock.
>
> [ 18.014514] Possible unsafe locking scenario:
> [ 18.014515] CPU0 CPU1
> [ 18.014517] ---- ----
> [ 18.045467] lock(&mtk_subdev->channels_lock);
> [ 18.045474] lock(&scp->ipi_desc[i].lock);
> [ 18.228399] lock(&mtk_subdev->channels_lock);
> [ 18.228405] lock(&scp->ipi_desc[i].lock);
> [ 18.264405]
>
> To solve this, simply unlock the channels_lock mutex before calling
> mtk_rpmsg_register_device() and relock it right after, as safety is
> still ensured by the locking mechanism that happens right after
> through SCP.
> Notably, mtk_rpmsg_register_device() does not even require locking.
>
> Fixes: 7017996951fd ("rpmsg: add rpmsg support for mt8183 SCP.")
> Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>

Friendly ping for an important fix... :)

> ---
> drivers/rpmsg/mtk_rpmsg.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/drivers/rpmsg/mtk_rpmsg.c b/drivers/rpmsg/mtk_rpmsg.c
> index 5b4404b8be4c..d1213c33da20 100644
> --- a/drivers/rpmsg/mtk_rpmsg.c
> +++ b/drivers/rpmsg/mtk_rpmsg.c
> @@ -234,7 +234,9 @@ static void mtk_register_device_work_function(struct work_struct *register_work)
> if (info->registered)
> continue;
>
> + mutex_unlock(&subdev->channels_lock);
> ret = mtk_rpmsg_register_device(subdev, &info->info);
> + mutex_lock(&subdev->channels_lock);
> if (ret) {
> dev_err(&pdev->dev, "Can't create rpmsg_device\n");
> continue;