Re: BUG spinlock lockup, rtc related, 3.0-rc7+

[Ben Greear <greearb@candelatech.com>]

On 07/19/2011 03:17 PM, john stultz wrote:
> On Wed, Jul 13, 2011 at 10:29 AM, Ben Greear<greearb@candelatech.com>  wrote:
>> This is on the same nfs testing machine I've been posting about.  This
>> has some additional nfs patches included, running tests to mount, do io,
>> unmount
>> over and over again.  Seems that the NFS bugs might be finally fixed, but
>> system is still un-stable in general when under load.
>>
>> This info was printed after several other warnings that I previously posted
>> to lkml.
>>
>> This one appears to lock up the machine pretty badly though...can't ssh into
>> it anymore, and similar messages keep spewing every few minutes.
>>
>> I *think* the BUG at the end of this email is the important part, but
>> maybe it's just a symptom of something else...
>
> Huh. So does this trigger frequently, or was this just a one time
> thing? I suspect the latter.

It seems I have been hitting a lot of rcu-boost locking issues
on this system with my nfs mount/unmount testing.

The system was having various lockups and bugs, but I don't think
I saw this particular one more than once or perhaps twice.

I plan to run some more tests with the rcu-boost locking fixes
applied to the kernel shortly.

At the time I reported this, I wasn't aware of the rcu boost bugs,
but perhaps that is root cause here as well...I don't know enough
about the code in question to make an educated guess.

>> From the looks of it, there's the btserver process (on cpu4) which
> during exit is caught up spinning trying to get the hrtimer base lock
> from hrtimer_cancel() in rtc_irq_set_state() when cleaning up from
> rtc_device_release().
>
> Meanwhile, On cpu0, a rtc periodic timer has fired and we're stuck in
> rtc_handle_legacy_irq(), likely waiting for the irq_task_lock held by
> cpu4 in rtc_irq_set_state().
>
> The rest of the cpus are idle, with the exception of the one that
> detected the stall from the normal timer tick.
>
> Hrmm.. It sounds like a circular lock between the rtc->irq_task_lock
> and the hrtimer base lock.
>
> rtc_irq_set_state: Grab irq_task_lock ->  call hrtimer_cancel ->  grab
> hrtimer_base_lock
>
> IRQ:  grab hrtimer_base_lock ->  run timers ->  rtc_handle_legacy_irq ->
> grab irq_task_lock
>
> But looking at __run_hrtimer(), the base lock should be released
> before the timer is run.
>
> So I'm not really sure what would be gumming up things here.
>
> Thomas: Any thoughts? There shouldn't be an issue calling
> hrtimer_cancel or other hrtimer operations from an  hrtimer handler
> right?
>
> thanks
> -john


-- 
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc  http://www.candelatech.com