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

Peter Zijlstra
2020-10-15T02:19:51Z

On Wed, Oct 14, 2020 at 03:11:52PM -0700, Paul E. McKenney wrote:
> 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?

rcu_cpu_starting();
raw_spin_lock_irqsave();
local_irq_save();
preempt_disable();
spin_acquire()
lock_acquire()
trace_lock_acquire() <--- *whoopsie-doodle*
/* uses RCU for tracing */
arch_spin_lock_flags() <--- the actual spinlock