Re: Leap second insertion causes futex to repeatedly timeout

From: John Stultz
Date: Sun Jul 01 2012 - 05:07:44 EST

On 07/01/2012 01:36 AM, Ben Blum wrote:
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 - - 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

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:

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

Quite right. The fix is a little awkward due to the need to call it outside of holding xtime_lock/timekeeper.lock.

I've just reproduced the issue w/ Thunderbird, and my fix seems to avoid the issue. Working up a patch now.


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at