Re: [PATCH 1/1] cpufreq: Fix show()/store() issue for hotplugging offline CPU

[guanjun <guanjun@linux.alibaba.com>]

Hi Rafael,

> 2022年10月19日 下午7:47，Rafael J. Wysocki <rafael@kernel.org> 写道：
> 
> On Wed, Oct 19, 2022 at 10:40 AM Guanjun <guanjun@linux.alibaba.com> wrote:
>> 
>> From: Zelin Deng <zelin.deng@linux.alibaba.com>
>> 
>> After brought one CPU offline, lscpu returned failure:
>> 
>> lscpu: cannot read /sys/devices/system/cpu/cpu64/cpufreq/cpuinfo_max_freq: Device or resource busy
>> 
>> which had blocked all outputs of lscpu.
> 
> OK, so the policy->cpus mask is empty and -EBUSY is returned.
> 
> What's wrong?

Here is all right.
The problem is that when I offline one cpu manually and lscpu will fail.
The reproduce process is as follows:
1. lscpu （success）
2. echo 0 > /sys/devices/system/cpu/cpu63/online （offline cpu63）
3. lscpu （fail， and print the error message, “lscpu: cannot read /sys/devices/system/cpu/cpu64/cpufreq/cpuinfo_max_freq: Device or resource busy”）

I think this failure doesn’t make sense.

Maybe I should make the commit message more readable.

Thanks,
Guanjun

> 
>> This is not the case mentioned in commit d4627a287e251, as the policy
>> had been created successfully but is inactive due to CPU gets offline.
> 
> Yes, that's when policy_is_inactive(policy) returns "true" IIUC.
> 
>> To fix this issue, just add an addtional check whether CPU is online or
>> not.
> 
> Which is racy.
> 
> Please explain the problem in the first place.
> 
>> Fixes: d4627a287e251 ("cpufreq: Abort show()/store() for half-initialized policies")
>> Signed-off-by: Zelin Deng <zelin.deng@linux.alibaba.com>
>> Signed-off-by: Guanjun <guanjun@linux.alibaba.com>
>> ---
>> drivers/cpufreq/cpufreq.c | 16 ++++++++++++----
>> 1 file changed, 12 insertions(+), 4 deletions(-)
>> 
>> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
>> index 69b3d61852ac..aa238ba7d2fe 100644
>> --- a/drivers/cpufreq/cpufreq.c
>> +++ b/drivers/cpufreq/cpufreq.c
>> @@ -956,8 +956,12 @@ static ssize_t show(struct kobject *kobj, struct attribute *attr, char *buf)
>>                return -EIO;
>> 
>>        down_read(&policy->rwsem);
>> -       if (likely(!policy_is_inactive(policy)))
>> -               ret = fattr->show(policy, buf);
>> +       if (unlikely(policy_is_inactive(policy) && cpu_online(policy->cpu)))
>> +               goto err;
>> +
>> +       ret = fattr->show(policy, buf);
>> +
>> +err:
>>        up_read(&policy->rwsem);
>> 
>>        return ret;
>> @@ -974,8 +978,12 @@ static ssize_t store(struct kobject *kobj, struct attribute *attr,
>>                return -EIO;
>> 
>>        down_write(&policy->rwsem);
>> -       if (likely(!policy_is_inactive(policy)))
>> -               ret = fattr->store(policy, buf, count);
>> +       if (unlikely(policy_is_inactive(policy) && cpu_online(policy->cpu)))
>> +               goto err;
>> +
>> +       ret = fattr->store(policy, buf, count);
>> +
>> +err:
>>        up_write(&policy->rwsem);
>> 
>>        return ret;
>> --
>> 2.32.0.GIT