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

[Akhil P Oommen]

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@xxxxxxxxxxxxx>
> > ---
> >
> > 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.