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

AngeloGioacchino Del Regno
2022-04-29T13:38:05Z

Il 28/04/22 19:31, Mathieu Poirier ha scritto:
> On Fri, Jan 14, 2022 at 03:47:37PM +0100, AngeloGioacchino Del Regno wrote:
>> 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]
>
> I finally understand the problem, something that would have been impossible
> without the pastebin you provided in your latest email. Please add the content
> of that pastebin to the changelog and send another revision (checkpatch
> warnings can be ignored).
>

Thanks for giving it another look... I'll add a cover letter with the contents
of the pastebin to avoid possible confusion for anyone looking at the patch.

>>
>> 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.
>
> The integrity of the subdev->channels list is guaranteed by relocking the
> mutex, I'm not sure what "through SCP" adds to the sentence.

I'll clarify the commit message.

>
>> Notably, mtk_rpmsg_register_device() does not even require locking.
>>
>
> I don't agree with the above sentence - if locking doesn't happen in
> mtk_rpmsg_create_device(), there can be two CPUs accessing the list at the same
> time.
>

That's right, I have largely underestimated that for some reason, sorry about that.

Regards,
Angelo

> Thanks,
> Mathieu
>