linux-kernel - Re: [PATCH 4/5] cpufreq, policy->initialized count must be atomic

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Date:	Sat, 08 Nov 2014 02:59:11 +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 4/5] cpufreq, policy->initialized count must be atomic

On Wednesday, November 05, 2014 09:53:58 AM Prarit Bhargava wrote:
> The policy->initialized value can be modified from several cpus concurrently if
> !CPUFREQ_HAVE_GOVERNOR_PER_POLICY.  This leads to a situation where a
> governor maybe switched out even though the governor->initialized is greater
> than one.  It must be switched to atomic_t and protected with a mutex to
> make sure that future read/writes obtain the correct data.

Same comments as for [3/5].  If you add a new mutex, you can avoid using
atomic_t.

> 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.c          |   12 +++++++++---
>  drivers/cpufreq/cpufreq_governor.c |    4 ++--
>  include/linux/cpufreq.h            |    3 ++-
>  3 files changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index e33cb15..cf11de6 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -2059,10 +2059,12 @@ static int __cpufreq_governor(struct cpufreq_policy *policy,
>  	ret = policy->governor->governor(policy, event);
>  
>  	if (!ret) {
> +		mutex_lock(&policy->governor->initialized_mutex);
>  		if (event == CPUFREQ_GOV_POLICY_INIT)
> -			policy->governor->initialized++;
> +			atomic_inc(&policy->governor->initialized);
>  		else if (event == CPUFREQ_GOV_POLICY_EXIT)
> -			policy->governor->initialized--;
> +			atomic_dec(&policy->governor->initialized);
> +		mutex_unlock(&policy->governor->initialized_mutex);
>  	} else {
>  		/* Restore original values */
>  		mutex_lock(&cpufreq_governor_lock);
> @@ -2092,7 +2094,11 @@ int cpufreq_register_governor(struct cpufreq_governor *governor)
>  
>  	mutex_lock(&cpufreq_governor_mutex);
>  
> -	governor->initialized = 0;
> +	mutex_init(&governor->initialized_mutex);
> +	mutex_lock(&governor->initialized_mutex);
> +	atomic_set(&governor->initialized, 0);
> +	mutex_unlock(&governor->initialized_mutex);
> +
>  	err = -EBUSY;
>  	if (__find_governor(governor->name) == NULL) {
>  		err = 0;
> diff --git a/drivers/cpufreq/cpufreq_governor.c b/drivers/cpufreq/cpufreq_governor.c
> index 32ad9db..b1ee597 100644
> --- a/drivers/cpufreq/cpufreq_governor.c
> +++ b/drivers/cpufreq/cpufreq_governor.c
> @@ -315,7 +315,7 @@ int cpufreq_governor_dbs(struct cpufreq_policy *policy,
>  					latency * LATENCY_MULTIPLIER));
>  
>  		if ((cdata->governor == GOV_CONSERVATIVE) &&
> -				(!policy->governor->initialized)) {
> +		    (!atomic_read(&policy->governor->initialized))) {
>  			struct cs_ops *cs_ops = dbs_data->cdata->gov_ops;
>  
>  			cpufreq_register_notifier(cs_ops->notifier_block,
> @@ -336,7 +336,7 @@ int cpufreq_governor_dbs(struct cpufreq_policy *policy,
>  				cpufreq_put_global_kobject();
>  
>  			if ((dbs_data->cdata->governor == GOV_CONSERVATIVE) &&
> -				(policy->governor->initialized == 1)) {
> +			    atomic_read(&policy->governor->initialized) == 1) {
>  				struct cs_ops *cs_ops = dbs_data->cdata->gov_ops;
>  
>  				cpufreq_unregister_notifier(cs_ops->notifier_block,
> diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
> index 43909ad..73cec45 100644
> --- a/include/linux/cpufreq.h
> +++ b/include/linux/cpufreq.h
> @@ -431,7 +431,8 @@ static inline unsigned long cpufreq_scale(unsigned long old, u_int div,
>  
>  struct cpufreq_governor {
>  	char	name[CPUFREQ_NAME_LEN];
> -	int	initialized;
> +	struct mutex initialized_mutex;
> +	atomic_t initialized;
>  	int	(*governor)	(struct cpufreq_policy *policy,
>  				 unsigned int event);
>  	ssize_t	(*show_setspeed)	(struct cpufreq_policy *policy,
> 

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