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

[Sudeep Holla <sudeep.holla@arm.com>]

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))
+		flush_work(&policy->update);
 	kfree(policy->min_freq_req);
 
 	cpufreq_policy_put_kobj(policy);
-- 
2.17.1