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

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Message-ID: <4FDF7161.5020108@us.ibm.com>
Date:	Mon, 18 Jun 2012 11:20:17 -0700
From:	John Stultz <johnstul@...ibm.com>
To:	Ben Hutchings <ben@...adent.org.uk>
CC:	Richard Cochran <richardcochran@...il.com>,
	Jonathan Nieder <jrnieder@...il.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 06/18/2012 06:55 AM, Ben Hutchings wrote:
> 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:
>>>
>>> 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.
Yes, I think Ben is right that before the ntp_lock split the potential 
deadlock couldn't happen.


>>> 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?

Yep. It just makes sure the TAI offset is adjusted at the same point 
that the leapsecond is inserted (as opposed to a second late).

>
> 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?
Attached is a simple leap second test you can play with.

thanks
-john


View attachment "leaptest.c" of type "text/x-csrc" (2077 bytes)