Il 17/02/22 20:03, Mathieu Poirier ha scritto:
Hi Angelo,
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);
I spent well over an hour tracing through the meanders of the code to end up in
scp_ipi_register() which, I think, leads to the above. But from there I don't
see how an IPI can come in and that tells me my assumption is wrong.
Can you give more details on the events that lead to the above? I'm not saying
there is no problem, I just need to understand it.
Hi Mathieu,
I understand that following this flow without the assistance of the actual
hardware may be a little confusing, so, no worries.
drivers/remoteproc/mtk_scp.c - this driver manages the SCP (obviously, a
remote processor)
drivers/remoteproc/mtk_scp_ipi.c - public functions for kernel SCP IPC
Flow:
- MediaTek SCP gets probed
- RPMSG starts, we start probing "something", like google,cros-ec-rpmsg
- mtk_rpmsg: creates endpoint; IPI handler is registered here.
( more flow )
- mtk_rpmsg: mtk_rpmsg_ns_cb() -> mtk_rpmsg_create_device(), channel is
added to the channels list, worker gets scheduled
Now for the part that produces the real issue:
label_a:
*** RPMSG MUTEX LOCK ***
- mtk_rpmsg: ## Go through multiple channels ##, call mtk_rpmsg_register_device()
- Registered device tries to communicate through RPMSG
- .send() or .trysend() (depending on the device) is called: send_ipi()
*** SCP MUTEX LOCK ***
- mtk_scp_ipi: Data written, ACK? ok -> return 0
*** SCP MUTEX UNLOCK ***
- mtk_scp_ipi: **** INTERRUPT!!! **** New RPMSG NS available? -> create channel
goto label_a;
*** RPMSG MUTEX UNLOCK ***
Pardon me for keeping some things in this flow implicit, but that was done to
simplify it as much as possible as to try to make you understand the situation.
Cheers,
Angelo
Thanks,
Mathieu
[ 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@xxxxxxxxxxxxx>
---
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
