kernel/watchdog.c: fix race between proc_watchdog_thresh() and watchdog_timer_fn()

Theoretically it is possible that the watchdog timer expires right at the
time when a user sets 'watchdog_thresh' to zero (note: this disables the
lockup detectors).  In this scenario, the is_softlockup() function - which
is called by the timer - could produce a false positive.

Fix this by checking the current value of 'watchdog_thresh'.
Signed-off-by: NUlrich Obergfell <uobergfe@redhat.com>
Acked-by: NDon Zickus <dzickus@redhat.com>
Reviewed-by: NAaron Tomlin <atomlin@redhat.com>
Cc: Ulrich Obergfell <uobergfe@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

kernel/watchdog.c

@@ -289,7 +289,7 @@ static int is_softlockup(unsigned long touch_ts)
 {
 	unsigned long now = get_timestamp();
 
-	if (watchdog_enabled & SOFT_WATCHDOG_ENABLED) {
+	if ((watchdog_enabled & SOFT_WATCHDOG_ENABLED) && watchdog_thresh){
 		/* Warn about unreasonable delays. */
 		if (time_after(now, touch_ts + get_softlockup_thresh()))
 			return now - touch_ts;