fix rounding problem during clock update

So I had assumed this was picked up already, but apparently it somehow got lost in the early 2.6.26 time-frame.

Anyway, I pinged Roman about it, but he hasn't responded, so I wanted to send this on for inclusion into -mm (I've not had much time to test with it, so it probably could use some wider testing) and then hopefully 2.6.27.

thanks
-john

From: Roman Zippel <[email protected]>

Due to a rounding problem during a clock update it's possible for readers to observe the clock jumping back by 1nsec. The following simplified example demonstrates the problem:

cycle xtime
0 0
1000 999999.6
2000 1999999.2
3000 2999998.8
...

1500 = 1499999.4
= 0.0 + 1499999.4
= 999999.6 + 499999.8

When reading the clock only the full nanosecond part is used, while timekeeping internally keeps nanosecond fractions. If the clock is now updated at cycle 1500 here, a nanosecond is missing due to the truncation.
The simple fix is to round up the xtime value during the update, this also changes the distance to the reference time, but the adjustment will automatically take care that it stays under control.

---
 kernel/time/timekeeping.c |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

Index: linux-2.6-mm/kernel/time/timekeeping.c
===================================================================
@@ -484,7 +484,7 @@ void update_wall_time(void)
 #else
 	offset = clock->cycle_interval;
 #endif
-	clock->xtime_nsec += (s64)xtime.tv_nsec << clock->shift;
+	clock->xtime_nsec = (s64)xtime.tv_nsec << clock->shift;
 
 	/* normally this loop will run just once, however in the
 	 * case of lost or late ticks, it will accumulate correctly.

@@ -515,9 +515,12 @@ void update_wall_time(void)
 	/* correct the clock when NTP error is too big */
 	clocksource_adjust(offset);
 
-	/* store full nanoseconds into xtime */
-	xtime.tv_nsec = (s64)clock->xtime_nsec >> clock->shift;
+	/* store full nanoseconds into xtime after rounding it up and
+	 * add the remainder to the error difference.
+	 */
+	xtime.tv_nsec = ((s64)clock->xtime_nsec >> clock->shift) + 1;
 	clock->xtime_nsec -= (s64)xtime.tv_nsec << clock->shift;
+	clock->error += clock->xtime_nsec << (NTP_SCALE_SHIFT - clock->shift);
 
 	update_xtime_cache(cyc2ns(clock, offset));
 

Hi,

Sorry about that, I wanted to update and test the patches over the weekend, but other things kept me busy. I'll sent the other patches out this week.

bye, Roman