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

Paul E. McKenney
2020-10-14T22:11:57Z

On Wed, Oct 14, 2020 at 11:53:19PM +0200, Peter Zijlstra wrote:
> 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?

You lost me on this one.

I am assuming that you are talking about rcu_cpu_starting(), because
that is the one where RCU is not initially watching, that is, the
case where tracing before the lock acquisition would be a problem.
You cannot be talking about rcu_cpu_starting() itself, because it does
not do any tracing before acquiring the lock. But if you are talking
about the caller of rcu_cpu_starting(), then that caller should put the
rcu_cpu_starting() before the tracing. But that would be the other
patch earlier in this thread that was proposing moving the call to
rcu_cpu_starting() much earlier in CPU bringup.

So what am I missing here?

Thanx, Paul