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Date:   Tue, 13 Oct 2020 12:34:06 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Paul E. McKenney" <paulmck@kernel.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
Message-ID: <20201013103406.GY2628@hirez.programming.kicks-ass.net>
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 <20201012031110.GA39540@debian-boqun.qqnc3lrjykvubdpftowmye0fmh.lx.internal.cloudapp.net>
 <20201012212812.GH3249@paulmck-ThinkPad-P72>
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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.