[PATCH] Subject: thermal: Fix potential race condition in suspend/resume

[Bo Ye]

From: "yugang.wang" <yugang.wang at mediatek.com>

Body:
This patch fixes a race condition during system resume. It occurs if
the system is exiting a suspend state and a user is trying to
register/unregister a thermal zone concurrently. The root cause is
that both actions access the `thermal_tz_list`.

In detail:

1. At PM_POST_SUSPEND during the resume, the system reads all thermal
   zones in `thermal_tz_list`, then resets and updates their
   temperatures.
2. When registering/unregistering a thermal zone, the
   `thermal_tz_list` gets manipulated.

These two actions might occur concurrently, causing a race condition.
To solve this issue, we introduce a mutex lock to protect
`thermal_tz_list` from being modified while it's being read and
updated during the resume from suspend.

Kernel oops excerpt related to this fix:

[ 5201.869845] [T316822] pc: [0xffffffeb7d4876f0] mutex_lock+0x34/0x170
[ 5201.869856] [T316822] lr: [0xffffffeb7ca98a84] thermal_pm_notify+0xd4/0x26c
[... cut for brevity ...]
[ 5201.871061] [T316822]  suspend_prepare+0x150/0x470
[ 5201.871067] [T316822]  enter_state+0x84/0x6f4
[ 5201.871076] [T316822]  state_store+0x15c/0x1e8

Change-Id: Ifdbdecba17093f91eab7e36ce04b46d311ca6568
Signed-off-by: yugang.wang <yugang.wang at mediatek.com>
Signed-off-by: Bo Ye <bo.ye at mediatek.com>
---
 drivers/thermal/thermal_core.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/thermal/thermal_core.c b/drivers/thermal/thermal_core.c
index 8717a3343512..a7a18ed57b6d 100644
--- a/drivers/thermal/thermal_core.c
+++ b/drivers/thermal/thermal_core.c
@@ -1529,12 +1529,14 @@ static int thermal_pm_notify(struct notifier_block *nb,
 	case PM_POST_HIBERNATION:
 	case PM_POST_RESTORE:
 	case PM_POST_SUSPEND:
+		mutex_lock(&thermal_list_lock);
 		atomic_set(&in_suspend, 0);
 		list_for_each_entry(tz, &thermal_tz_list, node) {
 			thermal_zone_device_init(tz);
 			thermal_zone_device_update(tz,
 						   THERMAL_EVENT_UNSPECIFIED);
 		}
+		mutex_unlock(&thermal_list_lock);
 		break;
 	default:
 		break;
-- 
2.17.0