Re: 2.6.39.4: Oops in rcu_read_unlock_special()/_raw_spin_lock()

["Will Simoneau" <simoneau@ele.uri.edu>]

On 07:07 Thu 25 Aug     , Paul E. McKenney wrote:
> On Thu, Aug 25, 2011 at 09:20:51AM -0400, Will Simoneau wrote:
> > On 14:27 Wed 24 Aug     , Paul E. McKenney wrote:
> > > On Wed, Aug 24, 2011 at 05:19:07PM -0400, Will Simoneau wrote:
> > > > The following commits from Linus' git seem vaguely related,
> > > > although I have no idea how relevant they are to 2.6.39.4:
> > > > 
> > > >    ec433f0c (softirq,rcu: Inform RCU of irq_exit() activity)
> > > >    10f39bb1 (rcu: protect __rcu_read_unlock() against scheduler-using
> > > >              irq handlers)
> > > 
> > > If this failure mechanism really is the culprit, you should be able
> > > to make failure happen much more frequently by inserting a delay in
> > > __rcu_read_unlock() just prior to the call to rcu_read_unlock_special().
> > > I would suggest starting with a few tens to hundreds of microseconds
> > > worth of delay.
> > > 
> > > If this does make the failure reproducible, then it would make sense
> > > to try applying the two patches you identified.
> > 
> > Hmm. I tried adding progressively larger delays in the spot you
> > indicated. I went from 100uS to an entire 1S (!) and got no crash or
> > deadlock. The target runs at 40MHz so the delays do need to be
> > relatively long compared to modern machines.
> > 
> > My hardware breakpoint as well as printk tests confirm that
> > rcu_read_unlock_special() really does get called multiple times per
> > second, and the 1S delay makes it painfully obvious as well. But, no
> > dice.
> 
> Well, you can always apply the two patches above anyway, but it is hard
> to prove what the underlying problem really is in your case.

I am still unable to reproduce the Oops so I have no way of knowing if
applying the patches has any effect. I did find and fix the issue with
booting post-2.6.39* kernels on my hardware, so I've moved on to
3.1-rc3. I guess I will get back to you if it happens again :-)