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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