linux-kernel - Re: [PATCH v1 2/2] thermal: Fail object registration if thermal class is not registered

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Message-ID: <CAJZ5v0heCTm+1sv9taAoMe8PGgLPEzsTxh9ZdOGjdhDLuvc-Zw@mail.gmail.com>
Date:   Mon, 23 Jan 2023 21:16:33 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Zhang Rui <rui.zhang@...el.com>
Subject: Re: [PATCH v1 2/2] thermal: Fail object registration if thermal class
 is not registered

On Sat, Jan 21, 2023 at 8:40 AM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
>
> On Fri, Jan 20, 2023 at 08:48:07PM +0100, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > If thermal_class is not registered with the driver core, there is no way
> > to expose the interfaces used by the thermal control framework, so
> > prevent thermal zones and cooling devices from being registered in
> > that case by returning an error from object registration functions.
> >
> > For this purpose, introduce class_is_registered() that checks the
> > private pointer of the given class and returns 'false' if it is NULL,
> > which means that the class has not been registered, and use it in the
> > thermal framework.
> >
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > ---
> >  drivers/thermal/thermal_core.c |    6 ++++++
> >  include/linux/device/class.h   |    5 +++++
> >  2 files changed, 11 insertions(+)
> >
> > Index: linux-pm/include/linux/device/class.h
> > ===================================================================
> > --- linux-pm.orig/include/linux/device/class.h
> > +++ linux-pm/include/linux/device/class.h
> > @@ -82,6 +82,11 @@ struct class_dev_iter {
> >       const struct device_type        *type;
> >  };
> >
> > +static inline bool class_is_registered(struct class *class)
> > +{
> > +     return !!class->p;
>
> I really do not like this as it is exposing internals to drivers and
> whenever we do that, it gets abused and we have to unwind the mess in a
> few years.
>
> Overall, I'm trying to remove the ->p usage, but that's a longterm goal
> of mine (to allow class and bus structures to be in read-only memory),
> which isn't your issue here, but it's good to think about why you want
> to know this information (more below.)
>
> > +}
> > +
> >  extern struct kobject *sysfs_dev_block_kobj;
> >  extern struct kobject *sysfs_dev_char_kobj;
> >  extern int __must_check __class_register(struct class *class,
> > Index: linux-pm/drivers/thermal/thermal_core.c
> > ===================================================================
> > --- linux-pm.orig/drivers/thermal/thermal_core.c
> > +++ linux-pm/drivers/thermal/thermal_core.c
> > @@ -880,6 +880,9 @@ __thermal_cooling_device_register(struct
> >           !ops->set_cur_state)
> >               return ERR_PTR(-EINVAL);
> >
> > +     if (!class_is_registered(&thermal_class))
> > +             return ERR_PTR(-ENODEV);
>
> If the class isn't registered, then sommething went wrong with the
> thermal core code, right?  So why isn't the thermal core keeping a local
> variable of "class was registered" and relying on the driver core to
> know this?
>
> The number of individual users that should be doing one thing or another
> if a class is not registered feels very very slim.  How come this code
> is being called at all if the thermal class was not registered in the
> first place?  What would have prevented that from happening?  Is it an
> ordering issue, or a kernel configuration issue?

It's basically a matter of class_register() returning an error.

Yes, we could use an extra variable for this purpose, but that would
be a bit wasteful, because thermal_class will then sit unused and
occupy memory in vain.

Oh well, we may as well just allocate it dynamically.