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From:   Joel Fernandes <joelaf@google.com>
Date:   Tue, 3 Sep 2019 10:21:59 -0400
Message-ID: <CAJWu+opnNT3bQwe+SsdR0Q+PSt7DA=JAQ_5sbZ6h2DXd4ZqwHA@mail.gmail.com>
Subject: Re: [PATCH v14 1/6] sched/core: uclamp: Extend CPU's cgroup controller
To:     =?UTF-8?Q?Michal_Koutn=C3=BD?= <mkoutny@suse.com>
Cc:     Suren Baghdasaryan <surenb@google.com>,
        Patrick Bellasi <patrick.bellasi@arm.com>,
        Alessio Balsini <balsini@android.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Quentin Perret <quentin.perret@arm.com>,
        Paul Turner <pjt@google.com>,
        Steve Muckle <smuckle@google.com>,
        Todd Kjos <tkjos@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Tejun Heo <tj@kernel.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Juri Lelli <juri.lelli@redhat.com>,
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On Tue, Sep 3, 2019 at 4:53 AM Michal Koutn=C3=BD <mkoutny@suse.com> wrote:
>
> On Mon, Sep 02, 2019 at 04:02:57PM -0700, Suren Baghdasaryan <surenb@goog=
le.com> wrote:
> > > +static inline void cpu_uclamp_print(struct seq_file *sf,
> > > +                                   enum uclamp_id clamp_id)
> > > [...]
> > > +       rcu_read_lock();
> > > +       tg =3D css_tg(seq_css(sf));
> > > +       util_clamp =3D tg->uclamp_req[clamp_id].value;
> > > +       rcu_read_unlock();
> > > +
> > > +       if (util_clamp =3D=3D SCHED_CAPACITY_SCALE) {
> > > +               seq_puts(sf, "max\n");
> > > +               return;
> > > +       }
> > > +
> > > +       percent =3D tg->uclamp_pct[clamp_id];
> >
> > You are taking RCU lock when accessing tg->uclamp_req but not when
> > accessing tg->uclamp_pct.
> Good point.
>
> > Is that intentional? Can tg be destroyed under you?
> Actually, the rcu_read{,un}lock should be unnecessary in the context of
> the kernfs file op handler -- the tg/css won't go away as long as its
> kernfs file is being worked with.
>

Also, add to that the fact that there is no rcu_dereference() call to
access any of the pointers in the reader or any of its callers. And, I
don't see any "wait for completion" type of pattern here so that
rcu_read_{lock, unlock}() pair does seem useless.

thanks,

 - Joel