[tip: timers/urgent] timekeeping: Use READ/WRITE_ONCE() for tick_do_timer_cpu

tip-bot2 for Thomas Gleixner
2024-04-10T08:23:03Z

The following commit has been merged into the timers/urgent branch of tip:

Commit-ID: f87cbcb345d059f0377b4fa0ba1b766a17fc3710
Gitweb: https://git.kernel.org/tip/f87cbcb345d059f0377b4fa0ba1b766a17fc3710
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Tue, 09 Apr 2024 12:29:12 +02:00
Committer: Ingo Molnar <mingo@kernel.org>
CommitterDate: Wed, 10 Apr 2024 10:13:42 +02:00

timekeeping: Use READ/WRITE_ONCE() for tick_do_timer_cpu

tick_do_timer_cpu is used lockless to check which CPU needs to take care
of the per tick timekeeping duty. This is done to avoid a thundering
herd problem on jiffies_lock.

The read and writes are not annotated so KCSAN complains about data races:

BUG: KCSAN: data-race in tick_nohz_idle_stop_tick / tick_nohz_next_event

write to 0xffffffff8a2bda30 of 4 bytes by task 0 on cpu 26:
tick_nohz_idle_stop_tick+0x3b1/0x4a0
do_idle+0x1e3/0x250

read to 0xffffffff8a2bda30 of 4 bytes by task 0 on cpu 16:
tick_nohz_next_event+0xe7/0x1e0
tick_nohz_get_sleep_length+0xa7/0xe0
menu_select+0x82/0xb90
cpuidle_select+0x44/0x60
do_idle+0x1c2/0x250

value changed: 0x0000001a -> 0xffffffff

Annotate them with READ/WRITE_ONCE() to document the intentional data race.

Reported-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Sean Anderson <sean.anderson@seco.com>
Link: https://lore.kernel.org/r/87cyqy7rt3.ffs@tglx
---
kernel/time/tick-common.c | 17 +++++++++--------
kernel/time/tick-sched.c | 36 ++++++++++++++++++++++--------------
2 files changed, 31 insertions(+), 22 deletions(-)

diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
index fb0fdec..d88b130 100644
--- a/kernel/time/tick-common.c
+++ b/kernel/time/tick-common.c
@@ -7,6 +7,7 @@
* Copyright(C) 2005-2007, Red Hat, Inc., Ingo Molnar
* Copyright(C) 2006-2007, Timesys Corp., Thomas Gleixner
*/
+#include <linux/compiler.h>
#include <linux/cpu.h>
#include <linux/err.h>
#include <linux/hrtimer.h>
@@ -84,7 +85,7 @@ int tick_is_oneshot_available(void)
*/
static void tick_periodic(int cpu)
{
- if (tick_do_timer_cpu == cpu) {
+ if (READ_ONCE(tick_do_timer_cpu) == cpu) {
raw_spin_lock(&jiffies_lock);
write_seqcount_begin(&jiffies_seq);

@@ -215,8 +216,8 @@ static void tick_setup_device(struct tick_device *td,
* If no cpu took the do_timer update, assign it to
* this cpu:
*/
- if (tick_do_timer_cpu == TICK_DO_TIMER_BOOT) {
- tick_do_timer_cpu = cpu;
+ if (READ_ONCE(tick_do_timer_cpu) == TICK_DO_TIMER_BOOT) {
+ WRITE_ONCE(tick_do_timer_cpu, cpu);
tick_next_period = ktime_get();
#ifdef CONFIG_NO_HZ_FULL
/*
@@ -232,7 +233,7 @@ static void tick_setup_device(struct tick_device *td,
!tick_nohz_full_cpu(cpu)) {
tick_take_do_timer_from_boot();
tick_do_timer_boot_cpu = -1;
- WARN_ON(tick_do_timer_cpu != cpu);
+ WARN_ON(READ_ONCE(tick_do_timer_cpu) != cpu);
#endif
}

@@ -406,10 +407,10 @@ void tick_assert_timekeeping_handover(void)
int tick_cpu_dying(unsigned int dying_cpu)
{
/*
- * If the current CPU is the timekeeper, it's the only one that
- * can safely hand over its duty. Also all online CPUs are in
- * stop machine, guaranteed not to be idle, therefore it's safe
- * to pick any online successor.
+ * If the current CPU is the timekeeper, it's the only one that can
+ * safely hand over its duty. Also all online CPUs are in stop
+ * machine, guaranteed not to be idle, therefore there is no
+ * concurrency and it's safe to pick any online successor.
*/
if (tick_do_timer_cpu == dying_cpu)
tick_do_timer_cpu = cpumask_first(cpu_online_mask);
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 1331216..71a792c 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -8,6 +8,7 @@
*
* Started by: Thomas Gleixner and Ingo Molnar
*/
+#include <linux/compiler.h>
#include <linux/cpu.h>
#include <linux/err.h>
#include <linux/hrtimer.h>
@@ -204,7 +205,7 @@ static inline void tick_sched_flag_clear(struct tick_sched *ts,

static void tick_sched_do_timer(struct tick_sched *ts, ktime_t now)
{
- int cpu = smp_processor_id();
+ int tick_cpu, cpu = smp_processor_id();

