linux-kernel - Re: [Xen-devel] 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:	Thu, 24 Mar 2011 08:27:55 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	tglx@...utronix.de, xen-devel@...ts.xensource.com,
	linux-kernel@...r.kernel.org
Subject: Re: [Xen-devel] 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 Tue, Mar 22, 2011 at 10:38:41AM -0400, Konrad Rzeszutek Wilk wrote:
> > > No. 2.6.38 vaniall works great.
> > 
> > Ok. Hrm. 
> > 
> > > > Any insight there?
> > > 
> > > I hoped you might have :-)
> > 
> > Could you help me understand where in the probe logic xen bombs out of
> > the cmos code?
> 
> Sure. The issue is that rtc_update_irq calls schedule_work with rtc->irqwork
> which has not been initialized. The reason for that is that rtc_device_register
> has never been called.. uh wait, that does not make sense, it is called in
> cmos_do_probe. Hmm, let get find out exactly on which variable queue_work_on
> bombs out on.

The problem is this:

cmos_do_probe does:

 	cmos_rtc.dev = dev; 
        dev_set_drvdata(dev, &cmos_rtc);

which means that dev->p->private_data contains cmos_rtc. And
dev->p->private_data->rtc is a NULL pointer. The next function:

        cmos_rtc.rtc = rtc_device_register(driver_name, dev, 
                                &cmos_rtc_ops, THIS_MODULE);

'rtc_device_register' creates an 'rtc' structure and sets 
its parent to be:
	rtc->dev.parent = dev;

and later on it does:
 if (!err && !rtc_valid_tm(&alrm.time))
                rtc_set_alarmtrtc, &alrm);

rtc_set_alarm calls rtc_timer_enqueue which calls __rtc_set_alarm.
__rtc_set_alarms calls 'cmos_set_alarm' via:
 err = rtc->ops->set_alarm(rtc->dev.parent, alarm);

which is basically passing in 'dev' to 'cmos_set_alarm', and
'cmos_set_alarm' uses the dev to:
        struct cmos_rtc *cmos = dev_get_drvdata(dev);

(so get the from dev->p->private_data the cmos_rtc).
get the 'cmos' (which is what 'cmos_rtc'). Great... except
then it ends up trying to dereference cmos->rtc.irqwork (via
cmos_irq_disable(cmos, .. and somehere in its chain calls
schedule_work(cmos->rtc) whcih ends up blowing up b/c
cmos_rtc.rtc has not been set yet.

The cmos_rtc.rtc is set when the when 'rtc_device_register'
finish, which it hadn't yet done.

git gui blame tells me to look at 
 f44f7f96a20af16f6f12e1c995576d6becf5f57b
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