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Subject: Re: [PATCH v3] PM / devfreq: Fix devfreq_add_device() when drivers
 are built as modules.
To:     Ezequiel Garcia <ezequiel@collabora.com>,
        Enric Balletbo i Serra <enric.balletbo@collabora.com>,
        linux-kernel@vger.kernel.org
Cc:     kernel@collabora.com, Chanwoo Choi <cw00.choi@samsung.com>,
        Kyungmin Park <kyungmin.park@samsung.com>,
        MyungJoo Ham <myungjoo.ham@samsung.com>,
        linux-pm@vger.kernel.org
References: <20180621220430.25644-1-enric.balletbo@collabora.com>
 <0383b4a703a0b619023c0654a4a7637419c72ebd.camel@collabora.com>
From:   Akhil P Oommen <akhilpo@codeaurora.org>
Message-ID: <addc4280-729f-85d7-a314-35afbb7c8bb1@codeaurora.org>
Date:   Fri, 22 Jun 2018 12:33:59 +0530
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On 6/22/2018 6:41 AM, Ezequiel Garcia wrote:
> Hey Enric,
>
> On Fri, 2018-06-22 at 00:04 +0200, Enric Balletbo i Serra wrote:
>> When the devfreq driver and the governor driver are built as modules,
>> the call to devfreq_add_device() or governor_store() fails because
>> the
>> governor driver is not loaded at the time the devfreq driver loads.
>> The
>> devfreq driver has a build dependency on the governor but also should
>> have a runtime dependency. We need to make sure that the governor
>> driver
>> is loaded before the devfreq driver.
>>
>> This patch fixes this bug by adding a try_then_request_governor()
>> function. First tries to find the governor, and then, if it is not
>> found,
>> it requests the module and tries again.
>>
>> Fixes: 1b5c1be2c88e (PM / devfreq: map devfreq drivers to governor
>> using name)
>> Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
>> ---
>>
>> Changes in v3:
>> - Remove unneded change in dev_err message.
>> - Fix err returned value in case to not find the governor.
>>
>> Changes in v2:
>> - Add a new function to request the module and call that function
>> from
>>    devfreq_add_device and governor_store.
>>
>>   drivers/devfreq/devfreq.c | 65 ++++++++++++++++++++++++++++++++-----
>> --
> [snip snip]
>> -	governor = find_devfreq_governor(devfreq->governor_name);
>> +	governor = try_then_request_governor(devfreq-
>>> governor_name);
>>   	if (IS_ERR(governor)) {
>>   		dev_err(dev, "%s: Unable to find governor for the
>> device\n",
>>   			__func__);
>>   		err = PTR_ERR(governor);
>> -		goto err_init;
>> +		goto err_unregister;
>>   	}
>>   
>> +	mutex_lock(&devfreq_list_lock);
>> +
> I know it's not something we are introducing in this patch,
> but still... calling a hook with a mutex held looks
> fishy to me.
>
> This lock should only protect the list, unless I am missing
> something.
>
>>   	devfreq->governor = governor;
>>   	err = devfreq->governor->event_handler(devfreq,
>> DEVFREQ_GOV_START,
>>   						NULL);
>> @@ -663,14 +703,16 @@ struct devfreq *devfreq_add_device(struct
>> device *dev,
>>   			__func__);
>>   		goto err_init;
>>   	}
>> +
>> +	list_add(&devfreq->node, &devfreq_list);
>> +
>>   	mutex_unlock(&devfreq_list_lock);
>>   
>>   	return devfreq;
>>   
>>   err_init:
>> -	list_del(&devfreq->node);
>>   	mutex_unlock(&devfreq_list_lock);
>> -
>> +err_unregister:
>>   	device_unregister(&devfreq->dev);
>>   err_dev:
>>   	if (devfreq)
>> @@ -988,12 +1030,13 @@ static ssize_t governor_store(struct device
>> *dev, struct device_attribute *attr,
>>   	if (ret != 1)
>>   		return -EINVAL;
>>   
>> -	mutex_lock(&devfreq_list_lock);
>> -	governor = find_devfreq_governor(str_governor);
>> +	governor = try_then_request_governor(str_governor);
>>   	if (IS_ERR(governor)) {
>> -		ret = PTR_ERR(governor);
>> -		goto out;
>> +		return PTR_ERR(governor);
>>   	}
>> +
>> +	mutex_lock(&devfreq_list_lock);
>> +
>>   	if (df->governor == governor) {
>>   		ret = 0;
>>   		goto out;
>> -- 
>> 2.17.1
>>
>>
>
> Regards,
> Eze

Adding to Ezequiel's point, shouldn't we take more granular lock 
(devfreq->lock) first and then call devfreq_list_lock at the time of 
adding to the list?

-Akhil.