Re: [PATCH v3] firmware: arm_scmi: Free mailbox channels if probe fails

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apologies for the delay in coming back to you.
A few comments below.

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thanks for looking into this kind of bad interactions.

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There seems to be another issue here. The response from agent can be delayed
causing a timeout during base protocol acquire,
which leads to the probe failure. What I have observed is sometimes the
failure of probe and rx_callback (due to a delayed message)
happens at the same time on different cpus.
Because of this race, the device memory may be cleared while the
interrupt(rx_callback) is executing on another cpu.

You are right that concurrency was not handled properly in this kind
of
context and moreover, if you think about it, even the case of out of
order reception of responses and delayed_responses (type2 SCMI
messages)
for asynchronous SCMI commands was not handled properly.

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How do you propose we solve this? Do you think it is better to take the
setting up of base and other protocols out of probe and
in some delayed work? That would imply the device memory is not released
until remove is called. Or should we add locking to
the interrupt handler(scmi_rx_callback) and the cleanup in probe to avoid
the race?

These issues were more easily exposed by SCMI Virtio transport, so in
the series where I introduced scmi-virtio:

https://lore.kernel.org/linux-arm-kernel/162848483974.232214.9506203742448269364.b4-ty@arm.com/

(which is now queued for v5.15 ...  now on -next I think...finger
crossed)

I took the chance to rectify a couple of other things in the SCMI core
in the initial commits.
As an example, in the above series

 [PATCH v7 05/15] firmware: arm_scmi: Handle concurrent and
out-of-order messages

cares to add a refcount to xfers and some locking on xfers between TX
and RX path to avoid that a timed out xfer can vanish while the rx
path
is concurrently working on it (as you said); moreover I handle the
condition (rare if not unplausible anyway) in which a transport
delivers
out of order responses and delayed responses.

I tested this scenarios on some fake emulated SCMI Virtio transport
where I could play any sort of mess and tricks to stress this limit
conditions, but you're more than welcome to verify if the race you are
seeing on Base protocol time out is solved (as I would hope :D) by
this
series of mine.

Let me know, any feedback is welcome.

Btw, in the series above there are also other minor changes, but there
is also another more radical change needed to ensure correctness and
protection against stale old messages which maybe could interest you
in general if you are looking into SCMI:

[PATCH v7 04/15] firmware: arm_scmi: Introduce monotonically
increasing tokens

Let me know if yo have other concerns.

Hi Rishabhb,

just a quick remark, thinking again about your fail @probe scenario
above
I realized that while the concurrency patch I mentioned above could help
on
races against vanishing xfers when late timed-out responses are
delivered,
here we really are then also shutting down everything on failure, so
there
could be further issues between a very late invokation of
scmi_rx_callback
and the core devm_ helpers freeing the underlying xfer/cinfo/etc..
structs
used by scmi-rx-callback itself (maybe this was already what you meant
and
I didn't get it,...sorry)

On the other side, I don't feel that delaying Base init to a deferred
worker is a viable solution since we need Base protocol init to be
initialized and we need to just give up if we cannot communicate with
the SCMI platform fw in such early stages. (Base protocol is really the
only mandatory proto is I remember correctly the spec)

Currenly I'm off and only glancing at mails but I'll have a thought
about
these issues once back in a few weeks time.

Thanks,
Cristian

Hi Cristian
I hope you enjoyed your vacation. Did you get a chance to look at
the issue
stated above and have some idea as to how to solve this?

Do you still see the issue with v5.15 ? Can you please check if haven't
already done that ?

Also 30ms delay we have is huge IMO and we typically expect the
communication
with remote processor or any entity that implements SCMI to happen in
terms
of one or few ms tops.

If there is a race, we need to fix that but I am interested in knowing
why the default time of 30ms not sufficient ? Did increasing that helps
and is this timeout happening only for the initial commands(guessing the
SCMI firmware is not yet ready) or does it happen even during run-time ?

Hi Sudeep
I haven't checked on 5.15 but after glancing at the code I believe
we should
see the same issue.
I agree 30ms is a big enough value and should be something that remote
firmware should resolve. But
if remote firmware goes into a bad state and not functioning
properly at
least kernel should not panic.

