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    SubjectRe: 2.6.39.4: Oops in rcu_read_unlock_special()/_raw_spin_lock()
    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 :-)


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