NOHZ: prevent multiplication overflow - stop timer for huge timeouts

get_next_timer_interrupt() returns a delta of (LONG_MAX > 1) in case
there is no timer pending. On 64 bit machines this results in a
multiplication overflow in tick_nohz_stop_sched_tick().

Reported by: Dave Miller <davem@davemloft.net>

Make the return value a constant and limit the return value to a 32 bit
value.

When the max timeout value is returned, we can safely stop the tick
timer device. The max jiffies delta results in a 12 days timeout for
HZ=1000.

In the long term the get_next_timer_interrupt() code needs to be
reworked to return ktime instead of jiffies, but we have to wait until
the last users of the original NO_IDLE_HZ code are converted.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 24 insertions, 2 deletions

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 3e7ebc4646b7..52db9e3c526e 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -247,6 +247,21 @@ void tick_nohz_stop_sched_tick(void)
                 if (cpu == tick_do_timer_cpu)
                         tick_do_timer_cpu = -1;
 
+                ts->idle_sleeps++;
+
+                /*
+                 * delta_jiffies >= NEXT_TIMER_MAX_DELTA signals that
+                 * there is no timer pending or at least extremly far
+                 * into the future (12 days for HZ=1000). In this case
+                 * we simply stop the tick timer:
+                 */
+                if (unlikely(delta_jiffies >= NEXT_TIMER_MAX_DELTA)) {
+                        ts->idle_expires.tv64 = KTIME_MAX;
+                        if (ts->nohz_mode == NOHZ_MODE_HIGHRES)
+                                hrtimer_cancel(&ts->sched_timer);
+                        goto out;
+                }
+
                 /*
                  * calculate the expiry time for the next timer wheel
                  * timer
@@ -254,7 +269,6 @@ void tick_nohz_stop_sched_tick(void)
                 expires = ktime_add_ns(last_update, tick_period.tv64 *
                                        delta_jiffies);
                 ts->idle_expires = expires;
-                ts->idle_sleeps++;
 
                 if (ts->nohz_mode == NOHZ_MODE_HIGHRES) {
                         hrtimer_start(&ts->sched_timer, expires,
diff --git a/kernel/timer.c b/kernel/timer.c
index 5ec5490f8d85..1a69705c2fb9 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -666,7 +666,7 @@ static inline void __run_timers(tvec_base_t *base)
 static unsigned long __next_timer_interrupt(tvec_base_t *base)
 {
         unsigned long timer_jiffies = base->timer_jiffies;
-        unsigned long expires = timer_jiffies + (LONG_MAX >> 1);
+        unsigned long expires = timer_jiffies + NEXT_TIMER_MAX_DELTA;
         int index, slot, array, found = 0;
         struct timer_list *nte;
         tvec_t *varray[4];
@@ -752,6 +752,14 @@ static unsigned long cmp_next_hrtimer_event(unsigned long now,
 
         tsdelta = ktime_to_timespec(hr_delta);
         delta = timespec_to_jiffies(&tsdelta);
+
+        /*
+         * Limit the delta to the max value, which is checked in
+         * tick_nohz_stop_sched_tick():
+         */
+        if (delta > NEXT_TIMER_MAX_DELTA)
+                delta = NEXT_TIMER_MAX_DELTA;
+
         /*
          * Take rounding errors in to account and make sure, that it
          * expires in the next tick. Otherwise we go into an endless