[patch 23/26] NTP: remove clock_was_set() call to prevent deadlock

Greg KH
2007-07-31T04:39:53Z

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-stable review patch. If anyone has any objections, please let us know.

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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/

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

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: john stultz <johnstul@us.ibm.com>
Cc: Dave Jones <davej@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Vincent Fortier <Vincent.Fortier1@EC.GC.CA>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
kernel/time/ntp.c | 2 --
1 file changed, 2 deletions(-)

--- linux-2.6.21.6.orig/kernel/time/ntp.c
+++ linux-2.6.21.6/kernel/time/ntp.c
@@ -120,7 +120,6 @@ void second_overflow(void)
*/
time_interpolator_update(-NSEC_PER_SEC);
time_state = TIME_OOP;
- clock_was_set();
printk(KERN_NOTICE "Clock: inserting leap second "
"23:59:60 UTC\n");
}
@@ -135,7 +134,6 @@ void second_overflow(void)
*/
time_interpolator_update(NSEC_PER_SEC);
time_state = TIME_WAIT;
- clock_was_set();
printk(KERN_NOTICE "Clock: deleting leap second "
"23:59:59 UTC\n");
}

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