The issue we see here happens during scmi probe. The response from the
remote agent can be delayed
causing a timeout during base protocol acquire, which leads to the
probe
failure.
What I have observed is sometimes the failure of probe and
rx_callback (due
to a delayed message)
happens around the same time on different cpus. Because of this
race, the
device memory may be cleared
while the interrupt(rx_callback) is executing on another cpu.

So I was looking at the failure path you mentioned: a late concurrent
reply on Base protocol from the fw, during the probe, leads to an
invocation
of scmi_rx_callback() on a different CPU while core data structs like
cinfo are being freed by the SCMI core on the probe failure path.
(v5.15-added SCMI concurrrency handling stuff I mentiond shuld help for
races regarding xfer but not for the cinfo stuff in this case ...)

We cannot defer Base proto init since we just wanna fail early while
probing if not even the Base protocol can work fine, and also because
Base protocol information are indeed needed for initial setup, so we
cannot juts proceed if we did not even got a Base reply on the number of
protos. (already said)

In my opinion, the proper way to address this kind of races at probe
failure should be to ensure that the transport you are using is properly
shut down completely before cleanup starts (same applies for a clean
remove), i.e. scmi_rx_callback should not even be possibly registered to
be called when the the final cleanup by the core is started (devm_ frees
I mean after scmi_probe exit failing...)

BUT indeed looking back at transport layers like mailbox and virtio,
this
should be happening already, because the flow is like

scmi_probe()
{
...

clean_tx_rx_setup:
	scmi_cleanup_txrx_channels()
		....
		--->>>  ret = idr_for_each(idr, info->desc->ops->chan_free, idr);
			-
	return ret;
}

.... only after this scmi_probe returns the core devm layer starts
freeing devm_
allocated stuff like cinfo, AND the above per-transport specific
.chan_free seems
to take care to 'deactivate/dregister' the scmi_rx_callback at the
transport layer:


e.g. MBOX transport
-------------------------
static int mailbox_chan_free(int id, void *p, void *data)
{
	struct scmi_chan_info *cinfo = p;
	struct scmi_mailbox *smbox = cinfo->transport_info;

	if (smbox && !IS_ERR(smbox->chan)) {
		mbox_free_channel(smbox->chan);    <<< THIS MBOX CORE CALL DEACTIVATE
		cinfo->transport_info = NULL;


e.g. VIRTIO Transport
-----------------------------
static int virtio_chan_free(int id, void *p, void *data)
{
	unsigned long flags;
	struct scmi_chan_info *cinfo = p;
	struct scmi_vio_channel *vioch = cinfo->transport_info;

	spin_lock_irqsave(&vioch->ready_lock, flags);
	vioch->ready = false;                     <<<< THIS VIRTIO FLAG
DEACTIVATE VIRTIO CBS INVOKCATION
	spin_unlock_irqrestore(&vioch->ready_lock, flags);


... AND both of the above call are indeed also spinlocked heavily, so
that
the 'deactivation' of the scmi_rx_callback should be visible properly;
in
other words I would expect that after the above .chan_free() have
completed the scmi_rx_callback() cannot be called anymore, because the
transport itself will properly drop any so-late fw reply.

So I am now wondering, which transport are you using in your tests ?
since at least for the above 2 example it seems to me that your
race-on-probe failure condition should be already addressed by the
transport layer itself....or am I getting wrong the nature of the race ?

Thanks
Cristian

Hi Cristian
You caught the scenario perfectly. But there is still a possibility of a
race. To be clear we use
the mbox transport. Let me explain in more detail.
Lets assume that the last command (base protocol acquire) kernel sent to
remote agent timed out.
This will lead to final cleanup before exiting probe like you mentioned.
Once cleanup is done(mailbox_chan_free)
no more responses from remote agent will acknowledged but if the response
comes in between the cleanup in probe
and the last command timing out we will see a race since the response can
come asynchronously. In this scenario cleanup
and scmi_rx_callback race with each other.
I believe to solve this we need to synchronize cleanup with
scmi_rx_callback. we can serialize these two paths
and exit early in rx_callback if cleanup has been completed.

Yes indeed, but my concern is also not to introduce to much contention
on the RX path (with irqsave spinlocking & friends), given that this racy
scenario has surely to be handled but it is also highly unlikely, so I don't
want to slow down all the rx path all the time.

