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SubjectRe: [PATCH 1/2] cpufreq: unlock correct rwsem while updating policy->cpu
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On 16 September 2013 21:57, Jon Medhurst (Tixy) <tixy@linaro.org> wrote:
> If I take mainline code and just change the line above to:

You meant this line by above line?

unlock_policy_rwsem_write(cpu);

> up_write(&per_cpu(cpu_policy_rwsem, (per_cpu(cpufreq_cpu_data,
> cpu))->last_cpu));
> then the big_little cpufreq driver works for me.

That would be same as: up_write(&per_cpu(cpu_policy_rwsem, policy->last_cpu));

>> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
>> index 43c24aa..c18bf7b 100644
>> --- a/drivers/cpufreq/cpufreq.c
>> +++ b/drivers/cpufreq/cpufreq.c
>> @@ -952,9 +952,16 @@ static void update_policy_cpu(struct cpufreq_policy *policy, unsigned int cpu)
>> if (cpu == policy->cpu)
>> return;
>>
>> + /* take direct locks as lock_policy_rwsem_write wouldn't work here */
>> + down_write(&per_cpu(cpu_policy_rwsem, policy->cpu));
>> + down_write(&per_cpu(cpu_policy_rwsem, cpu));
>> +
>> policy->last_cpu = policy->cpu;
>> policy->cpu = cpu;
>>
>> + up_write(&per_cpu(cpu_policy_rwsem, cpu));
>> + up_write(&per_cpu(cpu_policy_rwsem, policy->cpu));
>
> You've just overwritten policy->cpu with cpu.

Stupid enough :)

> I tried using
> policy->last_cpu to fix that, but it still doesn't work for me (giving
> the lockdep warning I mentioned.) If I change the code to just lock the
> original policy->cpu lock only, then all is fine.

Fixed my patch now.. find attached.. It mentions why lock for last cpu is
enough here. Copied that here too..

+ /*
+ * Take direct locks as lock_policy_rwsem_write wouldn't work here.
+ * Also lock for last cpu is enough here as contention will happen only
+ * after policy->cpu is changed and after it is changed, other threads
+ * will try to acquire lock for new cpu. And policy is already updated
+ * by then.
+ */

> Also, this locking is now just happening around policy->cpu update,
> whereas before this change, it was locked for the whole of
> update_policy_cpu, i.e. cpufreq_frequency_table_update_policy_cpu and
> the notifier callbacks. Is that change of lock coverage OK?

Yeah, the rwsem is only required for updating policy's fields and so that
should be okay.
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