[PATCH RFC] schedutil: Address the r/w ordering race in kthread

[Joel Fernandes (Google)]

Currently there is a race in schedutil code for slow-switch single-CPU
systems. Fix it by enforcing ordering the write to work_in_progress to
happen before the read of next_freq.

Kthread                                       Sched update

sugov_work()				      sugov_update_single()

      lock();
      // The CPU is free to rearrange below
      // two in any order, so it may clear
      // the flag first and then read next
      // freq. Lets assume it does.
      work_in_progress = false

                                               if (work_in_progress)
                                                     return;

                                               sg_policy->next_freq = 0;
      freq = sg_policy->next_freq;
                                               sg_policy->next_freq = real-freq;
      unlock();

Reported-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
CC: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: Patrick Bellasi <patrick.bellasi@xxxxxxx>
CC: Juri Lelli <juri.lelli@xxxxxxxxxx>
Cc: Luca Abeni <luca.abeni@xxxxxxxxxxxxxxx>
CC: Todd Kjos <tkjos@xxxxxxxxxx>
CC: claudio@xxxxxxxxxxxxxxx
CC: kernel-team@xxxxxxxxxxx
CC: linux-pm@xxxxxxxxxxxxxxx
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
I split this into separate patch, because this race can also happen in
mainline.

 kernel/sched/cpufreq_schedutil.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
index 5c482ec38610..ce7749da7a44 100644
--- a/kernel/sched/cpufreq_schedutil.c
+++ b/kernel/sched/cpufreq_schedutil.c
@@ -401,6 +401,13 @@ static void sugov_work(struct kthread_work *work)
 	 */
 	raw_spin_lock_irqsave(&sg_policy->update_lock, flags);
 	freq = sg_policy->next_freq;
+
+	/*
+	 * sugov_update_single can access work_in_progress without update_lock,
+	 * make sure next_freq is read before work_in_progress is set.
+	 */
+	smp_mb();
+
 	sg_policy->work_in_progress = false;
 	raw_spin_unlock_irqrestore(&sg_policy->update_lock, flags);
 
-- 
2.17.0.441.gb46fe60e1d-goog