linux-kernel - Re: [PATCH v2] cpufreq: fix governor start/stop race condition

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Date:	Mon, 10 Jun 2013 10:45:21 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Xiaoguang Chen <chenxg@...vell.com>
Cc:	rjw@...k.pl, cpufreq@...r.kernel.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, njiang1@...vell.com,
	zjwu@...vell.com, ylmao@...vell.com, chenxg.marvell@...il.com
Subject: Re: [PATCH v2] cpufreq: fix governor start/stop race condition

On 9 June 2013 13:20, Xiaoguang Chen <chenxg@...vell.com> wrote:
> cpufreq governor stop and start should be kept in sequence.
> If not, there will be unexpected behavior, for example:
>
> we have 4 cpus and policy->cpu=cpu0, cpu1/2/3 are linked to cpu0.
> the normal sequence is as below:
>
> 1) Current governor is userspace, one application tries to set
> governor to ondemand. it will call __cpufreq_set_policy in which it
> will stop userspace governor and then start ondemand governor.
>
> 2) Current governor is userspace, now cpu0 hotplugs in cpu3, it will
> call cpufreq_add_policy_cpu. on which it first stops userspace
> governor, and then starts userspace governor.
>
> Now if the sequence of above two cases interleaves, it becames
> below sequence:
>
> 1) application stops userspace governor
> 2)                                      hotplug stops userspace governor
> 3) application starts ondemand governor
> 4)                                      hotplug starts a governor
>
> in step 4, hotplug is supposed to start userspace governor, but now
> the governor has been changed by application to ondemand, so hotplug
> starts ondemand governor again !!!!
>
> The solution is: do not allow stop governor multi-times
> Governor stop should only do once, after it is stopped,
> no other governor stop should be executed.
>
> Signed-off-by: Xiaoguang Chen <chenxg@...vell.com>
> ---
>  drivers/cpufreq/cpufreq.c | 10 +++++++++-
>  include/linux/cpufreq.h   |  1 +
>  2 files changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 2d53f47..c8d7cb2 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -1562,6 +1562,11 @@ static int __cpufreq_governor(struct cpufreq_policy *policy,
>
>         pr_debug("__cpufreq_governor for CPU %u, event %u\n",
>                                                 policy->cpu, event);
> +
> +       if ((!policy->governor->enabled && (event == CPUFREQ_GOV_STOP)) ||
> +           (policy->governor->enabled && (event == CPUFREQ_GOV_START)))
> +               return 0;
> +
>         ret = policy->governor->governor(policy, event);
>
>         if (!ret) {
> @@ -1569,6 +1574,10 @@ static int __cpufreq_governor(struct cpufreq_policy *policy,
>                         policy->governor->initialized++;
>                 else if (event == CPUFREQ_GOV_POLICY_EXIT)
>                         policy->governor->initialized--;
> +               else if (event == CPUFREQ_GOV_STOP)
> +                       policy->governor->enabled = 0;
> +               else if (event == CPUFREQ_GOV_START)
> +                       policy->governor->enabled = 1;
>         }
>
>         /* we keep one module reference alive for
> @@ -1581,7 +1590,6 @@ static int __cpufreq_governor(struct cpufreq_policy *policy,
>         return ret;
>  }
>
> -
>  int cpufreq_register_governor(struct cpufreq_governor *governor)
>  {
>         int err;
> diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
> index 037d36a..16c5b70 100644
> --- a/include/linux/cpufreq.h
> +++ b/include/linux/cpufreq.h
> @@ -199,6 +199,7 @@ struct cpufreq_governor {
>                         will fallback to performance governor */
>         struct list_head        governor_list;
>         struct module           *owner;
> +       int enabled;
>  };

This isn't sufficient.

If there are two groups of clk-sharing-cpus, i.e. if we have multiple
policies and they are using same governor, then these functions
gets called twice for governor x. And you will return 0 for the second
policy.
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