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In drivers/cpufreq/cpufreq.c the function cpufreq_add_dev() takes the
error exit 'err_out_unregister' from different places once with the
'cpu_policy_rwsem' lock held, once with the lock released:
|		if (ret)
|			goto err_out_unregister;
|	}
|
|	policy->governor = NULL; /* to assure that the starting sequence is
|				  * run in cpufreq_set_policy */
|
|	/* set default policy */
|	ret = __cpufreq_set_policy(policy, &new_policy);
|	policy->user_policy.policy = policy->policy;
|	policy->user_policy.governor = policy->governor;
|
|	unlock_policy_rwsem_write(cpu);
|
|	if (ret) {
|		dprintk("setting policy failed\n");
|		goto err_out_unregister;
|	}

This leads to the following error message in case of a failing
__cpufreq_set_policy() call:
=====================================
[ BUG: bad unlock balance detected! ]
-------------------------------------
swapper/1 is trying to release lock (&per_cpu(cpu_policy_rwsem, cpu)) at:
[<c01b4564...