linux-kernel - Re: [PATCH v2 2/2] cpufreq: Specify default governor on command line

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Message-ID: <20200624153259.GA2844@google.com>
Date:   Wed, 24 Jun 2020 16:32:59 +0100
From:   Quentin Perret <qperret@...gle.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Arnd Bergmann <arnd@...db.de>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        "Cc: Android Kernel" <kernel-team@...roid.com>,
        Todd Kjos <tkjos@...gle.com>, adharmap@...eaurora.org
Subject: Re: [PATCH v2 2/2] cpufreq: Specify default governor on command line

On Wednesday 24 Jun 2020 at 14:51:04 (+0200), Rafael J. Wysocki wrote:
> On Wed, Jun 24, 2020 at 7:50 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
> > > @@ -2789,7 +2796,13 @@ static int __init cpufreq_core_init(void)
> > >       cpufreq_global_kobject = kobject_create_and_add("cpufreq", &cpu_subsys.dev_root->kobj);
> > >       BUG_ON(!cpufreq_global_kobject);
> > >
> > > +     mutex_lock(&cpufreq_governor_mutex);
> > > +     if (!default_governor)
> > > +             default_governor = cpufreq_default_governor();
> > > +     mutex_unlock(&cpufreq_governor_mutex);
> >
> > I don't think locking is required here at core-initcall level.
> 
> It isn't necessary AFAICS, but it may as well be regarded as
> annotation (kind of instead of having a comment explaining why it need
> not be used).

Right, but I must admit that, looking at this more, I'm getting a bit
confused with the overall locking for governors :/

When in cpufreq_init_policy() we find a governor using
find_governor(policy->last_governor), what guarantees this governor is
not concurrently unregistered? That is, what guarantees this governor
doesn't go away between that find_governor() call, and the subsequent
call to try_module_get() in cpufreq_set_policy() down the line?

Can we somewhat assume that whatever governor is referred to by
policy->last_governor will have a non-null refcount? Or are the
cpufreq_online() and cpufreq_unregister_governor() path mutually
exclusive? Or is there something else?

Thanks,
Quentin