linux-kernel - Re: [PATCH -stable] ntp: Correct TAI offset during leap second

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Message-ID: <1340027711.9372.29.camel@deadeye.wl.decadent.org.uk>
Date:	Mon, 18 Jun 2012 14:55:11 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Richard Cochran <richardcochran@...il.com>
Cc:	Jonathan Nieder <jrnieder@...il.com>,
	John Stultz <johnstul@...ibm.com>, stable@...r.kernel.org,
	Sasha Levin <levinsasha928@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Dave Jones <davej@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -stable]  ntp: Correct TAI offset during leap second

On Sun, 2012-06-17 at 19:34 +0200, Richard Cochran wrote:
> On Sun, Jun 17, 2012 at 11:47:51AM -0500, Jonathan Nieder wrote:
> > Ben Hutchings wrote:
> > > On Fri, 2012-06-15 at 11:56 -0700, John Stultz wrote:
> > 
> > >> commit dd48d708ff3e917f6d6b6c2b696c3f18c019feed upstream.
> > [...]
> > > This doesn't apply to 3.2.y, unsurprisingly.  Let me know if there are
> > > any urgent leap second fixes that will be needed there.
> > 
> > 6b43ae8a619d (ntp: Fix leap-second hrtimer livelock) sounds important,
> > but the patch depends on bd3312681f69 (ntp: Add ntp_lock to replace
> > xtime_locking) which does not have a commit message explaining its
> > purpose (and that patch in turn depends on ea7cf49a7633).

If I understand the commit message for 6b43ae8a619d correctly, the
livelock results from ntp_lock and xtime_lock being acquired in opposite
orders in two threads.  Which means it wasn't possible before ntp_lock
was introduced in bd3312681f69.

> > John, is that bug present in 3.2.y and 3.0.y, too?  Any hints for
> > fixing it?
> 
> It looks like incrementing the TAI offset was wrong even before 
> 
>    6b43ae8a ntp: Fix leap-second hrtimer livelock  v3.4-rc1~44^2~9
> 
> The offset should change upon entering state OOP, so something like
> the following (untested) patch should fix it for 3.2.9.
[...]

It looks like this patch just changes the offset reported by adjtimex()
during an inserted second; is that right?

Other than that, is 3.2.y likely to be OK?  Is there a good way to test
that in advance; does
<http://codemonkey.org.uk/2012/06/15/testing-leap-code/> look
reasonable?

Ben.

-- 
Ben Hutchings
If more than one person is responsible for a bug, no one is at fault.

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