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

["Paul E. McKenney" <paulmck@kernel.org>]

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@gmail.com>

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

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

> ---
> 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
>