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SubjectRe: [PATCH v3] PM / devfreq: Fix devfreq_add_device() when drivers are built as modules.
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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.

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