[PATCH] rpmsg: mtk_rpmsg: Fix circular locking dependency

[Mathieu Poirier]

On Wed, Feb 16, 2022 at 05:06:22PM +0100, AngeloGioacchino Del Regno wrote:
> 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 at collabora.com>
> 
> Friendly ping for an important fix... :)
>

This seems to have slipped through the cracks...  I will take a look later
today.

Thanks,
Mathieu

> > ---
> >   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;
> 
>