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    SubjectRe: 2.6.39-rc6-mmotm0506 - lockdep splat in RCU code on page fault
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    On Tue, 10 May 2011 01:20:29 PDT, "Paul E. McKenney" said:

    Would test, but it doesn't apply cleanly to my -mmotm0506 tree:

    > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
    > index 5616b17..20c22c5 100644
    > --- a/kernel/rcutree.c
    > +++ b/kernel/rcutree.c
    > @@ -1525,13 +1525,15 @@ static void rcu_cpu_kthread_setrt(int cpu, int to_rt)
    > */
    > static void rcu_cpu_kthread_timer(unsigned long arg)
    > {
    > - unsigned long flags;
    > + unsigned long old;
    > + unsigned long new;
    > struct rcu_data *rdp = per_cpu_ptr(rcu_state->rda, arg);
    > struct rcu_node *rnp = rdp->mynode;
    >
    > - raw_spin_lock_irqsave(&rnp->lock, flags);
    > - rnp->wakemask |= rdp->grpmask;
    > - raw_spin_unlock_irqrestore(&rnp->lock, flags);
    > + do {
    > + old = rnp->wakemask;
    > + new = old | rdp->grpmask;
    > + } while (cmpxchg(&rnp->wakemask, old, new) != old);
    > invoke_rcu_node_kthread(rnp);
    > }

    My source has this:

    raw_spin_lock_irqsave(&rnp->lock, flags);
    rnp->wakemask |= rdp->grpmask;
    invoke_rcu_node_kthread(rnp);
    raw_spin_unlock_irqrestore(&rnp->lock, flags);

    the last 2 lines swapped from what you diffed against. I can easily work around
    that, except it's unclear what the implications of the invoke_rcu moving outside
    of the irq save/restore pair (or if it being inside is the actual root cause)...

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