[PATCH net] eth: fbnic: fix race between concurrent hwmon sensor reads

[Zinc Lim <limzhineng2@gmail.com>]

From: Zinc Lim <limzhineng2@gmail.com>

Reading an hwmon sensor issues a TSENE firmware mailbox transaction that
uses a shared completion slot. Concurrent reads (e.g. parallel
"cat .../temp1_input" or a monitoring agent polling all attributes) race
over that slot, and the second transmit fails because a completion is
already pending:

  fbnic 0000:41:00.0: Failed to transmit TSENE read msg, err -17

Serialize the hwmon read path with a per-device mutex so only one TSENE
transaction is in flight at a time.

Fixes: 880630734102 ("eth: fbnic: Add hardware monitoring support via HWMON interface")
Signed-off-by: Zinc Lim <limzhineng2@gmail.com>
---
 drivers/net/ethernet/meta/fbnic/fbnic.h       |  2 ++
 drivers/net/ethernet/meta/fbnic/fbnic_hwmon.c | 15 +++++++++++++--
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/meta/fbnic/fbnic.h b/drivers/net/ethernet/meta/fbnic/fbnic.h
index d0715695c43e..e31d6f88b746 100644
--- a/drivers/net/ethernet/meta/fbnic/fbnic.h
+++ b/drivers/net/ethernet/meta/fbnic/fbnic.h
@@ -6,6 +6,7 @@
 
 #include <linux/interrupt.h>
 #include <linux/io.h>
+#include <linux/mutex.h>
 #include <linux/ptp_clock_kernel.h>
 #include <linux/types.h>
 #include <linux/workqueue.h>
@@ -27,6 +28,7 @@ struct fbnic_dev {
 	struct net_device *netdev;
 	struct dentry *dbg_fbd;
 	struct device *hwmon;
+	struct mutex hwmon_mutex; /* Serializes hwmon sensor reads */
 	struct devlink_health_reporter *fw_reporter;
 	struct devlink_health_reporter *otp_reporter;
 
diff --git a/drivers/net/ethernet/meta/fbnic/fbnic_hwmon.c b/drivers/net/ethernet/meta/fbnic/fbnic_hwmon.c
index def8598aceec..ac1b4a422677 100644
--- a/drivers/net/ethernet/meta/fbnic/fbnic_hwmon.c
+++ b/drivers/net/ethernet/meta/fbnic/fbnic_hwmon.c
@@ -33,10 +33,17 @@ static int fbnic_hwmon_read(struct device *dev, enum hwmon_sensor_types type,
 {
 	struct fbnic_dev *fbd = dev_get_drvdata(dev);
 	const struct fbnic_mac *mac = fbd->mac;
-	int id;
+	int id, err;
 
 	id = fbnic_hwmon_sensor_id(type);
-	return id < 0 ? id : mac->get_sensor(fbd, id, val);
+	if (id < 0)
+		return id;
+
+	mutex_lock(&fbd->hwmon_mutex);
+	err = mac->get_sensor(fbd, id, val);
+	mutex_unlock(&fbd->hwmon_mutex);
+
+	return err;
 }
 
 static const struct hwmon_ops fbnic_hwmon_ops = {
@@ -60,6 +67,8 @@ void fbnic_hwmon_register(struct fbnic_dev *fbd)
 	if (!IS_REACHABLE(CONFIG_HWMON))
 		return;
 
+	mutex_init(&fbd->hwmon_mutex);
+
 	fbd->hwmon = hwmon_device_register_with_info(fbd->dev, "fbnic",
 						     fbd, &fbnic_chip_info,
 						     NULL);
@@ -68,6 +77,7 @@ void fbnic_hwmon_register(struct fbnic_dev *fbd)
 			   "Failed to register hwmon device %pe\n",
 			   fbd->hwmon);
 		fbd->hwmon = NULL;
+		mutex_destroy(&fbd->hwmon_mutex);
 	}
 }
 
@@ -78,4 +88,5 @@ void fbnic_hwmon_unregister(struct fbnic_dev *fbd)
 
 	hwmon_device_unregister(fbd->hwmon);
 	fbd->hwmon = NULL;
+	mutex_destroy(&fbd->hwmon_mutex);
 }
-- 
2.53.0-Meta