touch-packages team mailing list archive

[Chris J Arges <1125726@xxxxxxxxxxxxxxxxxx>]

The verification of the Stable Release Update for ntp has completed
successfully and the package has now been released to -updates.
Subsequently, the Ubuntu Stable Release Updates Team is being
unsubscribed and will not receive messages about this bug report.  In
the event that you encounter a regression using the package from
-updates please report a new bug using ubuntu-bug and tag the bug report
regression-update so we can easily find any regressions.

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https://bugs.launchpad.net/bugs/1125726

Title:
  boot-time race between /etc/network/if-up.d/ntpdate and
  "/etc/init.d/ntp start"

Status in ntp package in Ubuntu:
  Fix Released
Status in ntp source package in Precise:
  Fix Released
Status in ntp source package in Trusty:
  Fix Released

Bug description:
  [Impact] 
  * Hardware clocks are not stepped at boot, which can prevent NTP from ever
    syncing the clock.
    Incorrect clocks can cause serious issues in distributed systems.

  * Upstream originally added a lock file to eliminate a race between the ntp
    service (which keeps the clock synchronized during normal operation) and
    ntpdate (which is used to step the clock by large intervals at boot time).
    That change had a flaw which introduced a deadlock. An Ubuntu patch was
    applied which broke the locking mechanism entirely, reintroducing the race
    condition.

  * This change undoes the Ubuntu patch and fixes the deadlock by unlocking
    before attempting to start the ntp service.

  [Test Case]

  * There are two bugs: The race, and the deadlock. To reproduce the race more
    consistently:
    - add 'sleep 30' to '/etc/network/if-up.d/ntpdate' on the line preceding
      '/usr/sbin/ntpdate-debian -s $OPTS 2>/dev/null || :', and comment out
      'invoke-rc.d --quiet $service stop >/dev/null 2>&1 || true'. This will
      reproduce the case where the ntp service starts between the stop command
      and the ntpdate command.
      The result will be that the ntpdate command fails. There will be a
      message in syslog like:
        'ntpdate[17660]: the NTP socket is in use, exiting'
    - Reintroducing the lock brings back the deadlock issue. Both the ntpdate
      if-up.d script and the ntp init script check the lock file, but the
      ntpdate script attempted to start the ntp init script before unlocking
      the lock. Moving the unlock before the init script invocation fixes
      the deadlock. The original deadlock behavior is described here:
        https://bugs.launchpad.net/ubuntu/+source/ntp/+bug/246203

  [Regression Potential]

  * Low. Out-of-sync clocks could be changed a large amount at boot time, but
    only for machines with static IP's. The clock is only likely to be in this
    state if the clock was very skewed at boot time, which is also unlikely
    since NTP usually keeps the software clock in sync during operation and
    the hardware clock is updated at shutdown.

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