[PATCH v4 1/3] hwmon: (pmbus) add spinlock to protect 64-bit timestamp

[Pradhan, Sanman]

From: Sanman Pradhan <psanman@xxxxxxxxxxx>

The next_access_backoff variable is a 64-bit ktime_t. On 32-bit
architectures, accesses to 64-bit variables are not atomic, which can
result in "torn" reads or writes if accessed concurrently from
different subsystems (e.g., hwmon sysfs reads vs. regulator updates).

Introduce a spinlock in struct pmbus_data to protect accesses to
next_access_backoff and prevent torn reads/writes on 32-bit systems.
This change addresses atomicity of the timestamp field itself as the
timing helpers become callable from more paths, including PMBus chip
drivers using the exported helpers.

This does not attempt to serialize the full wait/transfer/update
sequence across concurrent callers; it only protects the timestamp
field from torn access.

Signed-off-by: Sanman Pradhan <psanman@xxxxxxxxxxx>
---
v4:
- New patch in the series. Introduces a spinlock to protect the 64-bit 
  next_access_backoff timestamp from torn reads/writes on 32-bit 
  architectures.
---
 drivers/hwmon/pmbus/pmbus_core.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/drivers/hwmon/pmbus/pmbus_core.c b/drivers/hwmon/pmbus/pmbus_core.c
index 4d7634ee61484..42bd62f1e2e40 100644
--- a/drivers/hwmon/pmbus/pmbus_core.c
+++ b/drivers/hwmon/pmbus/pmbus_core.c
@@ -15,6 +15,7 @@
 #include <linux/init.h>
 #include <linux/err.h>
 #include <linux/slab.h>
+#include <linux/spinlock.h>
 #include <linux/i2c.h>
 #include <linux/hwmon.h>
 #include <linux/hwmon-sysfs.h>
@@ -122,6 +123,7 @@ struct pmbus_data {
 	int vout_high[PMBUS_PAGES];	/* voltage high margin */
 
 	ktime_t next_access_backoff;	/* Wait until at least this time */
+	spinlock_t timestamp_lock;	/* Protects next_access_backoff */
 };
 
 struct pmbus_debugfs_entry {
@@ -176,8 +178,14 @@ EXPORT_SYMBOL_NS_GPL(pmbus_set_update, "PMBUS");
 static void pmbus_wait(struct i2c_client *client)
 {
 	struct pmbus_data *data = i2c_get_clientdata(client);
-	s64 delay = ktime_us_delta(data->next_access_backoff, ktime_get());
+	ktime_t backoff;
+	s64 delay;
 
+	spin_lock(&data->timestamp_lock);
+	backoff = data->next_access_backoff;
+	spin_unlock(&data->timestamp_lock);
+
+	delay = ktime_us_delta(backoff, ktime_get());
 	if (delay > 0)
 		fsleep(delay);
 }
@@ -194,8 +202,11 @@ static void pmbus_update_ts(struct i2c_client *client, int op)
 	if (op & PMBUS_OP_PAGE_CHANGE)
 		delay = max(delay, info->page_change_delay);
 
-	if (delay > 0)
+	if (delay > 0) {
+		spin_lock(&data->timestamp_lock);
 		data->next_access_backoff = ktime_add_us(ktime_get(), delay);
+		spin_unlock(&data->timestamp_lock);
+	}
 }
 
 int pmbus_set_page(struct i2c_client *client, int page, int phase)
@@ -3687,6 +3698,7 @@ int pmbus_do_probe(struct i2c_client *client, struct pmbus_driver_info *info)
 
 	i2c_set_clientdata(client, data);
 	mutex_init(&data->update_lock);
+	spin_lock_init(&data->timestamp_lock);
 	data->dev = dev;
 
 	if (pdata)
-- 
2.34.1