linux-kernel - Re: [PATCH] NTP: remove clock_was

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Message-ID: <20070703194106.GA493@elte.hu>
Date:	Tue, 3 Jul 2007 21:41:06 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Linus Torvalds <torvalds@...l.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>, Andrew Morton <akpm@...l.org>,
	john stultz <johnstul@...ibm.com>,
	Stable Team <stable@...nel.org>,
	"Fortier,Vincent [Montreal]" <Vincent.Fortier1@...GC.CA>
Subject: Re: [PATCH] NTP: remove clock_was_set() call to prevent deadlock


* Thomas Gleixner <tglx@...utronix.de> wrote:

> 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>

looks good to me.

Acked-by: Ingo Molnar <mingo@...e.hu>

	Ingo
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