linux-kernel - Re: 2.6.39 crashes BUG: unable to handle kernel NULL pointer dereference at 000000000000042 .. cmos_checkintr+0x4d/0x55 under Xen as PV guest.

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Date:	Fri, 18 Mar 2011 22:51:35 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	tglx@...utronix.de, linux-kernel@...r.kernel.org,
	xen-devel@...ts.xensource.com
Subject: Re: 2.6.39 crashes BUG: unable to handle kernel NULL pointer
 dereference at 000000000000042 .. cmos_checkintr+0x4d/0x55 under Xen as PV
 guest.

On Fri, Mar 18, 2011 at 02:59:26PM -0700, John Stultz wrote:
> On Fri, 2011-03-18 at 16:38 -0400, Konrad Rzeszutek Wilk wrote:
> > Haven't done any bisection, but looking at the latest set of
> > patches John's name is on them. (John, congrats on a new job)
> 
> No new job, I'm still at IBM. Just a new email address, as I'm working
> as part of the Linaro effort. My old address still work too, and I'll
> continue to use them for non-Linaro work.
> 

Aah, OK.
> 
> > With the latest linus/master I get this when starting a Xen Linux PV
> > guest:
> > 
> > [    0.404760] initcall psmouse_init+0x0/0x79 returned 0 after 59 usecs
> > [    0.404767] calling  cmos_init+0x0/0x6a @ 1
> > [    0.464855] BUG: unable to handle kernel NULL pointer dereference at 0000000000000428
> > [    0.464867] IP: [<ffffffff8105d347>] queue_work_on+0x4/0x1d
> [snip]
> > [    0.465018] Call Trace:
> > [    0.465023]  [<ffffffff8105d38f>] queue_work+0x1a/0x1c
> > [    0.465029]  [<ffffffff8105d3a4>] schedule_work+0x13/0x15
> > [    0.465035]  [<ffffffff81331b2e>] rtc_update_irq+0x10/0x12
> > [    0.465041]  [<ffffffff81333939>] cmos_checkintr+0x4d/0x55
> > [    0.465047]  [<ffffffff81333987>] cmos_irq_disable+0x46/0x4e
> > [    0.465051]  [<ffffffff8133481d>] cmos_set_alarm+0xd9/0x16e
> > [    0.465051]  [<ffffffff813320a4>] __rtc_set_alarm+0x7d/0x88
> > [    0.465051]  [<ffffffff813321fa>] rtc_timer_enqueue+0x71/0xb8
> > [    0.465051]  [<ffffffff81331707>] ? rtc_tm_to_time+0x2f/0x38
> > 
> > ... full log at the end.
> > 
> > From a brief look it looks as if rtc_device_register was never
> > called, so
> > 
> > INIT_WORK(&rtc->irqwork, rtc_timer_do_work);
> > 
> > was never called.. and hence schedule_work tries to derefence an
> > unitialized rtc->irqwork.
> > 
> > Which actually sounds right - the rtc_device_register should not
> > be called since there are no RTC clocks exposed.
> 
> 
> Huh. Did you see this with 2.6.38 vanilla? Just want to clarify if this

No. 2.6.38 vaniall works great.
> is 2.6.39 only or not.

It is something new.
> 
> 
> > There are probably two ways of fixing this - making rtc_update_irq
> > check the rtc->irqwork (not attempted) or inhibit cmos_pnp_probe from
> > setting this up. Looking at the cmos_pnp_probe and its friend they all
> > call cmos_wake_setup, but never checks whether that function works properly.
> > 
> > The cmos_wake_setup checks for ACPI (which is disabled for PV guests)
> > and just returns.
> > 
> > This little patch seems to work, but not sure if that is the correct
> > way to do it?
> 
> So I'm still trying to get my head around this (sorry, just back from
> vacation).

No problem. I just noticed it today.
> 
> So the issue is that somehow the cmos code is calling rtc_update_irq
> even though there is no cmos rtc device registered. That clearly seems
> problematic.
> 
> However, its unclear from both the code and your patch if
> cmos_wake_setup or cmos_do_probe is causing the rtc_update_irq to be
> called.

> cmos_do_probe() has lots of checks for the hardware and even registers
> the rtc device (which should init the irqwork), so I don't see how the
> null irqwork would trip after that point.

It isn't a pointer actually. I thought it would but then I realized it
was a 'struct workqueue' which just hadn't been initialized.

> 
> Any insight there?

I hoped you might have :-)
> 
> thanks
> -john
> 
> 
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