[PATCH v4 4/4] devfreq: Refcount governor modules while in use

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

Add a refcount mechanism for governor modules to prevent the governor
module from being removed (except for force unload):
1. Add an optional 'owner' member to struct devfreq_governor so the devfreq
   core can identify the module that holds the governor code.
2. Get and put a refcount of the governor module when starting and stopping
   the governor.

The new 'owner' field is optional:
- Common governor modules (performance, powersave, simple_ondemand,
  userspace, passive) set 'owner' to THIS_MODULE.
- Governors that are bundled into a device driver module must leave 'owner'
  NULL.  The device's lifetime already pins that module, and setting
  'owner' would create a self-reference that blocks the driver from being
  unloaded.

As a result, a non-forced rmmod of an in-use stand-alone governor now
fails with -EBUSY, e.g.:

  $ cat governor
  performance
  $ rmmod governor_performance
  rmmod: ERROR: Module governor_performance is in use

Force unloads (rmmod -f, if configured) can't be blocked.

Signed-off-by: Jie Zhan <[email protected]>
---
 drivers/devfreq/devfreq.c                 | 17 ++++++++++++++++-
 drivers/devfreq/governor_passive.c        |  1 +
 drivers/devfreq/governor_performance.c    |  1 +
 drivers/devfreq/governor_powersave.c      |  1 +
 drivers/devfreq/governor_simpleondemand.c |  1 +
 drivers/devfreq/governor_userspace.c      |  1 +
 include/linux/devfreq-governor.h          | 11 +++++++++++
 7 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
index 959f0b08ed36..4a01dad21c16 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;
 		}
@@ -1051,9 +1064,11 @@ 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;
diff --git a/drivers/devfreq/governor_passive.c b/drivers/devfreq/governor_passive.c
index d7feecd900f1..3e63dd200a04 100644
--- a/drivers/devfreq/governor_passive.c
+++ b/drivers/devfreq/governor_passive.c
@@ -449,6 +449,7 @@ static int devfreq_passive_event_handler(struct devfreq *devfreq,
 static struct devfreq_governor devfreq_passive = {
 	.name = DEVFREQ_GOV_PASSIVE,
 	.flags = DEVFREQ_GOV_FLAG_IMMUTABLE,
+	.owner = THIS_MODULE,
 	.get_target_freq = devfreq_passive_get_target_freq,
 	.event_handler = devfreq_passive_event_handler,
 };
diff --git a/drivers/devfreq/governor_performance.c b/drivers/devfreq/governor_performance.c
index fdb22bf512cf..0a08b067ea6b 100644
--- a/drivers/devfreq/governor_performance.c
+++ b/drivers/devfreq/governor_performance.c
@@ -37,6 +37,7 @@ static int devfreq_performance_handler(struct devfreq *devfreq,
 
 static struct devfreq_governor devfreq_performance = {
 	.name = DEVFREQ_GOV_PERFORMANCE,
+	.owner = THIS_MODULE,
 	.get_target_freq = devfreq_performance_func,
 	.event_handler = devfreq_performance_handler,
 };
diff --git a/drivers/devfreq/governor_powersave.c b/drivers/devfreq/governor_powersave.c
index ee2d6ec8a512..3ae296d55e3e 100644
--- a/drivers/devfreq/governor_powersave.c
+++ b/drivers/devfreq/governor_powersave.c
@@ -37,6 +37,7 @@ static int devfreq_powersave_handler(struct devfreq *devfreq,
 
 static struct devfreq_governor devfreq_powersave = {
 	.name = DEVFREQ_GOV_POWERSAVE,
+	.owner = THIS_MODULE,
 	.get_target_freq = devfreq_powersave_func,
 	.event_handler = devfreq_powersave_handler,
 };
diff --git a/drivers/devfreq/governor_simpleondemand.c b/drivers/devfreq/governor_simpleondemand.c
index ac9c5e9e51a4..46287b279ded 100644
--- a/drivers/devfreq/governor_simpleondemand.c
+++ b/drivers/devfreq/governor_simpleondemand.c
@@ -119,6 +119,7 @@ static struct devfreq_governor devfreq_simple_ondemand = {
 	.name = DEVFREQ_GOV_SIMPLE_ONDEMAND,
 	.attrs = DEVFREQ_GOV_ATTR_POLLING_INTERVAL
 		| DEVFREQ_GOV_ATTR_TIMER,
+	.owner = THIS_MODULE,
 	.get_target_freq = devfreq_simple_ondemand_func,
 	.event_handler = devfreq_simple_ondemand_handler,
 };
diff --git a/drivers/devfreq/governor_userspace.c b/drivers/devfreq/governor_userspace.c
index 3906ebedbae8..b1acccc79f7f 100644
--- a/drivers/devfreq/governor_userspace.c
+++ b/drivers/devfreq/governor_userspace.c
@@ -135,6 +135,7 @@ static int devfreq_userspace_handler(struct devfreq *devfreq,
 
 static struct devfreq_governor devfreq_userspace = {
 	.name = DEVFREQ_GOV_USERSPACE,
+	.owner = THIS_MODULE,
 	.get_target_freq = devfreq_userspace_func,
 	.event_handler = devfreq_userspace_handler,
 };
diff --git a/include/linux/devfreq-governor.h b/include/linux/devfreq-governor.h
index dfdd0160a29f..ae1721e58401 100644
--- a/include/linux/devfreq-governor.h
+++ b/include/linux/devfreq-governor.h
@@ -12,6 +12,7 @@
 #define __LINUX_DEVFREQ_DEVFREQ_H__
 
 #include <linux/devfreq.h>
+struct module;
 
 #define DEVFREQ_NAME_LEN			16
 
@@ -53,6 +54,15 @@
  * @name:		Governor's name
  * @attrs:		Governor's sysfs attribute flags
  * @flags:		Governor's feature flags
+ * @owner:		Optional, module that owns this governor.
+ *			When set (typically to THIS_MODULE), the devfreq core
+ *			takes a reference on @owner while this governor is in
+ *			use so that the governor module cannot be removed,
+ *			except by a forced unload. Governors that are bundled
+ *			into a device driver module must leave @owner NULL: the
+ *			device's lifetime already pins that module, and setting
+ *			@owner would create a self-reference that prevents the
+ *			driver from being unloaded.
  * @get_target_freq:	Returns desired operating frequency for the device.
  *			Basically, get_target_freq will run
  *			devfreq_dev_profile.get_dev_status() to get the
@@ -70,6 +80,7 @@ struct devfreq_governor {
 	const char name[DEVFREQ_NAME_LEN];
 	const u64 attrs;
 	const u64 flags;
+	struct module *owner;
 	int (*get_target_freq)(struct devfreq *this, unsigned long *freq);
 	int (*event_handler)(struct devfreq *devfreq,
 				unsigned int event, void *data);
-- 
2.43.0