linux-kernel - Re: [PATCH V2 2/3] cpufreq: Panic if policy is active in cpufreq_policy

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Date:   Wed, 15 Jun 2022 10:29:57 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        kernel test robot <oliver.sang@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 2/3] cpufreq: Panic if policy is active in
 cpufreq_policy_free()

On 14-06-22, 15:59, Rafael J. Wysocki wrote:
> On Fri, May 27, 2022 at 5:53 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
> >
> > With the new design in place, to avoid potential races show() and
> > store() callbacks check if the policy is active or not before proceeding
> > any further. And in order to guarantee that cpufreq_policy_free() must
> > be called after clearing the policy->cpus mask, i.e. by marking it
> > inactive.
> >
> > Lets make sure we don't get a bug around this later and catch this early
> > by putting a BUG_ON() within cpufreq_policy_free().
> >
> > Also update cpufreq_online() a bit to make sure we clear the cpus mask
> > for each error case before calling cpufreq_policy_free().
> >
> > Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> > ---
> > V2: Update cpufreq_online() and changelog.
> >
> >  drivers/cpufreq/cpufreq.c | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> > index e24aa5d4bca5..0f8245731783 100644
> > --- a/drivers/cpufreq/cpufreq.c
> > +++ b/drivers/cpufreq/cpufreq.c
> > @@ -1284,6 +1284,12 @@ static void cpufreq_policy_free(struct cpufreq_policy *policy)
> >         unsigned long flags;
> >         int cpu;
> >
> > +       /*
> > +        * The callers must ensure the policy is inactive by now, to avoid any
> > +        * races with show()/store() callbacks.
> > +        */
> > +       BUG_ON(!policy_is_inactive(policy));
> 
> I'm not a super-big fan of this change.
> 
> First off, crashing the kernel outright here because of possible races
> appears a bit excessive to me.
> 
> Second, it looks like we are worrying about the code running before
> the wait_for_completion() call in cpufreq_policy_put_kobj(), because
> after that call no one can be running show() or store().  So why don't
> we reorder the wait_for_completion() call with respect to the code in
> question instead?

No, I am not worrying about that race. I am just trying to make sure some change
in future doesn't break this assumption (that policy should be inactive by this
point). That's all. It all looks good for now.

May be a WARN instead of BUG if we don't want to crash.

-- 
viresh