Re: [tip: locking/core] lockdep: Fix lockdep recursion

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Oct 12, 2020 at 02:28:12PM -0700, Paul E. McKenney wrote:
> It is certainly an accident waiting to happen.  Would something like
> the following make sense?

Sadly no.

> ------------------------------------------------------------------------
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index bfd38f2..52a63bc 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -4067,6 +4067,7 @@ void rcu_cpu_starting(unsigned int cpu)
>  
>  	rnp = rdp->mynode;
>  	mask = rdp->grpmask;
> +	lockdep_off();
>  	raw_spin_lock_irqsave_rcu_node(rnp, flags);
>  	WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext | mask);
>  	newcpu = !(rnp->expmaskinitnext & mask);
> @@ -4086,6 +4087,7 @@ void rcu_cpu_starting(unsigned int cpu)
>  	} else {
>  		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>  	}
> +	lockdep_on();
>  	smp_mb(); /* Ensure RCU read-side usage follows above initialization. */
>  }

This will just shut it up, but will not fix the actual problem of that
spin-lock ending up in trace_lock_acquire() which relies on RCU which
isn't looking.

What we need here is to supress tracing not lockdep. Let me consider.