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

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Message-ID: <20201015095235.GT2651@hirez.programming.kicks-ass.net>
Date:   Thu, 15 Oct 2020 11:52:35 +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
> @@ -1764,8 +1764,7 @@ static bool rcu_gp_init(void)
>  		smp_mb(); // Pair with barriers used when updating ->ofl_seq to odd values.
>  		firstseq = READ_ONCE(rnp->ofl_seq);
>  		if (firstseq & 0x1)
> -			while (firstseq == smp_load_acquire(&rnp->ofl_seq))
> -				schedule_timeout_idle(1);  // Can't wake unless RCU is watching.
> +			smp_cond_load_relaxed(&rnp->ofl_seq, VAL == firstseq);

My bad, that should be: VAL != firstseq.

>  		smp_mb(); // Pair with barriers used when updating ->ofl_seq to even values.
>  		raw_spin_lock(&rcu_state.ofl_lock);
>  		raw_spin_lock_irq_rcu_node(rnp);