[PATCH] timers/nohz: Annotate lockless accesses to got_idle_tick

[PATCH] timers/nohz: Annotate lockless accesses to got_idle_tick

[Kunwu Chan]

tick_sched_do_timer(), called from the tick interrupt handler, sets
ts->got_idle_tick when a tick fires while the CPU is idle.  The idle
path reads and clears it via tick_nohz_idle_got_tick() to detect
whether the tick handler has run.

The flag is accessed locklessly from hardirq and task context.  A
concurrent set can be overwritten by the clear.

Use READ_ONCE() and WRITE_ONCE() to annotate the lockless accesses.

Signed-off-by: Kunwu Chan <kunwu.chan@gmail.com>
---
 kernel/time/tick-sched.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 6c3fea386713..c0ab32e543b3 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -269,7 +269,7 @@ static void tick_sched_do_timer(struct tick_sched *ts, ktime_t now)
 	}
 
 	if (tick_sched_flag_test(ts, TS_FLAG_INIDLE))
-		ts->got_idle_tick = 1;
+		WRITE_ONCE(ts->got_idle_tick, 1);
 }
 
 static void tick_sched_handle(struct tick_sched *ts, struct pt_regs *regs)
@@ -1248,8 +1248,8 @@ bool tick_nohz_idle_got_tick(void)
 {
 	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);
 
-	if (ts->got_idle_tick) {
-		ts->got_idle_tick = 0;
+	if (READ_ONCE(ts->got_idle_tick)) {
+		WRITE_ONCE(ts->got_idle_tick, 0);
 		return true;
 	}
 	return false;
-- 
2.43.0

Re: [PATCH] timers/nohz: Annotate lockless accesses to got_idle_tick

[Thomas Gleixner]

On Sun, Sep 20 2026 at 16:54, Kunwu Chan wrote:
> tick_sched_do_timer(), called from the tick interrupt handler, sets
> ts->got_idle_tick when a tick fires while the CPU is idle.  The idle
> path reads and clears it via tick_nohz_idle_got_tick() to detect
> whether the tick handler has run.
>
> The flag is accessed locklessly from hardirq and task context.  A
> concurrent set can be overwritten by the clear.
>
> Use READ_ONCE() and WRITE_ONCE() to annotate the lockless accesses.

The following scenarios still happen:

A)

              if (READ_ONCE(got_tick))      // reads true
-> interrupt       
       WRITE_ONCE(got_tick, true);   

                   WRITE_ONCE(got_tick, false);

B)

              if (READ_ONCE(got_tick))      // reads false
-> interrupt       
       WRITE_ONCE(got_tick, true);   

#A is harmless because got_tick is already true and #B is harmless as
 well because the got_tick read result is only a hint for the next idle
 invocation.

As this is stricly per CPU the READ/WRITE_ONCE() does not change
anything in terms of ordering. So what exactly is solved by the
READ/WRITE_ONCE()?

Thanks,

        tglx