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

Paul E. McKenney
2020-10-15T16:15:21Z

On Thu, Oct 15, 2020 at 11:50:33AM +0200, Peter Zijlstra wrote:
> On Thu, Oct 15, 2020 at 11:49:26AM +0200, Peter Zijlstra wrote:
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1143,13 +1143,15 @@ bool rcu_lockdep_current_cpu_online(void
> > struct rcu_data *rdp;
> > struct rcu_node *rnp;
> > bool ret = false;
> > + unsigned long seq;
> >
> > if (in_nmi() || !rcu_scheduler_fully_active)
> > return true;
> > preempt_disable_notrace();
> > rdp = this_cpu_ptr(&rcu_data);
> > rnp = rdp->mynode;
> > - if (rdp->grpmask & rcu_rnp_online_cpus(rnp))
> > + seq = READ_ONCE(rnp->ofl_seq) & ~0x1;
> > + if (rdp->grpmask & rcu_rnp_online_cpus(rnp) || seq != READ_ONCE(rnp->ofl_seq))
> > ret = true;
> > preempt_enable_notrace();
> > return ret;
>
> Also, here, are the two loads important? Wouldn't:
>
> || READ_ONCE(rnp->ofl_seq) & 0x1
>
> be sufficient?

Indeed it would! Good catch, thank you!!!

Thanx, Paul