lkml.org 
[lkml]   [2012]   [Apr]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Subject[PATCH 0/1] ntp bug fix
Date
I have been cooking up a patch series to keep the core time in TAI
instead of UTC. In the course of that work I noticed a bug in the
current NTP code. During a leap second insertion, the kernel will
advertise the wrong TAI offset.

There is a nice explanation of this issue in the NIST leap second
list, which I quote below. With reference to that, the Linux kernel
handles the UTC time value like in the first case, but sets the TAI
offset as in the second case, resulting in a discontinuity in the
apparent TAI time.

* Excerpt from ftp://time.nist.gov/pub/leap-seconds.list

If your system keeps time as the number of seconds since some epoch
(e.g., NTP timestamps), then the algorithm for assigning a UTC time
stamp to an event that happens during a positive leap second is not
well defined. The official name of that leap second is 23:59:60,
but there is no way of representing that time in these systems.

Many systems of this type effectively stop the system clock for one
second during the leap second and use a time that is equivalent to
23:59:59 UTC twice. For these systems, the corresponding TAI
timestamp would be obtained by advancing to the next entry in the
following table when the time equivalent to 23:59:59 UTC is used
for the second time. Thus the leap second which occurred on 30 June
1972 at 23:59:59 UTC would have TAI timestamps computed as follows:

30 June 1972 23:59:59 (2287785599, first time): TAI= UTC + 10 seconds
30 June 1972 23:59:60 (2287785599,second time): TAI= UTC + 11 seconds
1 July 1972 00:00:00 (2287785600) TAI= UTC + 11 seconds

If your system realizes the leap second by repeating 00:00:00 UTC
twice (this is possible but not usual), then the advance to the
next entry in the table must occur the second time that a time
equivlent to 00:00:00 UTC is used. Thus, using the same example as
above:

30 June 1972 23:59:59 (2287785599): TAI= UTC + 10 seconds
30 June 1972 23:59:60 (2287785600, first time): TAI= UTC + 10 seconds
1 July 1972 00:00:00 (2287785600,second time): TAI= UTC + 11 seconds

in both cases the use of timestamps based on TAI produces a smooth
time scale with no discontinuity in the time interval.


Richard Cochran (1):
ntp: advertise correct TAI offset during leap second

kernel/time/ntp.c | 2 +-
1 files changed, 1 insertions(+), 1 deletions(-)

--
1.7.2.5



\
 
 \ /
  Last update: 2012-04-26 14:15    [W:0.057 / U:2.312 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site