linux-kernel - Re: [RFC v3] cpufreq: Make sure frequency transitions are serialized

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Date:	Thu, 20 Mar 2014 10:09:26 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Amit Daniel <amit.daniel@...sung.com>
Subject: Re: [RFC v3] cpufreq: Make sure frequency transitions are serialized

On 19 March 2014 17:45, Srivatsa S. Bhat
<srivatsa.bhat@...ux.vnet.ibm.com> wrote:
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index 199b52b..e90388f 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -349,6 +349,38 @@ void cpufreq_notify_post_transition(struct cpufreq_policy *policy,
>  EXPORT_SYMBOL_GPL(cpufreq_notify_post_transition);
>
>
> +void cpufreq_freq_transition_begin(struct cpufreq_policy *policy,
> +               struct cpufreq_freqs *freqs, unsigned int state)
> +{
> +wait:
> +       wait_event(&policy->transition_wait, !policy->transition_ongoing);

I think its broken here. At this point another thread can come take lock,
update transition_ongoing, send notification and finally unlock..

And after that we can take the lock and send another notification..

Correct?

> +       if (!mutex_trylock(&policy->transition_lock))
> +               goto wait;
> +
> +       policy->transition_ongoing++;

s/++/ = true

> +       cpufreq_notify_transition(policy, freqs, CPUFREQ_PRECHANGE);
> +
> +       mutex_unlock(&policy->transition_lock);

We can release the lock before sending notifications, its there just to
protect transition_ongoing.
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