[PATCH v2 5/6] devfreq: Get and put module refcount when switching governor

[Jie Zhan]

A governor module can be dynamically inserted or removed if compiled as a
kernel module.  'devfreq->governor' would become NULL if the governor
module is removed when it's in use.

To prevent the governor module from being removed (except for force
unload) when it's in use, get and put a refcount of the governor module
when starting and stopping the governor.

As a result, unloading a governor module in use returns an error, e.g.:
  $ cat governor
  performance
  $ rmmod governor_performance
  rmmod: ERROR: Module governor_performance is in use

Signed-off-by: Jie Zhan <zhanjie9 at hisilicon.com>
---
 drivers/devfreq/devfreq.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
index e1363ab69173..2ea42325d030 100644
--- a/drivers/devfreq/devfreq.c
+++ b/drivers/devfreq/devfreq.c
@@ -345,24 +345,37 @@ static int devfreq_set_governor(struct devfreq *df,
 				 __func__, df->governor->name, ret);
 			return ret;
 		}
+		module_put(old_gov->owner);
 	}
 
 	/* Start the new governor */
+	if (!try_module_get(new_gov->owner)) {
+		df->governor = NULL;
+		return -EINVAL;
+	}
+
 	df->governor = new_gov;
 	ret = df->governor->event_handler(df, DEVFREQ_GOV_START, NULL);
 	if (ret) {
 		dev_warn(dev, "%s: Governor %s not started(%d)\n",
 			 __func__, df->governor->name, ret);
+		module_put(new_gov->owner);
 
 		/* Restore previous governor */
 		df->governor = old_gov;
 		if (!df->governor)
 			return ret;
 
+		if (!try_module_get(old_gov->owner)) {
+			df->governor = NULL;
+			return -EINVAL;
+		}
+
 		ret = df->governor->event_handler(df, DEVFREQ_GOV_START, NULL);
 		if (ret) {
 			dev_err(dev, "%s: restore Governor %s failed (%d)\n",
 				__func__, df->governor->name, ret);
+			module_put(old_gov->owner);
 			df->governor = NULL;
 			return ret;
 		}
@@ -1040,9 +1053,11 @@ int devfreq_remove_device(struct devfreq *devfreq)
 
 	devfreq_cooling_unregister(devfreq->cdev);
 
-	if (devfreq->governor)
+	if (devfreq->governor) {
 		devfreq->governor->event_handler(devfreq,
 						 DEVFREQ_GOV_STOP, NULL);
+		module_put(devfreq->governor->owner);
+	}
 	device_unregister(&devfreq->dev);
 
 	return 0;
-- 
2.43.0