thermal: power_allocator: fix one race condition issue for thermal_instances list

When invoking allow_maximum_power and traverse tz->thermal_instances,
we should grab thermal_zone_device->lock to avoid race condition. For
example, during the system reboot, if the mali GPU device implements
device shutdown callback and unregister GPU devfreq cooling device,
the deleted list head may be accessed to cause panic, as the following
log shows:

[   33.551070] c3 25 (kworker/3:0) Unable to handle kernel paging request at virtual address dead000000000070
[   33.566708] c3 25 (kworker/3:0) pgd = ffffffc0ed290000
[   33.572071] c3 25 (kworker/3:0) [dead000000000070] *pgd=00000001ed292003, *pud=00000001ed292003, *pmd=0000000000000000
[   33.581515] c3 25 (kworker/3:0) Internal error: Oops: 96000004 [#1] PREEMPT SMP
[   33.599761] c3 25 (kworker/3:0) CPU: 3 PID: 25 Comm: kworker/3:0 Not tainted 4.4.35+ #912
[   33.614137] c3 25 (kworker/3:0) Workqueue: events_freezable thermal_zone_device_check
[   33.620245] c3 25 (kworker/3:0) task: ffffffc0f32e4200 ti: ffffffc0f32f0000 task.ti: ffffffc0f32f0000
[   33.629466] c3 25 (kworker/3:0) PC is at power_allocator_throttle+0x7c8/0x8a4
[   33.636609] c3 25 (kworker/3:0) LR is at power_allocator_throttle+0x808/0x8a4
[   33.643742] c3 25 (kworker/3:0) pc : [<ffffff8008683dd0>] lr : [<ffffff8008683e10>] pstate: 20000145
[   33.652874] c3 25 (kworker/3:0) sp : ffffffc0f32f3bb0
[   34.468519] c3 25 (kworker/3:0) Process kworker/3:0 (pid: 25, stack limit = 0xffffffc0f32f0020)
[   34.477220] c3 25 (kworker/3:0) Stack: (0xffffffc0f32f3bb0 to 0xffffffc0f32f4000)
[   34.819822] c3 25 (kworker/3:0) Call trace:
[   34.824021] c3 25 (kworker/3:0) Exception stack(0xffffffc0f32f39c0 to 0xffffffc0f32f3af0)
[   34.924993] c3 25 (kworker/3:0) [<ffffff8008683dd0>] power_allocator_throttle+0x7c8/0x8a4
[   34.933184] c3 25 (kworker/3:0) [<ffffff80086807f4>] handle_thermal_trip.part.25+0x70/0x224
[   34.941545] c3 25 (kworker/3:0) [<ffffff8008680a68>] thermal_zone_device_update+0xc0/0x20c
[   34.949818] c3 25 (kworker/3:0) [<ffffff8008680bd4>] thermal_zone_device_check+0x20/0x2c
[   34.957924] c3 25 (kworker/3:0) [<ffffff80080b93a4>] process_one_work+0x168/0x458
[   34.965414] c3 25 (kworker/3:0) [<ffffff80080ba068>] worker_thread+0x13c/0x4b4
[   34.972650] c3 25 (kworker/3:0) [<ffffff80080c0a4c>] kthread+0xe8/0xfc
[   34.979187] c3 25 (kworker/3:0) [<ffffff8008084e90>] ret_from_fork+0x10/0x40
[   34.986244] c3 25 (kworker/3:0) Code: f9405e73 eb1302bf d102e273 54ffc460 (b9402a61)
[   34.994339] c3 25 (kworker/3:0) ---[ end trace 32057901e3b7e1db ]---
Signed-off-by: Yi Zeng <yizeng@asrmicro.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>

drivers/thermal/power_allocator.c

@@ -523,6 +523,7 @@ static void allow_maximum_power(struct thermal_zone_device *tz)
 	struct thermal_instance *instance;
 	struct power_allocator_params *params = tz->governor_data;
 
+	mutex_lock(&tz->lock);
 	list_for_each_entry(instance, &tz->thermal_instances, tz_node) {
 		if ((instance->trip != params->trip_max_desired_temperature) ||
 		    (!cdev_is_power_actor(instance->cdev)))
@@ -534,6 +535,7 @@ static void allow_maximum_power(struct thermal_zone_device *tz)
 		mutex_unlock(&instance->cdev->lock);
 		thermal_cdev_update(instance->cdev);
 	}
+	mutex_unlock(&tz->lock);
 }
 
 /**