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

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Date:	Wed, 04 Jul 2007 13:15:16 +0200
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Roman Zippel <zippel@...ux-m68k.org>
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>, Ingo Molnar <mingo@...e.hu>,
	Stable Team <stable@...nel.org>,
	"Fortier,Vincent [Montreal]" <Vincent.Fortier1@...gc.ca>
Subject: Re: [PATCH] NTP: remove clock_was_set() call to prevent deadlock

On Wed, 2007-07-04 at 13:06 +0200, Roman Zippel wrote:
> Hi,
> 
> On Tuesday 03 July 2007, Thomas Gleixner 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.
> 
> That's a bit of an easy solution and doesn't fix the real problem. The 
> clock_was_set() calls were correct, what's broken is the locking. Why wasn't 
> that fixed instead?
> I would at least like to see a comment there, why these calls were removed.

We can not fix the locking that late in the cycle. I doubt that we can
fix it at all, as smp_call_function _must_ run with interrupts enabled,
therefor it can not run in interrupt context. It needs to run from
thread context only.

	tglx




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