Commit 746976a3 authored Jul 03, 2007 by Thomas Gleixner Committed by Linus Torvalds Jul 03, 2007

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.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 872aad45

kernel/time/ntp.c

+0 −2

Original line number	Diff line number	Diff line
		@@ -122,7 +122,6 @@ 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");
		}
		@@ -137,7 +136,6 @@ 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");
		}