[PATCH] clocksource: Fix thinko in watchdog selection

The watchdog implementation excludes low res / non continuous
clocksources from being selected as a watchdog reference
unintentionally.

Allow using jiffies/PIT as a watchdog reference as long as no better
clocksource is available. This is necessary to detect TSC breakage on
systems, which have no pmtimer/hpet.

The main goal of the initial patch (preventing to switch to highres/nohz
when no reliable fallback clocksource is available) is still guaranteed
by the checks in clocksource_watchdog().
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/time/clocksource.c

@@ -151,7 +151,8 @@ static void clocksource_check_watchdog(struct clocksource *cs)
 			watchdog_timer.expires = jiffies + WATCHDOG_INTERVAL;
 			add_timer(&watchdog_timer);
 		}
-	} else if (cs->flags & CLOCK_SOURCE_IS_CONTINUOUS) {
+	} else {
+		if (cs->flags & CLOCK_SOURCE_IS_CONTINUOUS)
 			cs->flags |= CLOCK_SOURCE_VALID_FOR_HRES;
 
 		if (!watchdog || cs->rating > watchdog->rating) {