linux-kernel - Re: [PATCH 3/5] cpufreq, dbs_data->usage count must be atomic

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Date:	Sat, 08 Nov 2014 02:57:41 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Prarit Bhargava <prarit@...hat.com>
Cc:	linux-kernel@...r.kernel.org, robert.schoene@...dresden.de,
	sboyd@...eaurora.org, Viresh Kumar <viresh.kumar@...aro.org>,
	linux-pm@...r.kernel.org
Subject: Re: [PATCH 3/5] cpufreq, dbs_data->usage count must be atomic

On Wednesday, November 05, 2014 09:53:57 AM Prarit Bhargava wrote:
> The usage_count value can be modified from several cpus concurrently if
> !CPUFREQ_HAVE_GOVERNOR_PER_POLICY.  This leads to a variety of panics in
> which the usage_count is negative or exceeds the number of cpus in the
> system.  It must be switched to atomic_t and protected with a mutex to make sure
> that future read/writes obtain the correct data.

Why do you need to use a new mutex and atomic_t at the same time?  Most likely
one of them is redundant.

> Cc: "Rafael J. Wysocki" <rjw@...ysocki.net>
> Cc: Viresh Kumar <viresh.kumar@...aro.org>
> Cc: linux-pm@...r.kernel.org
> Signed-off-by: Prarit Bhargava <prarit@...hat.com>
> ---
>  drivers/cpufreq/cpufreq_governor.c |   16 ++++++++++++----
>  drivers/cpufreq/cpufreq_governor.h |    3 ++-
>  2 files changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/cpufreq/cpufreq_governor.c b/drivers/cpufreq/cpufreq_governor.c
> index 1b44496..32ad9db 100644
> --- a/drivers/cpufreq/cpufreq_governor.c
> +++ b/drivers/cpufreq/cpufreq_governor.c
> @@ -265,7 +265,9 @@ int cpufreq_governor_dbs(struct cpufreq_policy *policy,
>  		if (have_governor_per_policy()) {
>  			WARN_ON(dbs_data);
>  		} else if (dbs_data) {
> -			dbs_data->usage_count++;
> +			mutex_lock(&dbs_data->usage_count_mutex);
> +			atomic_inc(&dbs_data->usage_count);
> +			mutex_unlock(&dbs_data->usage_count_mutex);
>  			policy->governor_data = dbs_data;
>  			return 0;
>  		}
> @@ -277,7 +279,10 @@ int cpufreq_governor_dbs(struct cpufreq_policy *policy,
>  		}
>  
>  		dbs_data->cdata = cdata;
> -		dbs_data->usage_count = 1;
> +		mutex_init(&dbs_data->usage_count_mutex);
> +		mutex_lock(&dbs_data->usage_count_mutex);
> +		atomic_set(&dbs_data->usage_count, 1);
> +		mutex_unlock(&dbs_data->usage_count_mutex);

The locking here isn't necessary (if it was, someone could be using an
uninitialized mutex).

>  		rc = cdata->init(dbs_data);
>  		if (rc) {
>  			pr_err("%s: POLICY_INIT: init() failed\n", __func__);
> @@ -322,7 +327,8 @@ int cpufreq_governor_dbs(struct cpufreq_policy *policy,
>  
>  		return 0;
>  	case CPUFREQ_GOV_POLICY_EXIT:
> -		if (!--dbs_data->usage_count) {
> +		mutex_lock(&dbs_data->usage_count_mutex);
> +		if (atomic_dec_and_test(&dbs_data->usage_count)) {
>  			sysfs_remove_group(get_governor_parent_kobj(policy),
>  					get_sysfs_attr(dbs_data));
>  
> @@ -338,9 +344,11 @@ int cpufreq_governor_dbs(struct cpufreq_policy *policy,
>  			}
>  
>  			cdata->exit(dbs_data);
> +			mutex_unlock(&dbs_data->usage_count_mutex);
>  			kfree(dbs_data);
>  			cdata->gdbs_data = NULL;
> -		}
> +		} else
> +			mutex_unlock(&dbs_data->usage_count_mutex);
>  
>  		policy->governor_data = NULL;
>  		return 0;
> diff --git a/drivers/cpufreq/cpufreq_governor.h b/drivers/cpufreq/cpufreq_governor.h
> index cc401d1..53ca449 100644
> --- a/drivers/cpufreq/cpufreq_governor.h
> +++ b/drivers/cpufreq/cpufreq_governor.h
> @@ -219,7 +219,8 @@ struct common_dbs_data {
>  struct dbs_data {
>  	struct common_dbs_data *cdata;
>  	unsigned int min_sampling_rate;
> -	int usage_count;
> +	struct mutex usage_count_mutex;
> +	atomic_t usage_count;
>  	void *tuners;
>  
>  	/* dbs_mutex protects dbs_enable in governor start/stop */
> 

-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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