linux-kernel - [PATCH] NTP: remove clock_was_set() call to prevent deadlock

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Date:	Tue, 03 Jul 2007 20:05:20 +0200
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Linus Torvalds <torvalds@...l.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, Dave Jones <davej@...hat.com>,
	Andrew Morton <akpm@...l.org>,
	john stultz <johnstul@...ibm.com>, Ingo Molnar <mingo@...e.hu>,
	Stable Team <stable@...nel.org>,
	"Fortier,Vincent [Montreal]" <Vincent.Fortier1@...GC.CA>
Subject: [PATCH] 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/

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@...utronix.de>

--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -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");
 		}

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