[PATCH] clocksource: Prevent immediate watchdog rerun after a delay

[Karl Mehltretter <kmehltretter@gmail.com>]

The watchdog rewrite made the clocksource check handle long intervals
between two measurements, but missed that the timer expiry needs to be
rebased after such a delay.

When a watchdog invocation is delayed by more than one interval, adding
WATCHDOG_INTERVAL to the previous expiry leaves the timer expiry in the
past. That causes an immediate second invocation. The resulting short
measurement interval can mark a clocksource unstable.

Cure this by rearming the timer based on jiffies, which places the next
expiry one full watchdog interval into the future.

Fixes: 763aacf86f1b ("clocksource: Rewrite watchdog code completely")
Closes: https://lore.kernel.org/r/20261010065509.72916-1-kmehltretter@gmail.com/
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
The baseline and patched builds used identical configs and initramfs
images. Tested on v7.3-rc5 (72d3fcf802c4) with four QEMU 8.2.2 TCG
vCPUs. PTRACE_ATTACH stopped only the vCPU 0 TCG thread for 800 ms,
six times 12 seconds apart per boot.

                              baseline   patched
TSC marked unstable           2/3 boots  0/3 boots
boots with gaps below 250 ms  3/3        0/3
minimum watchdog gap          0.231 ms   505.484 ms

All 18 injected stalls completed for each kernel.

 kernel/time/clocksource.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index f1253f5795c6e..58c0ca11cdc5a 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -673,7 +673,7 @@ static void clocksource_watchdog(struct timer_list *unused)
 
 	/* Could have been rearmed by a stop/start cycle */
 	if (!timer_pending(&watchdog_timer)) {
-		watchdog_timer.expires += WATCHDOG_INTERVAL;
+		watchdog_timer.expires = jiffies + WATCHDOG_INTERVAL;
 		add_timer_local(&watchdog_timer);
 	}
 }

base-commit: 5b0764a53235d3c77e1ed674e836e1b3fe1a4b1f
-- 
2.39.5 (Apple Git-154)