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Date:   Thu, 15 Oct 2020 09:15:19 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Boqun Feng <boqun.feng@gmail.com>, Qian Cai <cai@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>, x86 <x86@kernel.org>,
        linux-kernel@vger.kernel.org, linux-tip-commits@vger.kernel.org,
        Linux Next Mailing List <linux-next@vger.kernel.org>,
        Stephen Rothwell <sfr@canb.auug.org.au>
Subject: Re: [tip: locking/core] lockdep: Fix lockdep recursion
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Reply-To: paulmck@kernel.org
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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