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

[akhilpo]

On 2018-06-22 22:43, Ezequiel Garcia wrote:
> 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
In governor_store(), we do 'df->governor = governor;' without taking 
df->lock. So it is possible to switch governor while update_devfreq() is 
in progress. I smell trouble there. Don't you think so?
I am assuming df->lock protects 'struct devfreq' and devfreq_list_lock 
protects both device and governor lists.

-Akhil.