linux-kernel - Re: Bug: Status/Summary of slashdot leap-second crash on new years 2008-2009

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Date:	Mon, 5 Jan 2009 22:35:29 -0600
From:	"Linas Vepstas" <linasvepstas@...il.com>
To:	"john stultz-lkml" <johnstul.lkml@...il.com>
Cc:	"Chris Adams" <cmadams@...aay.net>, linux-kernel@...r.kernel.org,
	"Thomas Gleixner" <tglx@...utronix.de>
Subject: Re: Bug: Status/Summary of slashdot leap-second crash on new years 2008-2009

2009/1/5 john stultz-lkml <johnstul.lkml@...il.com>:
> On Fri, Jan 2, 2009 at 4:21 PM, Chris Adams <cmadams@...aay.net> wrote:
>> Basically (to my untrained eye), the leap second code is called from the
>> timer interrupt handler, which holds xtime_lock.  The leap second code
>> does a printk to notify about the leap second.  The printk code tries to
>> wake up klogd (I assume to prioritize kernel messages), and (under some
>> conditions), the scheduler attempts to get the current time, which tries
>> to get xtime_lock => deadlock.
>
> This analysis looks correct to me.
>
> Grrrr. This has bit us a few times since the "no printk while holding
> the xtime lock" restriction was added.
>
> Thomas: Do you think this warrents adding a check to the printk path
> to make sure the xtime lock isn't held?

No.

> This way we can at least get a
> warning when someone accidentally adds a printk or calls a function
> that does while holding the xtime_lock.

This seems like a basic mistake, that should be avoidable
with code review.  I'm sort-of surprised to even see it; anyone
even vaguely familiar with that code would spot it quickly.
Heh. Take that with a grain of salt -- not like I never make
mistakes ;-/

I mean, how many more times can the mistake be made?
I'm arguing its gonna be zero.

--linas
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