/*
* Check if the do_timer duty was dropped. We don't care about
@@ -216,16 +217,18 @@ static void tick_sched_do_timer(struct tick_sched *ts, ktime_t now)
* If nohz_full is enabled, this should not happen because the
* 'tick_do_timer_cpu' CPU never relinquishes.
*/
- if (IS_ENABLED(CONFIG_NO_HZ_COMMON) &&
- unlikely(tick_do_timer_cpu == TICK_DO_TIMER_NONE)) {
+ tick_cpu = READ_ONCE(tick_do_timer_cpu);
+
+ if (IS_ENABLED(CONFIG_NO_HZ_COMMON) && unlikely(tick_cpu == TICK_DO_TIMER_NONE)) {
#ifdef CONFIG_NO_HZ_FULL
WARN_ON_ONCE(tick_nohz_full_running);
#endif
- tick_do_timer_cpu = cpu;
+ WRITE_ONCE(tick_do_timer_cpu, cpu);
+ tick_cpu = cpu;
}

/* Check if jiffies need an update */
- if (tick_do_timer_cpu == cpu)
+ if (tick_cpu == cpu)
tick_do_update_jiffies64(now);

/*
@@ -610,7 +613,7 @@ bool tick_nohz_cpu_hotpluggable(unsigned int cpu)
* timers, workqueues, timekeeping, ...) on behalf of full dynticks
* CPUs. It must remain online when nohz full is enabled.
*/
- if (tick_nohz_full_running && tick_do_timer_cpu == cpu)
+ if (tick_nohz_full_running && READ_ONCE(tick_do_timer_cpu) == cpu)
return false;
return true;
}
@@ -891,6 +894,7 @@ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
{
u64 basemono, next_tick, delta, expires;
unsigned long basejiff;
+ int tick_cpu;

basemono = get_jiffies_update(&basejiff);
ts->last_jiffies = basejiff;
@@ -947,9 +951,9 @@ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
* Otherwise we can sleep as long as we want.
*/
delta = timekeeping_max_deferment();
- if (cpu != tick_do_timer_cpu &&
- (tick_do_timer_cpu != TICK_DO_TIMER_NONE ||
- !tick_sched_flag_test(ts, TS_FLAG_DO_TIMER_LAST)))
+ tick_cpu = READ_ONCE(tick_do_timer_cpu);
+ if (tick_cpu != cpu &&
+ (tick_cpu != TICK_DO_TIMER_NONE || !tick_sched_flag_test(ts, TS_FLAG_DO_TIMER_LAST)))
delta = KTIME_MAX;

/* Calculate the next expiry time */
@@ -970,6 +974,7 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
unsigned long basejiff = ts->last_jiffies;
u64 basemono = ts->timer_expires_base;
bool timer_idle = tick_sched_flag_test(ts, TS_FLAG_STOPPED);
+ int tick_cpu;
u64 expires;

/* Make sure we won't be trying to stop it twice in a row. */
@@ -1007,10 +1012,11 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
* do_timer() never gets invoked. Keep track of the fact that it
* was the one which had the do_timer() duty last.
*/
- if (cpu == tick_do_timer_cpu) {
- tick_do_timer_cpu = TICK_DO_TIMER_NONE;
+ tick_cpu = READ_ONCE(tick_do_timer_cpu);
+ if (tick_cpu == cpu) {
+ WRITE_ONCE(tick_do_timer_cpu, TICK_DO_TIMER_NONE);
tick_sched_flag_set(ts, TS_FLAG_DO_TIMER_LAST);
- } else if (tick_do_timer_cpu != TICK_DO_TIMER_NONE) {
+ } else if (tick_cpu != TICK_DO_TIMER_NONE) {
tick_sched_flag_clear(ts, TS_FLAG_DO_TIMER_LAST);
}

@@ -1173,15 +1179,17 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
return false;

if (tick_nohz_full_enabled()) {
+ int tick_cpu = READ_ONCE(tick_do_timer_cpu);
+
/*
* Keep the tick alive to guarantee timekeeping progression
* if there are full dynticks CPUs around
*/
- if (tick_do_timer_cpu == cpu)
+ if (tick_cpu == cpu)
return false;

/* Should not happen for nohz-full */
- if (WARN_ON_ONCE(tick_do_timer_cpu == TICK_DO_TIMER_NONE))
+ if (WARN_ON_ONCE(tick_cpu == TICK_DO_TIMER_NONE))
return false;
}