linux-kernel - Re: [tip: locking/core] lockdep: Fix lockdep recursion

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Message-ID: <20201015095033.GS2651@hirez.programming.kicks-ass.net>
Date:   Thu, 15 Oct 2020 11:50:33 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Boqun Feng <boqun.feng@...il.com>, Qian Cai <cai@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>, x86 <x86@...nel.org>,
        linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        Linux Next Mailing List <linux-next@...r.kernel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [tip: locking/core] lockdep: Fix lockdep recursion

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?