[PATCH v2] 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.

Set the timer expiry before handling the queue. Use its stored hard
expiry for the in-place ordering check and both updates to
base->expires_next.

The early update should be safe because remove_and_enqueue_same_base()
runs with base->cpu_base->lock held. The lock keeps the queue stable
while hrtimer_can_update_in_place() checks the new expiry against both
neighbours.  If the check fails, timerqueue_linked_del() removes the
node without comparing expiry values before it is reinserted.

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>
---
Changes in v2:
- Set the timer expiry before handling the queue and reuse its stored
  hard expiry, as suggested by Peter Zijlstra.

Link to v1: https://lore.kernel.org/r/20260907211134.3854-1-parri.andrea@gmail.com/
---
 kernel/time/hrtimer.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 530d61257b9a0..af22a2fec4904 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -1263,13 +1263,16 @@ remove_and_enqueue_same_base(struct hrtimer *timer, struct hrtimer_clock_base *b
 {
 	bool was_first = false;
 
+	/* Set the new expiry time */
+	hrtimer_set_expires_range_ns(timer, expires, delta_ns);
+	expires = hrtimer_get_expires(timer);
+
 	/* Remove it from the timer queue if active */
 	if (timer->is_queued) {
 		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)) {
-			hrtimer_set_expires_range_ns(timer, expires, delta_ns);
 			trace_hrtimer_start(timer, mode, true);
 			if (was_first)
 				base->expires_next = expires;
@@ -1280,9 +1283,6 @@ remove_and_enqueue_same_base(struct hrtimer *timer, struct hrtimer_clock_base *b
 		timerqueue_linked_del(&base->active, &timer->node);
 	}
 
-	/* Set the new expiry time */
-	hrtimer_set_expires_range_ns(timer, expires, delta_ns);
-
 	debug_activate(timer, mode, timer->is_queued);
 	base->cpu_base->active_bases |= 1 << base->index;
 
-- 
2.53.0