[PATCH] rpmsg: mtk_rpmsg: Fix circular locking dependency

AngeloGioacchino Del Regno
2022-01-14T14:47:53Z

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>
---
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;
--
2.33.1