[PATCH RFC] tick/nohz: fix data races in get_cpu_idle_time_us()

[Yu Liao]

selftest/proc/proc-uptime-001 complains:
  Euler:/mnt # while true; do ./proc-uptime-001; done
  proc-uptime-001: proc-uptime-001.c:41: main: Assertion `i1 >= i0' failed.
  proc-uptime-001: proc-uptime-001.c:41: main: Assertion `i1 >= i0' failed.

/proc/uptime should be monotonically increasing. This occurs because
the data races between get_cpu_idle_time_us and
tick_nohz_stop_idle/tick_nohz_start_idle, for example:

CPU0                        CPU1
get_cpu_idle_time_us

                            tick_nohz_idle_exit
                              now = ktime_get();
                              tick_nohz_stop_idle
                                update_ts_time_stats
                                  delta = ktime_sub(now, ts->idle_entrytime);
                                  ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta)
                                  ts->idle_entrytime = now

now = ktime_get();
if (ts->idle_active && !nr_iowait_cpu(cpu)) {
    ktime_t delta = ktime_sub(now, ts->idle_entrytime);
    idle = ktime_add(ts->idle_sleeptime, delta);
    //idle is slightly greater than the actual value
} else {
    idle = ts->idle_sleeptime;
}
                            ts->idle_active = 0

After this, idle = idle_sleeptime(actual idle value) + now(CPU0) - now(CPU1).
If get_cpu_idle_time_us() is called immediately after ts->idle_active = 0,
only ts->idle_sleeptime is returned, which is smaller than the previously
read one, resulting in a non-monotonically increasing idle time. In
addition, there are other data race scenarios not listed here.

This patch introduce a lock to prevent data races.

Fixes: a130e8fbc7de ("fs/proc/uptime.c: Fix idle time reporting in /proc/uptime")
Signed-off-by: Yu Liao <liaoyu15@xxxxxxxxxx>
---
 kernel/time/tick-sched.c | 15 ++++++++++++++-
 kernel/time/tick-sched.h |  1 +
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index b0e3c9205946..ad7d47098a58 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -36,7 +36,9 @@
 /*
  * Per-CPU nohz control structure
  */
-static DEFINE_PER_CPU(struct tick_sched, tick_cpu_sched);
+static DEFINE_PER_CPU(struct tick_sched, tick_cpu_sched) = {
+	.idle_time_lock = __SPIN_LOCK_UNLOCKED(tick_cpu_sched.idle_time_lock),
+};
 
 struct tick_sched *tick_get_tick_sched(int cpu)
 {
@@ -661,16 +663,24 @@ update_ts_time_stats(int cpu, struct tick_sched *ts, ktime_t now, u64 *last_upda
 
 static void tick_nohz_stop_idle(struct tick_sched *ts, ktime_t now)
 {
+	unsigned long flags;
+
+	spin_lock_irqsave(&ts->idle_time_lock, flags);
 	update_ts_time_stats(smp_processor_id(), ts, now, NULL);
 	ts->idle_active = 0;
+	spin_unlock_irqrestore(&ts->idle_time_lock, flags);
 
 	sched_clock_idle_wakeup_event();
 }
 
 static void tick_nohz_start_idle(struct tick_sched *ts)
 {
+	unsigned long flags;
+
+	spin_lock_irqsave(&ts->idle_time_lock, flags);
 	ts->idle_entrytime = ktime_get();
 	ts->idle_active = 1;
+	spin_unlock_irqrestore(&ts->idle_time_lock, flags);
 	sched_clock_idle_sleep_event();
 }
 
@@ -691,12 +701,14 @@ static void tick_nohz_start_idle(struct tick_sched *ts)
 u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
 {
 	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
+	unsigned long flags;
 	ktime_t now, idle;
 
 	if (!tick_nohz_active)
 		return -1;
 
 	now = ktime_get();
+	spin_lock_irqsave(&ts->idle_time_lock, flags);
 	if (last_update_time) {
 		update_ts_time_stats(cpu, ts, now, last_update_time);
 		idle = ts->idle_sleeptime;
@@ -709,6 +721,7 @@ u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
 			idle = ts->idle_sleeptime;
 		}
 	}
+	spin_unlock_irqrestore(&ts->idle_time_lock, flags);
 
 	return ktime_to_us(idle);
 
diff --git a/kernel/time/tick-sched.h b/kernel/time/tick-sched.h
index 504649513399..a64d4781e7af 100644
--- a/kernel/time/tick-sched.h
+++ b/kernel/time/tick-sched.h
@@ -81,6 +81,7 @@ struct tick_sched {
 	atomic_t			tick_dep_mask;
 	unsigned long			last_tick_jiffies;
 	unsigned int			stalled_jiffies;
+	spinlock_t			idle_time_lock;
 };
 
 extern struct tick_sched *tick_get_tick_sched(int cpu);
-- 
2.25.1