Re: [tip: locking/core] lockdep: Fix lockdep recursion

Peter Zijlstra
2020-10-15T01:30:36Z

On Wed, Oct 14, 2020 at 11:34:05AM -0700, Paul E. McKenney wrote:
> commit 7deaa04b02298001426730ed0e6214ac20d1a1c1
> Author: Paul E. McKenney <paulmck@kernel.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?