linux-kernel - Re: [Patch v4 3/6] cpufreq: qcom-cpufreq-hw: Add dcvs interrupt support

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Date:   Wed, 28 Jul 2021 18:19:05 -0400
From:   Thara Gopinath <thara.gopinath@...aro.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     agross@...nel.org, bjorn.andersson@...aro.org, rui.zhang@...el.com,
        daniel.lezcano@...aro.org, rjw@...ysocki.net, robh+dt@...nel.org,
        steev@...i.org, linux-arm-msm@...r.kernel.org,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org
Subject: Re: [Patch v4 3/6] cpufreq: qcom-cpufreq-hw: Add dcvs interrupt
 support



On 7/27/21 11:50 PM, Viresh Kumar wrote:
> On 27-07-21, 11:25, Thara Gopinath wrote:
>> +static void qcom_lmh_dcvs_notify(struct qcom_cpufreq_data *data)
>> +{
> 
>> +	/* In the unlikely case cpufreq is de-registered do not enable polling or h/w interrupt */
>> +
>> +	spin_lock(&data->throttle_lock);
>> +	if (data->cancel_throttle) {
>> +		spin_unlock(&data->throttle_lock);
>> +		return;
>> +	}
>> +	spin_unlock(&data->throttle_lock);
>> +
>> +	/*
>> +	 * If h/w throttled frequency is higher than what cpufreq has requested for, stop
>> +	 * polling and switch back to interrupt mechanism
>> +	 */
>> +
>> +	if (throttled_freq >= qcom_cpufreq_hw_get(cpumask_first(policy->cpus)))
>> +		/* Clear the existing interrupts and enable it back */
>> +		enable_irq(data->throttle_irq);
>> +	else
>> +		mod_delayed_work(system_highpri_wq, &data->throttle_work,
>> +				 msecs_to_jiffies(10));
>> +}
> 
>> +static void qcom_cpufreq_hw_lmh_exit(struct qcom_cpufreq_data *data)
>> +{
>> +	if (data->throttle_irq <= 0)
>> +		return;
>> +
>> +	spin_lock(&data->throttle_lock);
>> +	data->cancel_throttle = true;
>> +	spin_unlock(&data->throttle_lock);
>> +	cancel_delayed_work_sync(&data->throttle_work);
>> +	free_irq(data->throttle_irq, data);
>> +}
> 
> Lets see if we can still make it break :)
> 
> CPU0                                            CPU1
> 
> qcom_lmh_dcvs_notify()                          qcom_cpufreq_hw_lmh_exit()
> 
> spin_unlock()
>                                                  spin_lock(),
>                                                  cancel_throttle = true
>                                                  spin_unlock()
> 
>                                                  cancel_delayed_work_sync()
> mod_delayed_work()
>                                                  free_irq()
>                                                  kfree(data)
> qcom_lmh_dcvs_poll()
> Uses data.
> 
> 
> Sorry, locking is fun :)

Ha! I was too lazy to write this down! So how about I make this a mutex 
and put mod_delayed_work() inside the lock. So it will be something like 
below

qcom_lmh_dcvs_notify()			qcom_cpufreq_hw_lmh_exit()

mutex_lock()				mutex_lock()
   if (data->cancel_throttle) {		cancel_throttle = true
	mutex_unlock()			mutex_unlock()
	return				cancel_delayed_work_sync()
   }					free_irq()
   enable_irq() / mod_delayed_work()
mutex_unlock()

I will let you break it!

> 

-- 
Warm Regards
Thara (She/Her/Hers)