Re: [BUG] KCSAN: data-race in tick_nohz_idle_stop_tick / tick_nohz_next_event

Sean Anderson
2024-03-11T16:30:50Z

On 3/11/24 10:48, Thomas Gleixner wrote:
> 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;
> }

This fixes the issue for me (tested on 6.6; tweaked a bit for conflicts).

--Sean

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