Subject: Re: [PATCH v1 1/6] rcu: Add support for consolidated-RCU reader checking

From: "Paul E. McKenney" <[email-masked]>
Date: Fri, 12 Jul 2019 09:45:31 -0700

On Fri, Jul 12, 2019 at 11:10:51AM -0400, Joel Fernandes wrote:
> On Fri, Jul 12, 2019 at 01:11:25PM +0200, Peter Zijlstra wrote:
> > On Thu, Jul 11, 2019 at 07:43:56PM -0400, Joel Fernandes (Google) wrote:
> > > +int rcu_read_lock_any_held(void)
> > > +{
> > > +	int lockdep_opinion = 0;
> > > +
> > > +	if (!debug_lockdep_rcu_enabled())
> > > +		return 1;
> > > +	if (!rcu_is_watching())
> > > +		return 0;
> > > +	if (!rcu_lockdep_current_cpu_online())
> > > +		return 0;
> > > +
> > > +	/* Preemptible RCU flavor */
> > > +	if (lock_is_held(&rcu_lock_map))
> > 
> > you forgot debug_locks here.
> 
> Actually, it turns out debug_locks checking is not even needed. If
> debug_locks == 0, then debug_lockdep_rcu_enabled() returns 0 and we would not
> get to this point.
> 
> > > +		return 1;
> > > +
> > > +	/* BH flavor */
> > > +	if (in_softirq() || irqs_disabled())
> > 
> > I'm not sure I'd put irqs_disabled() under BH, also this entire
> > condition is superfluous, see below.
> > 
> > > +		return 1;
> > > +
> > > +	/* Sched flavor */
> > > +	if (debug_locks)
> > > +		lockdep_opinion = lock_is_held(&rcu_sched_lock_map);
> > > +	return lockdep_opinion || !preemptible();
> > 
> > that !preemptible() turns into:
> > 
> >   !(preempt_count()==0 && !irqs_disabled())
> > 
> > which is:
> > 
> >   preempt_count() != 0 || irqs_disabled()
> > 
> > and already includes irqs_disabled() and in_softirq().
> > 
> > > +}
> > 
> > So maybe something lke:
> > 
> > 	if (debug_locks && (lock_is_held(&rcu_lock_map) ||
> > 			    lock_is_held(&rcu_sched_lock_map)))
> > 		return true;
> 
> Agreed, I will do it this way (without the debug_locks) like:
> 
> ---8<-----------------------
> 
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index ba861d1716d3..339aebc330db 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -296,27 +296,15 @@ EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held);
>  
>  int rcu_read_lock_any_held(void)
>  {
> -	int lockdep_opinion = 0;
> -
>  	if (!debug_lockdep_rcu_enabled())
>  		return 1;
>  	if (!rcu_is_watching())
>  		return 0;
>  	if (!rcu_lockdep_current_cpu_online())
>  		return 0;
> -
> -	/* Preemptible RCU flavor */
> -	if (lock_is_held(&rcu_lock_map))
> -		return 1;
> -
> -	/* BH flavor */
> -	if (in_softirq() || irqs_disabled())
> -		return 1;
> -
> -	/* Sched flavor */
> -	if (debug_locks)
> -		lockdep_opinion = lock_is_held(&rcu_sched_lock_map);
> -	return lockdep_opinion || !preemptible();
> +	if (lock_is_held(&rcu_lock_map) || lock_is_held(&rcu_sched_lock_map))

OK, I will bite...  Why not also lock_is_held(&rcu_bh_lock_map)?

							Thanx, Paul

> +		return 1;
> +	return !preemptible();
>  }
>  EXPORT_SYMBOL_GPL(rcu_read_lock_any_held);
>  
>