[PATCH] timer_list: Annotate the clock event deadline snapshot

[PATCH] timer_list: Annotate the clock event deadline snapshot

[Zhu Jun]

print_tickdevice() reads next_event while the remote CPU can update it
from its timer interrupt. The value is only printed to /proc/timer_list,
so synchronizing the clock event device is unnecessary.

Use a single diagnostic snapshot and mark the intentional race, as is
already done for the tick scheduler statistics in print_cpu().

Fixes: 289f480af87e ("[PATCH] Add debugging feature /proc/timer_list")
Signed-off-by: Zhu Jun <zhujun2@cmss.chinamobile.com>
---
 kernel/time/timer_list.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/time/timer_list.c b/kernel/time/timer_list.c
index 0406bf4488e9..5f80af00729e 100644
--- a/kernel/time/timer_list.c
+++ b/kernel/time/timer_list.c
@@ -177,6 +177,7 @@ static void
 print_tickdevice(struct seq_file *m, struct tick_device *td, int cpu)
 {
 	struct clock_event_device *dev = td->evtdev;
+	ktime_t next_event;
 
 	touch_nmi_watchdog();
 
@@ -199,7 +200,9 @@ print_tickdevice(struct seq_file *m, struct tick_device *td, int cpu)
 	SEQ_printf(m, " mult:           %u\n", dev->mult);
 	SEQ_printf(m, " shift:          %u\n", dev->shift);
 	SEQ_printf(m, " mode:           %d\n", clockevent_get_state(dev));
-	SEQ_printf(m, " next_event:     %lld nsecs\n", (long long)dev->next_event);
+	/* Take a diagnostic snapshot without locking the clock event device. */
+	next_event = data_race(READ_ONCE(dev->next_event));
+	SEQ_printf(m, " next_event:     %lld nsecs\n", (long long)next_event);
 
 	SEQ_printf(m, " set_next_event: %ps\n", dev->set_next_event);
 
-- 
2.51.0

Re: [PATCH] timer_list: Annotate the clock event deadline snapshot

[Thomas Gleixner]

On Wed, Nov 11 2026 at 16:41, Zhu Jun wrote:
> print_tickdevice() reads next_event while the remote CPU can update it
> from its timer interrupt. The value is only printed to /proc/timer_list,
> so synchronizing the clock event device is unnecessary.
>
> Use a single diagnostic snapshot and mark the intentional race, as is
> already done for the tick scheduler statistics in print_cpu().
>
> Fixes: 289f480af87e ("[PATCH] Add debugging feature /proc/timer_list")

This really fixes nothing.

> Signed-off-by: Zhu Jun <zhujun2@cmss.chinamobile.com>
> ---
>  kernel/time/timer_list.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/time/timer_list.c b/kernel/time/timer_list.c
> index 0406bf4488e9..5f80af00729e 100644
> --- a/kernel/time/timer_list.c
> +++ b/kernel/time/timer_list.c
> @@ -177,6 +177,7 @@ static void
>  print_tickdevice(struct seq_file *m, struct tick_device *td, int cpu)
>  {
>  	struct clock_event_device *dev = td->evtdev;
> +	ktime_t next_event;
>  
>  	touch_nmi_watchdog();
>  
> @@ -199,7 +200,9 @@ print_tickdevice(struct seq_file *m, struct tick_device *td, int cpu)
>  	SEQ_printf(m, " mult:           %u\n", dev->mult);
>  	SEQ_printf(m, " shift:          %u\n", dev->shift);
>  	SEQ_printf(m, " mode:           %d\n", clockevent_get_state(dev));
> -	SEQ_printf(m, " next_event:     %lld nsecs\n", (long long)dev->next_event);
> +	/* Take a diagnostic snapshot without locking the clock event device. */
> +	next_event = data_race(READ_ONCE(dev->next_event));

data_race(READ_ONCE()) ?

READ_ONCE() needs to be paired with WRITE_ONCE(), which is not there to
write dev->next_event. And no, we don't add it as there is zero value to
it for the regular usage outside of this debug code.

data_race() is sufficient to shut KCSAN up and you neither need an extra
variable nor a comment. data_race() in debug code is selfexplaining, no?

Thanks,

        tglx