linux-kernel - Re: [PATCH] hwmon/coretemp: Simplify platform device antics

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Date:   Fri, 11 Nov 2022 13:37:53 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Robin Murphy <robin.murphy@....com>
Cc:     fenghua.yu@...el.com, jdelvare@...e.com,
        linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux.dev, janusz.krzysztofik@...ux.intel.com,
        lucas.demarchi@...el.com
Subject: Re: [PATCH] hwmon/coretemp: Simplify platform device antics

On Thu, Nov 10, 2022 at 04:20:25PM +0000, Robin Murphy wrote:
> Coretemp's vestigial platform driver is odd. All the real work is done
> globally by the initcall and CPU hotplug notifiers, while the "driver"
> effectively just wraps an allocation and the registration of the hwmon
> interface in a long-winded round-trip through the driver core. The whole
> logic of dynamically creating and destroying platform devices to bring
> the interfaces up and down is fatally flawed right away, since it
> assumes platform_device_add() will synchronously bind the driver and set
> drvdata before it returns, thus results in a NULL dereference if
> drivers_autoprobe is turned off for the platform bus. Furthermore, the
> unusual approach of doing that from within a CPU hotplug notifier is
> also problematic. It's already commented in the code that it deadlocks
> suspend, but it also causes lockdep issues for other drivers or
> subsystems which may want to legitimately register a CPU hotplug
> notifier from a platform bus notifier.
> 
> All of these issues can be solved by ripping this questionable behaviour
> out completely, simply tying the platform devices to the lifetime of the
> module itself, and directly managing the hwmon interfaces from the
> hotplug notifiers. There is a slight user-visible change in that
> /sys/bus/platform/drivers/coretemp will no longer appear, and
> /sys/devices/platform/coretemp.n will remain present if package n is
> hotplugged off, but hwmon users should really only be looking for the
> presence of the hwmon interfaces, whose behaviour remains unchanged.
> 
> Signed-off-by: Robin Murphy <robin.murphy@....com>
> ---
> 
> I haven't been able to fully test hotplug since I only have a
> single-socket Intel system to hand.

Someone with access to hardware will have to validate this.

For both subject and description, please avoid terms like "antics",
"odd", or "questionable". Please describe the problem in neutral terms.

Thanks,
Guenter