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SubjectRe: 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, 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.

> 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
> [ 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
is 2.6.39 only or not.

> 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

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

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

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.

Any insight there?


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