linux-kernel - Re: [PATCH] PM / devfreq: use put

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Date:   Fri, 13 Apr 2018 11:43:03 +0900
From:   Chanwoo Choi <cw00.choi@...sung.com>
To:     arvindY <arvind.yadav.cs@...il.com>, myungjoo.ham@...sung.com,
        kyungmin.park@...sung.com
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH] PM / devfreq: use put_device() instead of kfree()

On 2018년 04월 13일 11:37, arvindY wrote:
> 
> 
> On Friday 13 April 2018 07:59 AM, Chanwoo Choi wrote:
>> On 2018년 04월 13일 11:15, arvindY wrote:
>>> Hi Chanwoo,
>>>
>>> On Friday 13 April 2018 06:43 AM, Chanwoo Choi wrote:
>>>> On 2018년 04월 13일 10:03, Chanwoo Choi wrote:
>>>>> Hi,
>>>>>
>>>>> I'm sorry for the late reply.
>>>>>
>>>>> On 2018년 03월 30일 20:44, Arvind Yadav wrote:
>>>>>> Never directly free @dev after calling device_register() or
>>>>>> device_unregister(), even if device_register() returned an error.
>>>>>> Always use put_device() to give up the reference initialized.
>>>>>>
>>>>>> Signed-off-by: Arvind Yadav <arvind.yadav.cs@...il.com>
>>>>>> ---
>>>>>>    drivers/devfreq/devfreq.c | 4 +++-
>>>>>>    1 file changed, 3 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
>>>>>> index fe2af6a..a225b94 100644
>>>>>> --- a/drivers/devfreq/devfreq.c
>>>>>> +++ b/drivers/devfreq/devfreq.c
>>>>>> @@ -625,7 +625,8 @@ struct devfreq *devfreq_add_device(struct device *dev,
>>>>>>        err = device_register(&devfreq->dev);
>>>>>>        if (err) {
>>>>>>            mutex_unlock(&devfreq->lock);
>>>>>> -        goto err_dev;
>>>>>> +        put_device(&devfreq->dev);
>>>>>> +        goto err_out;
>>>>> why do you change the goto postion?
>>>>> err_out is correct to free the memory of devfreq instance.
>>>> Sorry. err_dev is correct instead of err_out.
>>> If you will see the comment for device_register(drivers/base/core.c)
>>> there is mentioned that 'NOTE: _Never_ directly free @dev
>>> after calling this function, even if it returned an error!
>>> Always use put_device() to give up the reference initialized
>>> in this function instead. Here  put_device() will decrement
>>> the last reference and then free the memory by calling dev->release.
>>> Internally put_device() -> kobject_put() -> kobject_cleanup() which
>>> is responsible to call 'dev -> release' and also free other kobject resources.
>>>
>>> We are releasing devfreq in devfreq_dev_release(). So no need
>>> to call kfree() again. It'll be redundant.  'err_out' is correct.
>> You're right. err_out is correct.
>> put_device() -> dev->release() -> devfreq_dev_release() -> kfree(devfreq)
>>
>>>>>>        }
>>>>>>          devfreq->trans_table =    devm_kzalloc(&devfreq->dev,
>>>>>> @@ -671,6 +672,7 @@ struct devfreq *devfreq_add_device(struct device *dev,
>>>>>>        mutex_unlock(&devfreq_list_lock);
>>>>>>          device_unregister(&devfreq->dev);
>>>>>> +    devfreq = NULL;
>>>>> It is wrong. If you initialize the devfreq as NULL,
>>>>> never free the 'devfreq' instance.
>>> No need to release memory after device_unregister().
>>> driver core will take care of this. It will release memory
>>> So no need to call free again.
>> If you have to initialize the devfreq instance as NULL,
>> I think that you better to init in the devfreq_dev_release()
>> as following:
>>
>> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
>> index fe2af6aa88fc..8c52a13d3887 100644
>> --- a/drivers/devfreq/devfreq.c
>> +++ b/drivers/devfreq/devfreq.c
>> @@ -543,6 +543,7 @@ static void devfreq_dev_release(struct device *dev)
>>            mutex_destroy(&devfreq->lock);
>>          kfree(devfreq);
>> +       devfreq = NULL;
>>   }
> 
> Yes, You are right. I will share a update patch.
> Thanks for reviewing.

It is my mistake. 'devfreq' is local variable in the devfreq_dev_release(0.  
Even if initializes 'devfreq = NULL' in the devfreq_dev_release(),
it cannot initialize the 'devfreq' local variable in the devfreq_add_device().

I think that your original patch is good.
Reviewed-by: Chanwoo Choi <cw00.choi@...sung.com>

>>
>>>>>>    err_dev:
>>>>>>        if (devfreq)
>>>>>>            kfree(devfreq);
>>>>>>
>>> ~arvind
>>>
>>>
>>>
>>
> 
> 
> 
> 


-- 
Best Regards,
Chanwoo Choi
Samsung Electronics