linux-kernel - Re: [v2.6.34-stable 109/165] ntp: Fix leap-second hrtimer livelock

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Message-ID: <502E8F0B.9000503@windriver.com>
Date:	Fri, 17 Aug 2012 14:35:55 -0400
From:	Paul Gortmaker <paul.gortmaker@...driver.com>
To:	Herton Ronaldo Krzesinski <herton.krzesinski@...onical.com>
CC:	<stable@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	John Stultz <john.stultz@...aro.org>,
	Sasha Levin <levinsasha928@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Prarit Bhargava <prarit@...hat.com>, Willy Tarreau <w@....eu>,
	Richard Cochran <richardcochran@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...nel.org>,
	Ben Hutchings <ben@...adent.org.uk>
Subject: Re: [v2.6.34-stable 109/165] ntp: Fix leap-second hrtimer livelock

On 12-08-17 12:17 PM, Herton Ronaldo Krzesinski wrote:
> On Wed, Aug 15, 2012 at 03:47:33PM -0400, Paul Gortmaker wrote:
>> From: John Stultz <john.stultz@...aro.org>
>>
>>                    -------------------
>>     This is a commit scheduled for the next v2.6.34 longterm release.
>>     http://git.kernel.org/?p=linux/kernel/git/paulg/longterm-queue-2.6.34.git
>>     If you see a problem with using this for longterm, please comment.
>>                    -------------------
>>
>> commit 6b43ae8a619d17c4935c3320d2ef9e92bdeed05d upstream.
>>
>> This should have been backported when it was commited, but I
>> mistook the problem as requiring the ntp_lock changes
>> that landed in 3.4 in order for it to occur.
>>
>> Unfortunately the same issue can happen (with only one cpu)
>> as follows:
>> do_adjtimex()
>>  write_seqlock_irq(&xtime_lock);
>>   process_adjtimex_modes()
>>    process_adj_status()
>>     ntp_start_leap_timer()
>>      hrtimer_start()
>>       hrtimer_reprogram()
>>        tick_program_event()
>>         clockevents_program_event()
>>          ktime_get()
>>           seq = req_seqbegin(xtime_lock); [DEADLOCK]
>>
>> This deadlock will no always occur, as it requires the
>> leap_timer to force a hrtimer_reprogram which only happens
>> if its set and there's no sooner timer to expire.
> [...]
> 
> I remember this came through the leapsecond deadlock & hrtimer/futex
> issue fixes. But I noted now commit 6b1859dba01c7d512b72d77e3fd7da8354235189
> ("ntp: Fix STA_INS/DEL clearing bug") was not included, and its changelog
> says it fixes a problem introduced with this. Wouldn't it be worth/right to
> also include it in 2.6.34? I didn't test etc., just detected through the
> changelog/description but would expect it would be affected too. I also
> noted it is missing from 3.2.y and 2.6.32.y stables, and perhaps should be
> scheduled for inclusion in their cases too.
> 

Thanks again Herton.  I've queued this and I'll keep an eye out for the
pending -tip commit from John.

Paul.

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