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authorRoman Zippel <zippel@linux-m68k.org>2008-09-22 14:42:43 -0700
committerThomas Gleixner <tglx@linutronix.de>2008-09-24 17:33:13 +0200
commit5cd1c9c5cf30d4b33df3d3f74d8142f278d536b7 (patch)
tree6d74ddeff7e9a044d961d36db13071c158f0557a /kernel/time
parenteb3f938fd6292dc79f43a5fe14784b044776e9f0 (diff)
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timekeeping: fix rounding problem during clock update
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. Signed-off-by: Roman Zippel <zippel@linux-m68k.org> Signed-off-by: John Stultz <johnstul@us.ibm.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Diffstat (limited to 'kernel/time')
-rw-r--r--kernel/time/timekeeping.c9
1 files changed, 6 insertions, 3 deletions
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index e91c29f..5ecbfc3 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -454,7 +454,7 @@ 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.
@@ -479,9 +479,12 @@ 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));
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