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

[Boqun Feng <boqun.feng@gmail.com>]

On Tue, Oct 13, 2020 at 12:27:15PM +0200, Peter Zijlstra wrote:
> On Mon, Oct 12, 2020 at 11:11:10AM +0800, Boqun Feng wrote:
> 
> > I think this happened because in this commit debug_lockdep_rcu_enabled()
> > didn't adopt to the change that made lockdep_recursion a percpu
> > variable?
> > 
> > Qian, mind to try the following?
> > 
> > Although, arguably the problem still exists, i.e. we still have an RCU
> > read-side critical section inside lock_acquire(), which may be called on
> 
> There is actual RCU usage from the trace_lock_acquire().
> 
> > a yet-to-online CPU, which RCU doesn't watch. I think this used to be OK
> > because we don't "free" anything from lockdep, IOW, there is no
> > synchronize_rcu() or call_rcu() that _needs_ to wait for the RCU
> > read-side critical sections inside lockdep. But now we lock class
> > recycling, so it might be a problem.
> > 
> > That said, currently validate_chain() and lock class recycling are
> > mutually excluded via graph_lock, so we are safe for this one ;-)
> 
> We should have a comment on that somewhere, could you write one?
> 

Sure, I will write something tomorrow.

Regards,
Boqun

> > ----------->8
> > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> > index 39334d2d2b37..35d9bab65b75 100644
> > --- a/kernel/rcu/update.c
> > +++ b/kernel/rcu/update.c
> > @@ -275,8 +275,8 @@ EXPORT_SYMBOL_GPL(rcu_callback_map);
> >  
> >  noinstr int notrace debug_lockdep_rcu_enabled(void)
> >  {
> > -	return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE && debug_locks &&
> > -	       current->lockdep_recursion == 0;
> > +	return rcu_scheduler_active != RCU_SCHEDULER_INACTIVE &&
> > +	       __lockdep_enabled;
> >  }
> >  EXPORT_SYMBOL_GPL(debug_lockdep_rcu_enabled);
> 
> Urgh, I didn't expect (and forgot to grep) lockdep_recursion users
> outside of lockdep itself :/ It looks like this is indeed the only one.
> 
>