Re: [PATCH] cpufreq: flush any pending policy update work scheduled before freeing

["Rafael J. Wysocki" <rafael@kernel.org>]

On Thu, Oct 17, 2019 at 9:36 PM Rafael J. Wysocki <rafael@kernel.org> wrote:
>
> On Thu, Oct 17, 2019 at 6:35 PM Sudeep Holla <sudeep.holla@arm.com> wrote:
> >
> > dev_pm_qos_remove_request ends calling {max,min}_freq_req QoS notifiers
> > which schedule policy update work. It may end up racing with the freeing
> > the policy and unregistering the driver.
> >
> > One possible race is as below where the cpufreq_driver is unregistered
> > but the scheduled work gets executed at later stage when cpufreq_driver
> > is NULL(i.e. after freeing the policy and driver)
> >
> > Unable to handle kernel NULL pointer dereference at virtual address 0000001c
> > pgd = (ptrval)
> > [0000001c] *pgd=80000080204003, *pmd=00000000
> > Internal error: Oops: 206 [#1] SMP THUMB2
> > Modules linked in:
> > CPU: 0 PID: 34 Comm: kworker/0:1 Not tainted 5.4.0-rc3-00006-g67f5a8081a4b #86
> > Hardware name: ARM-Versatile Express
> > Workqueue: events handle_update
> > PC is at cpufreq_set_policy+0x58/0x228
> > LR is at dev_pm_qos_read_value+0x77/0xac
> > Control: 70c5387d  Table: 80203000  DAC: fffffffd
> > Process kworker/0:1 (pid: 34, stack limit = 0x(ptrval))
> >         (cpufreq_set_policy) from (refresh_frequency_limits.part.24+0x37/0x48)
> >         (refresh_frequency_limits.part.24) from (handle_update+0x2f/0x38)
> >         (handle_update) from (process_one_work+0x16d/0x3cc)
> >         (process_one_work) from (worker_thread+0xff/0x414)
> >         (worker_thread) from (kthread+0xff/0x100)
> >         (kthread) from (ret_from_fork+0x11/0x28)
> >
> > Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
> > Cc: Viresh Kumar <viresh.kumar@linaro.org>
> > Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
> > ---
> >  drivers/cpufreq/cpufreq.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > Hi Rafael, Viresh,
> >
> > This fixed the boot issue I reported[1] on TC2 with bL switcher enabled.
> > I have based this patch on -rc3 and not on top of your patches. This
> > only fixes the boot issue but I hit the other crashes while continuously
> > switching on and off the bL switcher that register/unregister the driver
> > Your patch series fixes them. I can based this on top of those if you
> > prefer.
> >
> > Regards,
> > Sudeep
> >
> > [1] https://lore.kernel.org/linux-pm/20191015155735.GA29105@bogus/
> >
> > diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> > index c52d6fa32aac..b703c29a84be 100644
> > --- a/drivers/cpufreq/cpufreq.c
> > +++ b/drivers/cpufreq/cpufreq.c
> > @@ -1278,6 +1278,9 @@ static void cpufreq_policy_free(struct cpufreq_policy *policy)
> >         }
> >
> >         dev_pm_qos_remove_request(policy->min_freq_req);
> > +       /* flush the pending policy->update work before freeing the policy */
> > +       if (work_pending(&policy->update))
>
> Isn't this racy?
>
> It still may be running if the pending bit is clear and we still need
> to wait for it then, don't we?
>
> Why don't you do an unconditional flush_work() here?

You may as well do a cancel_work_sync() here, because whether or not
the last update of the policy happens before it goes away is a matter
of timing in any case