Re: [PATCH] cpufreq, store_scaling_governor requires policy->rwsem to be held for duration of changing governors [v2]

[Prarit Bhargava]

On 08/13/2014 03:39 AM, Viresh Kumar wrote:
> On 12 August 2014 17:03, Prarit Bhargava <prarit@xxxxxxxxxx> wrote:
>> Okay, this is what I have and I can reproduce this *easily* 100% of the time.
>>
>> I've used your above config options and have enabled LOCKDEP.
>>
>> In order to restore the locking, I've applied the following patch to the cpufreq
>> core (sorry for the cut-and-paste):
>>
>> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
>> index d9fdedd..dfda238 100644
>> --- a/drivers/cpufreq/cpufreq.c
>> +++ b/drivers/cpufreq/cpufreq.c
>> @@ -2192,9 +2192,7 @@ static int cpufreq_set_policy(struct cpufreq_policy *polic
>>         /* end old governor */
>>         if (old_gov) {
>>                 __cpufreq_governor(policy, CPUFREQ_GOV_STOP);
>> -               up_write(&policy->rwsem);
>>                 __cpufreq_governor(policy, CPUFREQ_GOV_POLICY_EXIT);
>> -               down_write(&policy->rwsem);
>>         }
>>
>>         /* start new governor */
>> @@ -2203,9 +2201,7 @@ static int cpufreq_set_policy(struct cpufreq_policy *polic
>>                 if (!__cpufreq_governor(policy, CPUFREQ_GOV_START))
>>                         goto out;
>>
>> -               up_write(&policy->rwsem);
>>                 __cpufreq_governor(policy, CPUFREQ_GOV_POLICY_EXIT);
>> -               down_write(&policy->rwsem);
>>         }
>>
>>         /* new governor failed, so re-start old one */
>>
>>
>> I've modified the acpi-cpufreq driver to include (sorry for the cut-and-paste)
>>
>> diff --git a/drivers/cpufreq/acpi-cpufreq.c b/drivers/cpufreq/acpi-cpufreq.c
>> index b0c18ed..97653c3 100644
>> --- a/drivers/cpufreq/acpi-cpufreq.c
>> +++ b/drivers/cpufreq/acpi-cpufreq.c
>> @@ -884,6 +884,9 @@ static struct freq_attr *acpi_cpufreq_attr[] = {
>>  };
>>
>>  static struct cpufreq_driver acpi_cpufreq_driver = {
>> +       .flags                  = CPUFREQ_STICKY |
>> +                                       CPUFREQ_HAVE_GOVERNOR_PER_POLICY |
>> +                                       CPUFREQ_NEED_INITIAL_FREQ_CHECK,
>>         .verify         = cpufreq_generic_frequency_table_verify,
>>         .target_index   = acpi_cpufreq_target,
>>         .bios_limit     = acpi_processor_get_bios_limit,
>>
>> I do a
>>
>> cat /sys/devices/system/cpu/cpu2/cpufreq/conservative/*
>> echo ondemand > /sys/devices/system/cpu/cpu2/cpufreq/scaling_governor
>>
>> and then I immediately see the stack trace.
> 
> What's your system configuration? How many clusters/cpus/etc..

Anywhere from 2-4 sockets, 8 - 240 cpus (depending on # of sockets), x86 arch.

P.
> 
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