linux-kernel - Re: [PATCH 13/13] cpufreq: make sure frequency transitions are serialized

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Message-ID: <1894928.On8nI7rMdZ@vostro.rjw.lan>
Date:	Mon, 24 Jun 2013 13:43:48 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	linaro-kernel@...ts.linaro.org, patches@...aro.org,
	cpufreq@...r.kernel.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org, robin.randhawa@....com,
	Steve.Bannister@....com, Liviu.Dudau@....com,
	charles.garcia-tobin@....com, arvind.chauhan@....com,
	dave.martin@....com
Subject: Re: [PATCH 13/13] cpufreq: make sure frequency transitions are serialized

On Wednesday, June 19, 2013 02:23:07 PM Viresh Kumar wrote:
> Whenever we are changing frequency of a cpu, we are calling PRECHANGE and
> POSTCHANGE notifiers. They must be serialized. i.e. PRECHANGE or POSTCHANGE
> shouldn't be called twice contiguously.
> 
> This can happen due to bugs in users of __cpufreq_driver_target() or actual
> cpufreq drivers who are sending these notifiers.
> 
> This patch adds some protection against this. Now, we keep track of the last
> transaction and see if something went wrong.
> 
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
>  drivers/cpufreq/cpufreq.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 2d53f47..92cb8b3 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -107,6 +107,9 @@ static void handle_update(struct work_struct *work);
>  static BLOCKING_NOTIFIER_HEAD(cpufreq_policy_notifier_list);
>  static struct srcu_notifier_head cpufreq_transition_notifier_list;
>  
> +/* Tracks status of transition */
> +static int transition_ongoing;
> +
>  static bool init_cpufreq_transition_notifier_list_called;
>  static int __init init_cpufreq_transition_notifier_list(void)
>  {
> @@ -264,6 +267,8 @@ void __cpufreq_notify_transition(struct cpufreq_policy *policy,
>  	switch (state) {
>  
>  	case CPUFREQ_PRECHANGE:
> +		WARN_ON(transition_ongoing++);
> +
>  		/* detect if the driver reported a value as "old frequency"
>  		 * which is not equal to what the cpufreq core thinks is
>  		 * "old frequency".
> @@ -283,6 +288,8 @@ void __cpufreq_notify_transition(struct cpufreq_policy *policy,
>  		break;
>  
>  	case CPUFREQ_POSTCHANGE:
> +		WARN_ON(!transition_ongoing--);

Shouldn't we try to avoid going into the negative range here?

> +
>  		adjust_jiffies(CPUFREQ_POSTCHANGE, freqs);
>  		pr_debug("FREQ: %lu - CPU: %lu", (unsigned long)freqs->new,
>  			(unsigned long)freqs->cpu);
> @@ -1458,6 +1465,8 @@ int __cpufreq_driver_target(struct cpufreq_policy *policy,
>  
>  	if (cpufreq_disabled())
>  		return -ENODEV;
> +	if (transition_ongoing)
> +		return -EBUSY;
>  
>  	/* Make sure that target_freq is within supported range */
>  	if (target_freq > policy->max)
> 

Rafael

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