Re: [PATCH linux-next] mark access to tick_do_timer_cpu with READ_ONCE/WRITE_ONCE

From: Paul E. McKenney
Date: Fri Jan 06 2023 - 09:35:00 EST

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On Mon, Dec 19, 2022 at 01:21:28PM +0800, Zhouyi Zhou wrote:
> mark access to tick_do_timer_cpu with READ_ONCE/WRITE_ONCE to fix concurrency bug
> reported by KCSAN.
>
> Signed-off-by: Zhouyi Zhou <zhouzhouyi@xxxxxxxxx>

This does indeed get rid of some KCSAN complaints, thank you!!!

Tested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

> ---
> During the rcutorture test on linux-next,
> ./tools/testing/selftests/rcutorture/bin/torture.sh --do-kcsan --kcsan-kmake-arg "CC=clang-12"
> following KCSAN BUG is reported:
> [ 35.397089] BUG: KCSAN: data-race in tick_nohz_idle_stop_tick / tick_nohz_next_event^M
> [ 35.400593] ^M
> [ 35.401377] write to 0xffffffffb64b1270 of 4 bytes by task 0 on cpu 3:^M
> [ 35.405325] tick_nohz_idle_stop_tick+0x14c/0x3e0^M
> [ 35.407162] do_idle+0xf3/0x2a0^M
> [ 35.408016] cpu_startup_entry+0x15/0x20^M
> [ 35.409084] start_secondary+0x8f/0x90^M
> [ 35.410207] secondary_startup_64_no_verify+0xe1/0xeb^M
> [ 35.411607] ^M
> [ 35.412042] no locks held by swapper/3/0.^M
> [ 35.413172] irq event stamp: 53048^M
> [ 35.414175] hardirqs last enabled at (53047): [<ffffffffb41f8404>] tick_nohz_idle_enter+0x104/0x140^M
> [ 35.416681] hardirqs last disabled at (53048): [<ffffffffb41229f1>] do_idle+0x91/0x2a0^M
> [ 35.418988] softirqs last enabled at (53038): [<ffffffffb40bf21e>] __irq_exit_rcu+0x6e/0xc0^M
> [ 35.421347] softirqs last disabled at (53029): [<ffffffffb40bf21e>] __irq_exit_rcu+0x6e/0xc0^M
> [ 35.423685] ^M
> [ 35.424119] read to 0xffffffffb64b1270 of 4 bytes by task 0 on cpu 0:^M
> [ 35.425870] tick_nohz_next_event+0x233/0x2b0^M
> [ 35.427119] tick_nohz_idle_stop_tick+0x8f/0x3e0^M
> [ 35.428386] do_idle+0xf3/0x2a0^M
> [ 35.429265] cpu_startup_entry+0x15/0x20^M
> [ 35.430429] rest_init+0x20c/0x210^M
> [ 35.431382] arch_call_rest_init+0xe/0x10^M
> [ 35.432508] start_kernel+0x544/0x600^M
> [ 35.433519] secondary_startup_64_no_verify+0xe1/0xeb^M
>
> fix above bug by marking access to tick_do_timer_cpu with READ_ONCE/WRITE_ONCE
>
> Thanks
> Zhouyi
> --
> kernel/time/tick-common.c | 16 ++++++++--------
> kernel/time/tick-sched.c | 28 +++++++++++++++-------------
> 2 files changed, 23 insertions(+), 21 deletions(-)
>
> diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
> index 46789356f856..a7753465a0be 100644
> --- a/kernel/time/tick-common.c
> +++ b/kernel/time/tick-common.c
> @@ -84,7 +84,7 @@ 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);
>
> @@ -184,9 +184,9 @@ static void giveup_do_timer(void *info)
> {
> int cpu = *(unsigned int *)info;
>
> - WARN_ON(tick_do_timer_cpu != smp_processor_id());
> + WARN_ON(READ_ONCE(tick_do_timer_cpu) != smp_processor_id());
>
> - tick_do_timer_cpu = cpu;
> + WRITE_ONCE(tick_do_timer_cpu, cpu);
> }
>
> static void tick_take_do_timer_from_boot(void)
> @@ -217,8 +217,8 @@ 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
> @@ -235,7 +235,7 @@ 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
> }
>
> @@ -407,8 +407,8 @@ EXPORT_SYMBOL_GPL(tick_broadcast_oneshot_control);
> */
> void tick_handover_do_timer(void)
> {
> - if (tick_do_timer_cpu == smp_processor_id())
> - tick_do_timer_cpu = cpumask_first(cpu_online_mask);
> + if (READ_ONCE(tick_do_timer_cpu) == smp_processor_id())
> + WRITE_ONCE(tick_do_timer_cpu, cpumask_first(cpu_online_mask));
> }
>
> /*
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index b0e3c9205946..2dac94b3187f 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -186,16 +186,16 @@ 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 never relinquishes.
> */
> - if (unlikely(tick_do_timer_cpu == TICK_DO_TIMER_NONE)) {
> + if (unlikely(READ_ONCE(tick_do_timer_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);
> }
> #endif
>
> /* Check, if the jiffies need an update */
> - if (tick_do_timer_cpu == cpu)
> + if (READ_ONCE(tick_do_timer_cpu) == cpu)
> tick_do_update_jiffies64(now);
>
> /*
> @@ -534,7 +534,7 @@ static int tick_nohz_cpu_down(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 -EBUSY;
> return 0;
> }
> @@ -849,8 +849,9 @@ 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 || !ts->do_timer_last))
> + if (cpu != READ_ONCE(tick_do_timer_cpu) &&
> + (READ_ONCE(tick_do_timer_cpu) != TICK_DO_TIMER_NONE
> + || !ts->do_timer_last))
> delta = KTIME_MAX;
>
> /* Calculate the next expiry time */
> @@ -883,10 +884,10 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
> * do_timer() never 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;
> + if (cpu == READ_ONCE(tick_do_timer_cpu)) {
> + WRITE_ONCE(tick_do_timer_cpu, TICK_DO_TIMER_NONE);
> ts->do_timer_last = 1;
> - } else if (tick_do_timer_cpu != TICK_DO_TIMER_NONE) {
> + } else if (READ_ONCE(tick_do_timer_cpu) != TICK_DO_TIMER_NONE) {
> ts->do_timer_last = 0;
> }
>
> @@ -1049,8 +1050,8 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
> * invoked.
> */
> if (unlikely(!cpu_online(cpu))) {
> - if (cpu == tick_do_timer_cpu)
> - tick_do_timer_cpu = TICK_DO_TIMER_NONE;
> + if (cpu == READ_ONCE(tick_do_timer_cpu))
> + WRITE_ONCE(tick_do_timer_cpu, TICK_DO_TIMER_NONE);
> /*
> * Make sure the CPU doesn't get fooled by obsolete tick
> * deadline if it comes back online later.
> @@ -1073,11 +1074,12 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
> * Keep the tick alive to guarantee timekeeping progression
> * if there are full dynticks CPUs around
> */
> - if (tick_do_timer_cpu == cpu)
> + if (READ_ONCE(tick_do_timer_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(READ_ONCE(tick_do_timer_cpu)
> + == TICK_DO_TIMER_NONE))
> return false;
> }
>
> --
> 2.34.1
>

Next message: Greg Kroah-Hartman: "Re: [PATCH 6.0 000/177] 6.0.18-rc1 review"
Previous message: kernel test robot: "kernel/trace/rv/monitors/wip/wip.c:20:1: sparse: sparse: symbol 'da_mon_wip' was not declared. Should it be static?"
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