linux-kernel - Re: 2.6.21.5 june 30th to july 1st date hang?

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Message-Id: <200707062003.l66K3McB001808@pasta.boston.redhat.com>
Date:	Fri, 06 Jul 2007 16:03:22 -0400
From:	Ernie Petrides <petrides@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
cc:	Chris Friesen <cfriesen@...tel.com>,
	Clemens Koller <clemens.koller@...gramm.de>,
	Chris Wright <chrisw@...s-sol.org>,
	Uli Luckas <u.luckas@...d.de>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: 2.6.21.5 june 30th to july 1st date hang?

On Friday, 6-Jul-2007 at 7:17 +0200, Thomas Gleixner wrote:

> On Thu, 2007-07-05 at 19:12 -0400, Ernie Petrides wrote:
> > On Thursday, 5-Jul-2007 at 16:49 MDT, Chris Friesen wrote:
> >
> > > Ernie Petrides wrote:
> > >
> > > > Only kernels built with the CONFIG_HIGH_RES_TIMERS option enabled were
> > > > vulnerable.
> > >
> > > As I mentioned in my post to Thomas, we have high res timers disabled
> > > and were still affected.  Granted, our kernel has been modified so it is
> > > possible that vanilla would not be affected....I haven't tested it.
> > >
> > > Chris
> >
> > That's odd, because Thomas's patch removed two calls to clock_was_set(),
> > which is a no-op when CONFIG_HIGH_RES_TIMERS is not enabled (at least in
> > the 2.6.21 source tree).
> >
> > Also, I personally tested with the reproducer you posted here, initially
> > on a box running 2.6.22-rc4, and there were no problems (but I'm not sure
> > what config options were enabled on that kernel).  I did reproduce the
> > problem on a stock 2.6.21 kernel with CONFIG_HIGH_RES_TIMERS enabled.
>
> It needs a running smp_call_function() to be interrupted by the timer
> interrupt, which calls clock_was_set(). So it's not that easy to
> reproduce.

I think it's reproducible at will when CONFIG_BUG is enabled, because the
WARN_ON() on line 546 of arch/i386/kernel/smp.c fires in smp_call_function(),
causing lots of console output.  By the time on_each_cpu() later reenables
interrupts, another clock interrupt is pending, and (I think) causes a
self-deadlock on the xtime_lock in vmi_account_real_cycles().

That's my (unproven) theory, anyway.  :)

At any rate, I reproduced it twice in two tries on stock 2.6.21.

Cheers.  -ernie
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