linux-kernel - Re: [PATCH v1] cpuidle: Fail cpuidle device registration if there is one already

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Message-ID: <CAJZ5v0hCaLebTwtm36dxxG0NBOc1DVPj9cjYHKOWm5kApNa1TQ@mail.gmail.com>
Date: Fri, 19 Sep 2025 13:14:10 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Dhruva Gole <d-gole@...com>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, Linux PM <linux-pm@...r.kernel.org>, 
	LKML <linux-kernel@...r.kernel.org>, Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [PATCH v1] cpuidle: Fail cpuidle device registration if there is
 one already

Hi,

On Fri, Sep 19, 2025 at 7:09 AM Dhruva Gole <d-gole@...com> wrote:
>
> Hi Rafael,
>
> On Sep 18, 2025 at 23:19:20 +0200, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > Refuse to register a cpuidle device if the given CPU has a cpuidle
> > device already and print a message regarding it.
> >
> > Without this, an attempt to register a new cpuidle device without
> > unregistering the existing one leads to the removal of the existing
> > cpuidle device without removing its sysfs interface.
> >
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > ---
> >  drivers/cpuidle/cpuidle.c |    8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > --- a/drivers/cpuidle/cpuidle.c
> > +++ b/drivers/cpuidle/cpuidle.c
> > @@ -635,11 +635,17 @@ static void __cpuidle_device_init(struct
> >  static int __cpuidle_register_device(struct cpuidle_device *dev)
> >  {
> >       struct cpuidle_driver *drv = cpuidle_get_cpu_driver(dev);
> > +     unsigned int cpu = dev->cpu;
> >       int i, ret;
> >
> >       if (!try_module_get(drv->owner))
> >               return -EINVAL;
> >
> > +     if (per_cpu(cpuidle_devices, cpu)) {
> > +             pr_info("CPU%d: cpuidle device already registered\n", cpu);
> > +             return -EEXIST;
>
> Here we return prematurely after a try_module_get right?
> Do we need a module_put() similar to how you do it later by calling
> unregister_device function by checking ret = cpuidle_coupled_register_device ?

Good catch, thanks!

I need to move the new check above the driver module reference counting.