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

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Date:   Sat, 21 Jan 2023 08:40:15 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     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 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?

thanks,

greg k-h