Re: 2.6.39-rc6-mmotm0506 - lockdep splat in RCU code on page fault

Valdis.Kletnieks
2011-05-11T23:12:50Z

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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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