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

[Viresh Kumar <viresh.kumar@linaro.org>]

On 12 August 2014 17:03, Prarit Bhargava <prarit@redhat.com> 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..