Messages in this thread | | | Date | Sun, 8 Jan 2012 21:48:12 +0100 | From | Sander Eikelenboom <> | Subject | Re: 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 |
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Friday, January 6, 2012, 9:41:15 PM, you wrote:
> On Wed, 2012-01-04 at 15:13 +0100, Stefan Bader wrote: >> On 04.01.2012 13:25, Stefan Bader 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 <john.stultz@linaro.org> 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... >> > >> >> Darn, guess I understand now... So cmos-rtc will call rtc_device_register from >> within do_cmos_probe and set cmos_rtc.rtc with the pointer that >> rtc_device_register returns. >> But when having rtc_initialize_alarm being called earlier in >> rtc_device_register, and that queues the work, there is a chance it will fire >> even before that function returns. >> And then cmos_checkintr will dereference the null pointer still stored in >> cmos_rtc.rtc for calling rtc_update_irq...
> Hey Stefan, Konrad, Sander, Neil, > Thanks again for the bug reporting and sorry for the earlier trouble. > So after the problematic patch was reverted for 3.2, I worked with Neil > to try to address the issue in a better way. It looks like its working > for Neil, so I was wondering if any of you could look over and maybe > give the following tree a whirl: > git://git.linaro.org/people/jstultz/linux.git dev/rtc-fixups
> The shortlog is here: > http://git.linaro.org/gitweb?p=people/jstultz/linux.git;a=shortlog;h=refs/heads/dev/rtc-fixups
> If this avoids the issue for you, and no one sees any other problems, > I'll queue these for 3.3
> thanks > -john
Boots both with single as with multiple CPU Xen guests
Tested-by: Sander Eikelenboom <linux@eikelenboom.it>
thanks,
-- Sander
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