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

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Message-ID: <20201014215319.GF2974@worktop.programming.kicks-ass.net>
Date:   Wed, 14 Oct 2020 23:53:19 +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 Wed, Oct 14, 2020 at 11:34:05AM -0700, Paul E. McKenney wrote:
> commit 7deaa04b02298001426730ed0e6214ac20d1a1c1
> Author: Paul E. McKenney <paulmck@...nel.org>
> Date:   Tue Oct 13 12:39:23 2020 -0700
> 
>     rcu: Prevent lockdep-RCU splats on lock acquisition/release
>     
>     The rcu_cpu_starting() and rcu_report_dead() functions transition the
>     current CPU between online and offline state from an RCU perspective.
>     Unfortunately, this means that the rcu_cpu_starting() function's lock
>     acquisition and the rcu_report_dead() function's lock releases happen
>     while the CPU is offline from an RCU perspective, which can result in
>     lockdep-RCU splats about using RCU from an offline CPU.  In reality,
>     aside from the splats, both transitions are safe because a new grace
>     period cannot start until these functions release their locks.

But we call the trace_* crud before we acquire the lock. Are you sure
that's a false-positive?