So I tried something along this lines:

----8<------

diff --git a/drivers/firmware/arm_scmi/common.h b/drivers/firmware/arm_scmi/common.h
index dea1bfbe1052..036f8ccff450 100644
--- a/drivers/firmware/arm_scmi/common.h
+++ b/drivers/firmware/arm_scmi/common.h

@@ -340,11 +340,13 @@ void scmi_protocol_release(const struct scmi_handle *handle, u8 protocol_id);
  *	 channel
  * @handle: Pointer to SCMI entity handle
  * @transport_info: Transport layer related information
+ * @users: A refcount to track active users of this channel
  */
 struct scmi_chan_info {
 	struct device *dev;
 	struct scmi_handle *handle;
 	void *transport_info;
+	refcount_t users;
 };
 
 /**

diff --git a/drivers/firmware/arm_scmi/driver.c b/drivers/firmware/arm_scmi/driver.c
index b406b3f78f46..5814ed3f444e 100644
--- a/drivers/firmware/arm_scmi/driver.c
+++ b/drivers/firmware/arm_scmi/driver.c

@@ -678,6 +678,16 @@ static void scmi_handle_response(struct scmi_chan_info *cinfo,
 	scmi_xfer_command_release(info, xfer);
 }
 
+static inline bool scmi_acquire_channel(struct scmi_chan_info *cinfo)
+{
+	return refcount_inc_not_zero(&cinfo->users);
+}
+
+static inline void scmi_release_channel(struct scmi_chan_info *cinfo)
+{
+	return refcount_dec(&cinfo->users);
+}
+
 /**
  * scmi_rx_callback() - callback for receiving messages
  *

@@ -695,6 +705,10 @@ void scmi_rx_callback(struct scmi_chan_info *cinfo, u32 msg_hdr, void *priv)
 {
 	u8 msg_type = MSG_XTRACT_TYPE(msg_hdr);
 
+	/* Bail out if channel freed already */
+	if (!scmi_acquire_channel(cinfo))
+		return;
+
 	switch (msg_type) {
 	case MSG_TYPE_NOTIFICATION:
 		scmi_handle_notification(cinfo, msg_hdr, priv);

@@ -707,6 +721,8 @@ void scmi_rx_callback(struct scmi_chan_info *cinfo, u32 msg_hdr, void *priv)
 		WARN_ONCE(1, "received unknown msg_type:%d\n", msg_type);
 		break;
 	}
+
+	scmi_release_channel(cinfo);
 }
 
 /**

@@ -1506,10 +1522,27 @@ static int scmi_chan_setup(struct scmi_info *info, struct device *dev,
 		return ret;
 	}
 
+	refcount_set(&cinfo->users, 1);
 	cinfo->handle = &info->handle;
 	return 0;
 }
 
+static int scmi_chan_free(int id, void *p, void *data)
+{
+	struct scmi_chan_info *cinfo = p;
+	struct scmi_info *info = handle_to_scmi_info(cinfo->handle);
+
+	if (refcount_dec_and_test(&cinfo->users)) {
+		info->desc->ops->chan_free(id, cinfo, data);
+	} else {
+		/* Stall till the ongoing rx_callback completes */
+		spin_until_cond(refcount_read(&cinfo->users) == 0);
+		info->desc->ops->chan_free(id, cinfo, data);
+	}
+
+	return 0;
+}
+
 static inline int
 scmi_txrx_setup(struct scmi_info *info, struct device *dev, int prot_id)
 {

@@ -1792,11 +1825,11 @@ static int scmi_cleanup_txrx_channels(struct scmi_info *info)
 	int ret;
 	struct idr *idr = &info->tx_idr;
 
-	ret = idr_for_each(idr, info->desc->ops->chan_free, idr);
+	ret = idr_for_each(idr, scmi_chan_free, idr);
 	idr_destroy(&info->tx_idr);
 
 	idr = &info->rx_idr;
-	ret = idr_for_each(idr, info->desc->ops->chan_free, idr);
+	ret = idr_for_each(idr, scmi_chan_free, idr);
 	idr_destroy(&info->rx_idr);
 
 	return ret;

------8<-----

Can you give it a go on your setup ?

Beware it is not really tested on the racy error path (:P) and I could have
still missed something regarding synchro (and I expect an undesired refcount
warn on the scmi_release_channel too when the race is hit....but just to
experiment a bit for now and see if something like this could be enough while
avoiding further locking)
o