Re: Hard lockups using 3.10.0

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jul 11, 2013 at 12:07:21PM +0200, Borislav Petkov wrote:
> On Thu, Jul 11, 2013 at 11:38:37AM +0200, Rolf Eike Beer wrote:
> > Hi,
> > 
> > I'm running 3.10.0 (from openSUSE packages) on an "Intel(R) Core(TM) i7-2600 
> > CPU @ 3.40GHz". I got a hard lockup on one of my CPUs twice, once with 
> > backtrace (see attached image). Graphics is the builtin Intel, used with X 7.6 
> > and KDE 4.10beta2 (basically current openSUSE 12.3+KDE).
> > 
> > I'm not aware that I had done anything special, just "normal" desktop and 
> > development usage, but no heavy compile work at the moment the lockups 
> > happened.
> 
> Hmm, I can see commit_creds() doing some rcu pointers assignment and rcu
> calling into the scheduler which screams about a cpu runqueue of the
> task we're about to reschedule not being locked. Let's add some more
> people who should know better.

Ok, for the other people too lazy to bother finding the picture:

  http://marc.info/?l=linux-kernel&m=137353587012001&q=p3

So we bug at:

kernel/sched/core.c:519 assert_raw_spin_locked(&task_rq(p)->lock);

and get there through:

  resched_task()
  check_preempt_wakeup()
  check_preempt_curr()
  try_to_wake_up()
  autoremove_wake_function()
  __call_rcu_nocb_enqueue()
  __call_rcu()
  commit_creds()
  ____call_usermodehelper()
  ret_from_fork()

That don't make much sense though. Since:

  try_to_wake_up()
    ttwu_queue()
      raw_spin_lock(&rq->lock)
      ttwu_do_activate()
        ttwu_do_wakeup()
          check_preempt_curr()
            check_preempt_wakeup()
              resched_task(rq->curr)
                assert_raw_spin_locked(task_rq(p)->lock)

It would somehow mean that 'task_rq(rq->curr) != rq', that's completely
bonkers, we do after all have rq->lock locked.

I must also say that I've _never_ seen this bug before.