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Date: Mon, 11 Mar 2024 15:48:45 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Sean Anderson <sean.anderson@...o.com>, Mirsad Todorovac
<mirsad.todorovac@....unizg.hr>, linux-kernel@...r.kernel.org
Cc: Frederic Weisbecker <frederic@...nel.org>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [BUG] KCSAN: data-race in tick_nohz_idle_stop_tick /
tick_nohz_next_event
On Thu, Mar 07 2024 at 14:43, Sean Anderson wrote:
> On 8/18/23 10:15, Mirsad Todorovac wrote:
>
> The justification seems to be in tick_sched_do_timer:
>
> /*
> * Check if the do_timer duty was dropped. We don't care about
> * concurrency: This happens only when the CPU in charge went
> * into a long sleep. If two CPUs happen to assign themselves to
> * this duty, then the jiffies update is still serialized by
> * jiffies_lock.
> *
> * If nohz_full is enabled, this should not happen because the
> * tick_do_timer_cpu never relinquishes.
> */
>
> with the other assignment being in tick_nohz_stop_tick. I'm not familiar
> enough with this code to say whether we should be using READ/WRITE_ONCE
> or maybe just data_race (as would be implied by the comment above).
It wants to be READ/WRITE_ONCE(). Something like the untested below,
which applies to -next or the master branch of the tip tree.
Thanks,
tglx
---
--- 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 tic
* 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 tic
!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_ON_ONCE(tick_do_timer_cpu) != cpu);
#endif
}
@@ -406,10 +407,10 @@ void tick_assert_timekeeping_handover(vo
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);
--- 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
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 t
* 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
* 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;
}
@@ -890,6 +893,7 @@ static ktime_t tick_nohz_next_event(stru
{
u64 basemono, next_tick, delta, expires;
unsigned long basejiff;
+ int tick_cpu;
basemono = get_jiffies_update(&basejiff);
ts->last_jiffies = basejiff;
@@ -946,9 +950,9 @@ static ktime_t tick_nohz_next_event(stru
* 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 */
@@ -969,6 +973,7 @@ static void tick_nohz_stop_tick(struct t
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. */
@@ -1006,10 +1011,11 @@ static void tick_nohz_stop_tick(struct t
* 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);
}
@@ -1172,15 +1178,17 @@ static bool can_stop_idle_tick(int cpu,
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;
}
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