Re: [PATCH] [RFC] Potential fix for leapsecond caused futex relatedload spikes

[Jan Ceuleers]

On 07/01/2012 11:36 AM, John Stultz wrote:
> I believe this issue is due to the leapsecond being added without
> calling clock_was_set() to notify the hrtimer subsystem of the
> change. (Although I've not yet chased all the way down to the
> hrtimer code to validate exactly what's going on there).

For the benefit of -stable:

Am I right in thinking that, if the analysis is confirmed, this was
caused by the following commit:

commit 746976a301ac9c9aa10d7d42454f8d6cdad8ff2b
Author: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Date:   Tue Jul 3 20:05:20 2007 +0200

NTP: remove clock_was_set() call to prevent deadlock

The clock_was_set() call in seconds_overflow() which happens only when
leap seconds are inserted / deleted is wrong in two aspects:

1. it results in a call to on_each_cpu() with interrupts disabled
2. it is potential deadlock source vs. call_lock in smp_call_function()

The only possible side effect of the removal might be, that an absolute
CLOCK_REALTIME timer fires 1 second too late, in the rare case of leap
second deletion and an absolute CLOCK_REALTIME timer which expires in
the affected time frame. It will never fire too early.

This was probably observed by the reporter of a June 30th -> July 1st
hang: http://lkml.org/lkml/2007/7/3/103

A similar problem was observed by Dave Jones, who provided a screen shot
with a lockdep back trace, which allowed to analyse the problem.

Sob: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Ab: Ingo Molnar <mingo@xxxxxxx>
Sob: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
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