[PATCH net v2] mctp i3c: fix MCTP I3C driver multi-thread issue

[Leo Yang <leo.yang.sy0@gmail.com>]

We found a timeout problem with the pldm command on our system.  The
reason is that the MCTP-I3C driver has a race condition when receiving
multiple-packet messages in multi-thread, resulting in a wrong packet
order problem.

We identified this problem by adding a debug message to the
mctp_i3c_read function.

According to the MCTP spec, a multiple-packet message must be composed
in sequence, and if there is a wrong sequence, the whole message will be
discarded and wait for the next SOM.
For example, SOM → Pkt Seq #2 → Pkt Seq #1 → Pkt Seq #3 → EOM.

Therefore, we try to solve this problem by adding a mutex to the
mctp_i3c_read function.  Before the modification, when a command
requesting a multiple-packet message response is sent consecutively, an
error usually occurs within 100 loops.  After the mutex, it can go
through 40000 loops without any error, and it seems to run well.

But I'm a little worried about the performance of mutex in high load
situation (as spec seems to allow different endpoints to respond at the
same time), do you think this is a feasible solution?

Fixes: c8755b29b58e ("mctp i3c: MCTP I3C driver")
Signed-off-by: Leo Yang <Leo-Yang@quantatw.com>

---
Change in v2:
    1. Add Fixes tag.
	2. Add mutex comment.
    - Link to v1: https://lore.kernel.org/netdev/20241226025319.1724209-1-Leo-Yang@quantatw.com/
---

 drivers/net/mctp/mctp-i3c.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/net/mctp/mctp-i3c.c b/drivers/net/mctp/mctp-i3c.c
index 9adad59b8676..d247fe483c58 100644
--- a/drivers/net/mctp/mctp-i3c.c
+++ b/drivers/net/mctp/mctp-i3c.c
@@ -125,6 +125,8 @@ static int mctp_i3c_read(struct mctp_i3c_device *mi)
 
 	xfer.data.in = skb_put(skb, mi->mrl);
 
+	/* Make sure netif_rx() is read in the same order as i3c. */
+	mutex_lock(&mi->lock);
 	rc = i3c_device_do_priv_xfers(mi->i3c, &xfer, 1);
 	if (rc < 0)
 		goto err;
@@ -166,8 +168,10 @@ static int mctp_i3c_read(struct mctp_i3c_device *mi)
 		stats->rx_dropped++;
 	}
 
+	mutex_unlock(&mi->lock);
 	return 0;
 err:
+	mutex_unlock(&mi->lock);
 	kfree_skb(skb);
 	return rc;
 }
-- 
2.39.2