[PATCH] hrtimer: Use hard expiry when updating timers on the same base

[Andrea Parri <parri.andrea@gmail.com>]

Rearming a queued timer with nonzero slack can leave the timerqueue out
of order. remove_and_enqueue_same_base() checks the new soft expiry
against its neighbours' hard expiries, then stores the new hard expiry
in the node without requeueing it.

For example, with A at 10 and B at 20, rearming A at 11 with slack 30
passes the neighbour check but leaves A's hard expiry of 41 before B's
20. The same function also caches the soft expiry in base->expires_next
when updating or inserting the first timer, giving next-event selection
an earlier deadline than the queue head's hard expiry.

Compute the hard expiry with the same saturating addition used by
hrtimer_set_expires_range_ns(). Use it for the in-place ordering check
and both updates to base->expires_next.

Fixes: eddffab8282e3 ("hrtimer: Keep track of first expiring timer per clock base")
Fixes: 343f2f4dc5425 ("hrtimer: Try to modify timers in place")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
---
 kernel/time/hrtimer.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 530d61257b9a0..c5940323746db 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -1261,6 +1261,7 @@ static inline bool
 remove_and_enqueue_same_base(struct hrtimer *timer, struct hrtimer_clock_base *base,
 			     const enum hrtimer_mode mode, ktime_t expires, u64 delta_ns)
 {
+	ktime_t hard_expires = ktime_add_safe(expires, ns_to_ktime(delta_ns));
 	bool was_first = false;
 
 	/* Remove it from the timer queue if active */
@@ -1268,11 +1269,11 @@ remove_and_enqueue_same_base(struct hrtimer *timer, struct hrtimer_clock_base *b
 		was_first = !timerqueue_linked_prev(&timer->node);
 
 		/* Try to update in place to avoid the de/enqueue dance */
-		if (hrtimer_can_update_in_place(timer, base, expires)) {
+		if (hrtimer_can_update_in_place(timer, base, hard_expires)) {
 			hrtimer_set_expires_range_ns(timer, expires, delta_ns);
 			trace_hrtimer_start(timer, mode, true);
 			if (was_first)
-				base->expires_next = expires;
+				base->expires_next = hard_expires;
 			return was_first;
 		}
 
@@ -1291,7 +1292,7 @@ remove_and_enqueue_same_base(struct hrtimer *timer, struct hrtimer_clock_base *b
 
 	/* If it's the first expiring timer now or again, update base */
 	if (timerqueue_linked_add(&base->active, &timer->node)) {
-		base->expires_next = expires;
+		base->expires_next = hard_expires;
 		return true;
 	}
 
-- 
2.53.0