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    SubjectRe: Regression: ONE CPU fails bootup at Re: [3.2.0-RC7] BUG: unable to handle kernel NULL pointer dereference at 0000000000000598 1.478005] IP: [<ffffffff8107a6c4>] queue_work_on+0x4/0x30
    Hello Stefan,

    Wednesday, January 4, 2012, 1:25:16 PM, you wrote:

    > On 04.01.2012 09:17, Stefan Bader wrote:
    >> On 04.01.2012 01:53, John Stultz wrote:
    >>> On Wed, 2012-01-04 at 11:31 +1100, NeilBrown wrote:
    >>>> On Tue, 03 Jan 2012 15:09:48 -0800 John Stultz <> wrote:
    >>>>> >From the stack trace, we've kicked off a rtc_timer_do_work, probably
    >>>>> from the rtc_initialize_alarm() schedule_work call added in Neil's
    >>>>> patch. From there, we call __rtc_set_alarm -> cmos_set_alarm ->
    >>>>> cmos_rq_disable -> cmos_checkintr -> rtc_update_irq -> schedule_work.
    >> Sorry, I was off for the evening a while after sending this out. And I just
    >> started, so a few thing I will be doing later but have not yet had time.
    >> Over night I had still be thinking on this and maybe one important fact I had
    >> been ignoring. This really has only been observed on paravirt guests on Xen as
    >> far as I know. And one thing that I should have pointed out is that
    >> [ 0.792634] rtc_cmos rtc_cmos: rtc core: registered rtc_cmos as rtc0
    >> [ 0.792725] rtc_cmos: probe of rtc_cmos failed with error -38
    >> So first the registration is done and the first line is the last thing printed
    >> in the registration function. Then, and that line always comes after, the probe,
    >> which looks like being done asynchronously, detects that the rtc is not
    >> implemented. I would assume that this causes the rtc to be unregistered again
    >> and that is probably the point where, under the right circumstances, the worker
    >> triggered by the initialize alarm is trying to set another alarm. Probably while
    >> some of the elements of the structure started to be torn down. I need to check
    >> on that code path, yet. So right now its more a guess.
    >>>>> So, what it looks to me is that in cmos_checkintr, we grab the cmos->rtc
    >>>>> and pass that along. Unfortunately, since the cmos->rtc value isn't set
    >>>>> until after rtc_device_register() returns its null at that point. So
    >>>>> your patch isn't really fixing the issue, but just reducing the race
    >>>>> window for the second cpu to schedule the work.
    >>>>> Sigh. I'd guess dropping the schedule_work call from
    >>>>> rtc_initialize_alarm() is the right approach (see below). When reviewing
    >>>>> Neil's patch it seemed like a good idea there, but it seems off to me
    >>>>> now.
    >>>>> Neil, any thoughts on the following? Can you expand on the condition you
    >>>>> were worried about in around that call?
    >>>> If you set an alarm in the future, then shutdown and boot again after that
    >>>> time, then you will end up with a timer_queue node which is in the past.
    >>> Thanks for explaining this again.
    >>> Hrm. It seems the easy answer is to simply not add alarms that are in
    >>> the past. Further, I'm a bit perplexed, as if they are in the past, the
    >>> enabled flag shouldn't be set. __rtc_read_alarm() does check the
    >>> current time, so maybe we can make sure we don't return old values? I
    >>> guess I assumed __rtc_read_alarm() avoided returning stale values, but
    >>> apparently not.
    >>>> When this happens the queue gets stuck. That entry-in-the-past won't get
    >>>> removed until and interrupt happens and an interrupt won't happen because the
    >>>> RTC only triggers an interrupt when the alarm is "now".
    >>>> So you'll find that e.g. "hwclock" will always tell you that 'select' timed
    >>>> out.
    >>>> So we force the interrupt work to happen at the start just in case.
    >>> Unfortunately its too early.
    >>>> Did you see my proposed patch which converted those calls to do the work
    >>>> in-process rather than passing it to a worker-thread? I think that is a
    >>>> clean fix.
    >>> I don't think I saw it today. Was it from before the holidays?
    >> I fear I caused a bit of confusion there. Neil responded to my initial mail
    >> which was done as a reply to the mail announcing this patch for stable (which
    >> just was the first thread I could get hold of).
    >> I will try Neil's patch as well. And in parallel try to see whether the theory I
    >> had this night makes sense. If it does, then it is only indirectly that the work
    >> is scheduled too early. In that case just the teardown needs to make sure that
    >> no work is being run while removal. Well, maybe the question is whether there
    >> should be a delay in running the irq work until the device really, really is
    >> completely set up... But that sounds a bit more complicated.

    > By now I tried Neil's proposed patch and unfortunately that makes things rather
    > worse. I also played around with the idea of the unregistration race. Maybe
    > there also is one (that cancel_work_sync should be called before unregistering
    > the device) but definitely it is not what happens at least in the one CPU case.
    > I added some more printk's and the crash happens before even the rtc core class
    > has been fully registered. And no unregister is call has been made either.

    > Which may point to execution of the irq worker (including a schedule_work)
    > before the rtc-cmos parts are finished... Would explain why moving the
    > initialize call further down does at least narrow the window for it to happen...
    > The only thing I do not understand then is why that seems only to happen on Xen
    > guests...

    > -Stefan

    Has there been a request to revert this for 3.2 final gone to Linus ?

    >>> Even so, at this point, I don't know if we have enough time for testing,
    >>> so I'm thinking we either just drop the problematic sched_work call or
    >>> revert the whole thing and try again for 3.3
    >> That was the reason I was in a bit of hurry to get this back to you. Especially
    >> since this patch had been marked as stable material and sooner or later will or
    >> would be added to all the stable releases it applies to.
    >> Thanks,
    >> Stefan
    >>> thanks
    >>> -john

    Best regards,

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