Commit f87cbcb3 authored by Thomas Gleixner's avatar Thomas Gleixner Committed by Ingo Molnar
Browse files

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: default avatarMirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Tested-by: default avatarSean Anderson <sean.anderson@seco.com>
Link: https://lore.kernel.org/r/87cyqy7rt3.ffs@tglx
parent 6d029c25

kernel/time/tick-common.c

+9 −8
Original line number Diff line number Diff line
@@ -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);

kernel/time/tick-sched.c

+22 −14
Original line number Diff line number Diff line
@@ -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;

	}