[PATCH v2 2/2] sched/loadavg: Use {READ,WRITE}_ONCE() for sample window

From: Matt Fleming
Date: Fri Feb 17 2017 - 07:07:42 EST


'calc_load_update' is accessed without any kind of locking and there's
a clear assumption in the code that only a single value is read or
written.

Make this explicit by using READ_ONCE() and WRITE_ONCE(), and avoid
unintentionally seeing multiple values, or having the load/stores
split.

Technically the loads in calc_global_*() don't require this since
those are the only functions that update 'calc_load_update', but I've
added the READ_ONCE() for consistency.

Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: Mike Galbraith <umgwanakikbuti@xxxxxxxxx>
Cc: Morten Rasmussen <morten.rasmussen@xxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Signed-off-by: Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx>
---
kernel/sched/loadavg.c | 18 +++++++++++-------
1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/loadavg.c b/kernel/sched/loadavg.c
index ec91fcc09bfe..3f96a7d014ce 100644
--- a/kernel/sched/loadavg.c
+++ b/kernel/sched/loadavg.c
@@ -168,7 +168,7 @@ static inline int calc_load_write_idx(void)
* If the folding window started, make sure we start writing in the
* next idle-delta.
*/
- if (!time_before(jiffies, calc_load_update))
+ if (!time_before(jiffies, READ_ONCE(calc_load_update)))
idx++;

return idx & 1;
@@ -203,7 +203,7 @@ void calc_load_exit_idle(void)
/*
* If we're still before the pending sample window, we're done.
*/
- this_rq->calc_load_update = calc_load_update;
+ this_rq->calc_load_update = READ_ONCE(calc_load_update);
if (time_before(jiffies, this_rq->calc_load_update))
return;

@@ -307,13 +307,15 @@ calc_load_n(unsigned long load, unsigned long exp,
*/
static void calc_global_nohz(void)
{
+ unsigned long sample_window;
long delta, active, n;

- if (!time_before(jiffies, calc_load_update + 10)) {
+ sample_window = READ_ONCE(calc_load_update);
+ if (!time_before(jiffies, sample_window + 10)) {
/*
* Catch-up, fold however many we are behind still
*/
- delta = jiffies - calc_load_update - 10;
+ delta = jiffies - sample_window - 10;
n = 1 + (delta / LOAD_FREQ);

active = atomic_long_read(&calc_load_tasks);
@@ -323,7 +325,7 @@ static void calc_global_nohz(void)
avenrun[1] = calc_load_n(avenrun[1], EXP_5, active, n);
avenrun[2] = calc_load_n(avenrun[2], EXP_15, active, n);

- calc_load_update += n * LOAD_FREQ;
+ WRITE_ONCE(calc_load_update, sample_window + n * LOAD_FREQ);
}

/*
@@ -351,9 +353,11 @@ static inline void calc_global_nohz(void) { }
*/
void calc_global_load(unsigned long ticks)
{
+ unsigned long sample_window;
long active, delta;

- if (time_before(jiffies, calc_load_update + 10))
+ sample_window = READ_ONCE(calc_load_update);
+ if (time_before(jiffies, sample_window + 10))
return;

/*
@@ -370,7 +374,7 @@ void calc_global_load(unsigned long ticks)
avenrun[1] = calc_load(avenrun[1], EXP_5, active);
avenrun[2] = calc_load(avenrun[2], EXP_15, active);

- calc_load_update += LOAD_FREQ;
+ WRITE_ONCE(calc_load_update, sample_window + LOAD_FREQ);

/*
* In case we idled for multiple LOAD_FREQ intervals, catch up in bulk.
--
2.10.0