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 <[email protected]>
CC: Rafael J. Wysocki <[email protected]>
CC: Peter Zijlstra <[email protected]>
CC: Ingo Molnar <[email protected]>
CC: Patrick Bellasi <[email protected]>
CC: Juri Lelli <[email protected]>
Cc: Luca Abeni <[email protected]>
CC: Todd Kjos <[email protected]>
CC: [email protected]
CC: [email protected]
CC: [email protected]
Signed-off-by: Joel Fernandes (Google) <[email protected]>
---
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
