When a user removes a governor using devfreq_remove_governor(), if
the current device is using this governor, devfreq->governor will
be set to NULL. When the user registers any governor
using devfreq_add_governor(), since devfreq->governor is NULL, a
null pointer error occurs in strncmp().

For example: A user loads the userspace gov through a module, then
a device selects userspace. When unloading the userspace module and
then loading it again, the null pointer error occurs:

Unable to handle kernel NULL pointer dereference at virtual address
0000000000000010
Mem abort info:
ESR = 0x0000000096000004
EC = 0x25: DABT (current EL), IL = 32 bits
*******************skip *********************
Call trace:
__pi_strncmp+0x20/0x1b8
devfreq_userspace_init+0x1c/0xff8 [governor_userspace]
do_one_initcall+0x4c/0x278
do_init_module+0x5c/0x218
load_module+0x1f1c/0x1fc8
init_module_from_file+0x8c/0xd0
__arm64_sys_finit_module+0x220/0x3d8
invoke_syscall+0x48/0x110
el0_svc_common.constprop.0+0xbc/0xe8
do_el0_svc+0x20/0x30
el0_svc+0x24/0xb8
el0t_64_sync_handler+0xb8/0xc0
el0t_64_sync+0x14c/0x150

To fix this issue, remove the list_for_each_entry() logic, as
find_devfreq_governor() has already checked for the existence of
governor with the same name. This makes it impossible to find a
duplicate governor in the list, so the subsequent logic is
unreachable and can be removed.

Fixes: 1b5c1be2c88e ("PM / devfreq: map devfreq drivers to governor using name")
Signed-off-by: Yaxiong Tian <tianyaxiong@kylinos.cn>
---
 drivers/devfreq/devfreq.c | 33 ---------------------------------
 1 file changed, 33 deletions(-)

Hi Jie Zhan: 
	If you're willing, I'd like to add your Co-developed-by tag.

diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
index 82dd9a43dc62..994984f7b6e1 100644
--- a/drivers/devfreq/devfreq.c
+++ b/drivers/devfreq/devfreq.c
@@ -1261,7 +1261,6 @@ void devfreq_resume(void)
 int devfreq_add_governor(struct devfreq_governor *governor)
 {
 	struct devfreq_governor *g;
-	struct devfreq *devfreq;
 	int err = 0;
 
 	if (!governor) {
@@ -1280,38 +1279,6 @@ int devfreq_add_governor(struct devfreq_governor *governor)
 
 	list_add(&governor->node, &devfreq_governor_list);
 
-	list_for_each_entry(devfreq, &devfreq_list, node) {
-		int ret = 0;
-		struct device *dev = devfreq->dev.parent;
-
-		if (!strncmp(devfreq->governor->name, governor->name,
-			     DEVFREQ_NAME_LEN)) {
-			/* The following should never occur */
-			if (devfreq->governor) {
-				dev_warn(dev,
-					 "%s: Governor %s already present\n",
-					 __func__, devfreq->governor->name);
-				ret = devfreq->governor->event_handler(devfreq,
-							DEVFREQ_GOV_STOP, NULL);
-				if (ret) {
-					dev_warn(dev,
-						 "%s: Governor %s stop = %d\n",
-						 __func__,
-						 devfreq->governor->name, ret);
-				}
-				/* Fall through */
-			}
-			devfreq->governor = governor;
-			ret = devfreq->governor->event_handler(devfreq,
-						DEVFREQ_GOV_START, NULL);
-			if (ret) {
-				dev_warn(dev, "%s: Governor %s start=%d\n",
-					 __func__, devfreq->governor->name,
-					 ret);
-			}
-		}
-	}
-
 err_out:
 	mutex_unlock(&devfreq_list_lock);
 
-- 
2.25.1