[tip: core/core] watchdog/softlockup: Enforce that timestamp is valid on boot

["tip-bot2 for Thomas Gleixner" <tip-bot2@linutronix.de>]

The following commit has been merged into the core/core branch of tip:

Commit-ID:     11e31f608b499f044f24b20be73f1dcab3e43f8a
Gitweb:        https://git.kernel.org/tip/11e31f608b499f044f24b20be73f1dcab3e43f8a
Author:        Thomas Gleixner <tglx@linutronix.de>
AuthorDate:    Thu, 16 Jan 2020 19:17:02 +01:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Fri, 17 Jan 2020 11:19:22 +01:00

watchdog/softlockup: Enforce that timestamp is valid on boot

Robert reported that during boot the watchdog timestamp is set to 0 for one
second which is the indicator for a watchdog reset.

The reason for this is that the timestamp is in seconds and the time is
taken from sched clock and divided by ~1e9. sched clock starts at 0 which
means that for the first second during boot the watchdog timestamp is 0,
i.e. reset.

Use ULONG_MAX as the reset indicator value so the watchdog works correctly
right from the start. ULONG_MAX would only conflict with a real timestamp
if the system reaches an uptime of 136 years on 32bit and almost eternity
on 64bit.

Reported-by: Robert Richter <rrichter@marvell.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/87o8v3uuzl.fsf@nanos.tec.linutronix.de

---
 kernel/watchdog.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index e3774e9..b6b1f54 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -161,6 +161,8 @@ static void lockup_detector_update_enable(void)
 
 #ifdef CONFIG_SOFTLOCKUP_DETECTOR
 
+#define SOFTLOCKUP_RESET	ULONG_MAX
+
 /* Global variables, exported for sysctl */
 unsigned int __read_mostly softlockup_panic =
 			CONFIG_BOOTPARAM_SOFTLOCKUP_PANIC_VALUE;
@@ -272,7 +274,7 @@ notrace void touch_softlockup_watchdog_sched(void)
 	 * Preemption can be enabled.  It doesn't matter which CPU's timestamp
 	 * gets zeroed here, so use the raw_ operation.
 	 */
-	raw_cpu_write(watchdog_touch_ts, 0);
+	raw_cpu_write(watchdog_touch_ts, SOFTLOCKUP_RESET);
 }
 
 notrace void touch_softlockup_watchdog(void)
@@ -296,14 +298,14 @@ void touch_all_softlockup_watchdogs(void)
 	 * the softlockup check.
 	 */
 	for_each_cpu(cpu, &watchdog_allowed_mask)
-		per_cpu(watchdog_touch_ts, cpu) = 0;
+		per_cpu(watchdog_touch_ts, cpu) = SOFTLOCKUP_RESET;
 	wq_watchdog_touch(-1);
 }
 
 void touch_softlockup_watchdog_sync(void)
 {
 	__this_cpu_write(softlockup_touch_sync, true);
-	__this_cpu_write(watchdog_touch_ts, 0);
+	__this_cpu_write(watchdog_touch_ts, SOFTLOCKUP_RESET);
 }
 
 static int is_softlockup(unsigned long touch_ts)
@@ -379,7 +381,7 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 	/* .. and repeat */
 	hrtimer_forward_now(hrtimer, ns_to_ktime(sample_period));
 
-	if (touch_ts == 0) {
+	if (touch_ts == SOFTLOCKUP_RESET) {
 		if (unlikely(__this_cpu_read(softlockup_touch_sync))) {
 			/*
 			 * If the time stamp was touched atomically