[PATCH 6.18.y 1/2] posix-timers: Handle the timer_[re]arm() return value

[Sasha Levin <sashal@kernel.org>]

From: Thomas Gleixner <tglx@kernel.org>

[ Upstream commit cfb7fe3fdd4ca1d37da1ed15a1897d4a27c47a8a ]

The [re]arm callbacks will return true when the timer was queued and false
if it was already expired at enqueue time.

In both cases the call sites can trivially queue the signal right there,
when the timer was already expired. That avoids a full round trip through
the hrtimer interrupt.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260408114952.198028466@kernel.org
Stable-dep-of: 141a27750ec3 ("posix-timers: Report zero remaining time for disarmed SIGEV_NONE timers")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/time/posix-timers.c | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index 3082129d836c2..3d39f77423afb 100644
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -306,6 +306,8 @@ static bool common_hrtimer_rearm(struct k_itimer *timr)
 
 static bool __posixtimer_deliver_signal(struct kernel_siginfo *info, struct k_itimer *timr)
 {
+	bool queued;
+
 	guard(spinlock)(&timr->it_lock);
 
 	/*
@@ -319,12 +321,18 @@ static bool __posixtimer_deliver_signal(struct kernel_siginfo *info, struct k_it
 	if (!timr->it_interval || WARN_ON_ONCE(timr->it_status != POSIX_TIMER_REQUEUE_PENDING))
 		return true;
 
-	timr->kclock->timer_rearm(timr);
-	timr->it_status = POSIX_TIMER_ARMED;
+	/* timer_rearm() updates timr::it_overrun */
+	queued = timr->kclock->timer_rearm(timr);
+
 	timr->it_overrun_last = timr->it_overrun;
 	timr->it_overrun = -1LL;
 	++timr->it_signal_seq;
 	info->si_overrun = timer_overrun_to_int(timr);
+
+	if (queued)
+		timr->it_status = POSIX_TIMER_ARMED;
+	else
+		posix_timer_queue_signal(timr);
 	return true;
 }
 
@@ -912,9 +920,13 @@ int common_timer_set(struct k_itimer *timr, int flags,
 		expires = timens_ktime_to_host(timr->it_clock, expires);
 	sigev_none = timr->it_sigev_notify == SIGEV_NONE;
 
-	kc->timer_arm(timr, expires, flags & TIMER_ABSTIME, sigev_none);
-	if (!sigev_none)
-		timr->it_status = POSIX_TIMER_ARMED;
+	if (kc->timer_arm(timr, expires, flags & TIMER_ABSTIME, sigev_none)) {
+		if (!sigev_none)
+			timr->it_status = POSIX_TIMER_ARMED;
+	} else {
+		/* Timer was already expired, queue the signal */
+		posix_timer_queue_signal(timr);
+	}
 	return 0;
 }
 
-- 
2.53.0