[PATCH 3/4 V2] Use get_online_cpus to avoid races involving CPU hotplug

[Silas Boyd-Wickizer <sbw@mit.edu>]

via_cputemp_init in drivers/hwmon/via-cputemp.c loops with
for_each_online_cpu, adding platform_devices, then calls
register_hotcpu_notifier.  If a CPU is offlined between the loop and
register_hotcpu_notifier, then later onlined, via_cputemp_device_add
will attempt to add platform devices with the same ID.  A similar race
occurs during via_cputemp_exit, after the module calls
unregister_hotcpu_notifier, a CPU might offline and a device will
exist for a CPU that is offline.

This fix surrounds for_each_online_cpu and register_hotcpu_notifier
with get_online_cpus+put_online_cpus; and surrounds
unregister_hotcpu_notifier and device unregistering with
get_online_cpus+put_online_cpus.

Build tested.

Signed-off-by: Silas Boyd-Wickizer <sbw@mit.edu>
---
 drivers/hwmon/via-cputemp.c |    5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/hwmon/via-cputemp.c b/drivers/hwmon/via-cputemp.c
index 8689664..54b01ac 100644
--- a/drivers/hwmon/via-cputemp.c
+++ b/drivers/hwmon/via-cputemp.c
@@ -328,6 +328,7 @@ static int __init via_cputemp_init(void)
 	if (err)
 		goto exit;
 
+	get_online_cpus();
 	for_each_online_cpu(i) {
 		struct cpuinfo_x86 *c = &cpu_data(i);
 
@@ -347,12 +348,14 @@ static int __init via_cputemp_init(void)
 
 #ifndef CONFIG_HOTPLUG_CPU
 	if (list_empty(&pdev_list)) {
+		put_online_cpus();
 		err = -ENODEV;
 		goto exit_driver_unreg;
 	}
 #endif
 
 	register_hotcpu_notifier(&via_cputemp_cpu_notifier);
+	put_online_cpus();
 	return 0;
 
 #ifndef CONFIG_HOTPLUG_CPU
@@ -367,6 +370,7 @@ static void __exit via_cputemp_exit(void)
 {
 	struct pdev_entry *p, *n;
 
+	get_online_cpus();
 	unregister_hotcpu_notifier(&via_cputemp_cpu_notifier);
 	mutex_lock(&pdev_list_mutex);
 	list_for_each_entry_safe(p, n, &pdev_list, list) {
@@ -375,6 +379,7 @@ static void __exit via_cputemp_exit(void)
 		kfree(p);
 	}
 	mutex_unlock(&pdev_list_mutex);
+	put_online_cpus();
 	platform_driver_unregister(&via_cputemp_driver);
 }
 
-- 
1.7.10.4