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SubjectRe: [v2.6.34-stable 109/165] ntp: Fix leap-second hrtimer livelock
On 08/17/2012 09:17 AM, Herton Ronaldo Krzesinski wrote:
> On Wed, Aug 15, 2012 at 03:47:33PM -0400, Paul Gortmaker wrote:
>> From: John Stultz <john.stultz@linaro.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.

Yes. I also have another change pending in -tip that I've marked for
stable. Once that lands I'm planning on reviewing the current stable
trees to check that they have all the needed changes.

thanks
-john



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