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

[Ezequiel Garcia]

Hey Akhil,

On Fri, 2018-06-22 at 12:33 +0530, Akhil P Oommen wrote:
> 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@xxxxxxxxxxx
> > > om>
> > > ---
> > > 
> > > 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?
> 

Not sure why we should do that. devfreq->lock should be used to
protect the struct devfreq state, while the devfreq_list_lock
is apparently protecting the two lists (which seem unrelated
lists).

So, the two locks are not correlated.

Regards,
Eze