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

[John Stultz]

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@xxxxxxxxxx>
> >
> >                     -------------------
> >      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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