[Patch] cpufreq: fix a deadlock during shutting down

[Amerigo Wang <amwang@redhat.com>]

Benjamin reported that, the machine deadlocks right after printing the
following when doing a shutdown:

halt/4071 is trying to acquire lock:
 (s_active){++++.+}, at: [<c0000000001ef868>] .sysfs_addrm_finish+0x58/0xc0

but task is already holding lock:
 (&per_cpu(cpu_policy_rwsem, cpu)){+.+.+.}, at: [<c0000000004cd6ac>] .lock_policy_rwsem_write+0x84/0xf4

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

<nothing else ... machine deadlocked here>


This is because we are trying to kobject_put() a kobject while
we are holding cpu policy rwsem. So just move kobject_put()
down after releasing the rwsem.

Totally untested.

Reported-by: Xiaotian Feng <xtfeng@gmail.com>
Reported-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: WANG Cong <amwang@redhat.com>
Cc: Dave Jones <davej@redhat.com>
Cc: Thomas Renninger <trenn@suse.de>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>

---
diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
index 67bc2ec..222b35f 100644
--- a/drivers/cpufreq/cpufreq.c
+++ b/drivers/cpufreq/cpufreq.c
@@ -1113,6 +1113,7 @@ static int __cpufreq_remove_dev(struct sys_device *sys_dev)
 	unsigned int cpu = sys_dev->id;
 	unsigned long flags;
 	struct cpufreq_policy *data;
+	struct kobject *kobj;
 #ifdef CONFIG_SMP
 	struct sys_device *cpu_sys_dev;
 	unsigned int j;
@@ -1192,7 +1193,7 @@ static int __cpufreq_remove_dev(struct sys_device *sys_dev)
 	if (cpufreq_driver->target)
 		__cpufreq_governor(data, CPUFREQ_GOV_STOP);
 
-	kobject_put(&data->kobj);
+	kobj = &data->kobj;
 
 	/* we need to make sure that the underlying kobj is actually
 	 * not referenced anymore by anybody before we proceed with
@@ -1207,6 +1208,7 @@ static int __cpufreq_remove_dev(struct sys_device *sys_dev)
 
 	unlock_policy_rwsem_write(cpu);
 
+	kobject_put(kobj);
 	free_cpumask_var(data->related_cpus);
 	free_cpumask_var(data->cpus);
 	kfree(data);