Re: Leap second insertion causes futex to repeatedly timeout

[Ben Blum]

On Sun, Jul 01, 2012 at 01:16:13AM -0700, john stultz wrote:
> On Sat, Jun 30, 2012 at 5:57 PM, Jan Engelhardt <jengelh@xxxxxxx> wrote:
> >
> > This year's leap second insertion has had the strange effect on at least
> > Linux versions 3.4.4 (my end) and 3.5-rc4 (Simon's box, Cc) that certain
> > processes use up all CPU power, because of futexes repeatedly timing
> > out. This seems to only affect certain processes.
> >
> > Simon observes - http://s85.org/owXfmLvt - that
> > Firefox/Thunderbird/Chrome/Java are affected.
> >
> > As for me, it affects VirtualBox, mysqld and ksoftirqd. The processes
> > continue to run and respond. Most weird: I can stop-start mysqld and the
> > issue persists. (I would have expected it to go away because the leap
> > second event would then be in the past that mysqld does not know about
> > anymore.)
> >
> >
> > Is this a kernel issue? glibc?
> 
> Some of the reports that the issue is resolved by calling:
>        $ date -s "`date`"
> suggests that it might be due to clock_was_set() not being called
> after the leap second was added, causing some hrtimer confusion.
> 
> Thomas: does that sound about right?
> 
> I've got an initial patch to add the clock_was_set() calls where
> needed, but so far have not been able to reproduce the issue (tried
> firefox and some simpler futex tests).  I'll keep trying and hopefully
> have something to send out tomorrow.
> 
> Again, my apologies for the trouble.

I can't vouch for whether this is the problem or not, but be very
careful with clock_was_set()! See this commit:

http://www.mail-archive.com/git-commits-head@xxxxxxxxxxxxxxx/msg15039.html

In short, clock_was_set() calls on_each_cpu() which is not allowed to be
called in atomic context. Watch out for xtime_lock.

Ben
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