linux-kernel - Re: [PATCH V2] acpi-cpufreq: replace per_cpu with driver

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Date:	Wed, 08 Jul 2015 20:28:28 +0800
From:	Pan Xinhui <xinhuix.pan@...el.com>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linux-pm@...r.kernel.org, rjw@...ysocki.net,
	viresh.kumar@...aro.org, "mnipxh@....com" <mnipxh@....com>,
	"yanmin_zhang@...ux.intel.com" <yanmin_zhang@...ux.intel.com>
Subject: Re: [PATCH V2] acpi-cpufreq: replace per_cpu with driver_data of
 policy

hi, Dmitry	
	thanks for your reply

On 2015年07月08日 01:11, Dmitry Torokhov wrote:
> Hi Pan,
> 
> On Tue, Jul 07, 2015 at 08:43:26PM +0800, Pan Xinhui wrote:
>> @@ -364,19 +363,24 @@ static u32 get_cur_val(const struct cpumask *mask)
>>  
>>  static unsigned int get_cur_freq_on_cpu(unsigned int cpu)
>>  {
>> -	struct acpi_cpufreq_data *data = per_cpu(acfreq_data, cpu);
>> +	struct acpi_cpufreq_data *data;
>> +	struct cpufreq_policy *policy;
>>  	unsigned int freq;
>>  	unsigned int cached_freq;
>>  
>>  	pr_debug("get_cur_freq_on_cpu (%d)\n", cpu);
>>  
>> -	if (unlikely(data == NULL ||
>> -		     data->acpi_data == NULL || data->freq_table == NULL)) {
>> +	policy = cpufreq_cpu_get(cpu);
>> +	if (unlikely(!policy))
>> +		return 0;
>> +
>> +	data = policy->driver_data;
>> +	cpufreq_cpu_put(policy);
> 
> If we put policy here can we guarantee that memory pointed to by data
> stays valid? Shoudln't we issue cpufreq_cpu_put(policy) after we done
> assessing the pointer?
> 

*driver_data* is used internal by acpi-cpufreq driver. So probably issuing
cpufreq_cpu_put(policy) after we get *driver_data* is OKay.

The worry you have is about the race. we set *driver_data* to NULL then 
free it in ->exit callback while ->get callback is using it.

CPU A                                   CPU B
->get					->exit
data = policy->driver_data;
if (!data ....)
					policy->driver_data = NULL;
					kfree(data);
access data ....

yes, it might happen in real world. As Viresh says, it is more like to be a core level work.
But this race exists in current codes, too. Maybe down_write policy->rwsem can avoid this race(need double check).

thanks for pointing out it. :)

thanks
xinhui

>> +	if (unlikely(!data || !data->acpi_data || !data->freq_table))
>>  		return 0;
>> -	}
>>  
>>  	cached_freq = data->freq_table[data->acpi_data->state].frequency;
>> -	freq = extract_freq(get_cur_val(cpumask_of(cpu)), data);
>> +	freq = extract_freq(get_cur_val(cpumask_of(cpu), data), data);
>>  	if (freq != cached_freq) {
>>  		/*
>>  		 * The dreaded BIOS frequency change behind our back.
> 
> Thanks.
> 
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