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

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Message-ID: <1f1b08da0901051821q31a5c98akc5165aac36c6201e@mail.gmail.com>
Date:	Mon, 5 Jan 2009 18:21:36 -0800
From:	"john stultz-lkml" <johnstul.lkml@...il.com>
To:	"Chris Adams" <cmadams@...aay.net>
Cc:	"Linas Vepstas" <linasvepstas@...il.com>,
	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

On Fri, Jan 2, 2009 at 4:21 PM, Chris Adams <cmadams@...aay.net> wrote:
> Once upon a time, Linas Vepstas <linasvepstas@...il.com> said:
>> Below follows a summary of the reported crashes. I'm ignoring the
>> zillions of "mine didn't crash" reports, or the "you're a paranoid
>> conspiracy theorist, its random chance" reports.
>
> I have reproduced this and got a stack trace (this is with Fedora 8 and
> kernel kernel-2.6.26.6-49.fc8.x86_64):
>
[snip]
> 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? 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.

thanks
-john
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