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

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Date:   Fri, 13 Apr 2018 07:45:39 +0530
From:   arvindY <arvind.yadav.cs@...il.com>
To:     Chanwoo Choi <cw00.choi@...sung.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()

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.
>
>>>   	}
>>>   
>>>   	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.
>>
>>>   err_dev:
>>>   	if (devfreq)
>>>   		kfree(devfreq);
>>>
>>
>

~arvind