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 16:27:49 -0700

On Fri, Jul 12, 2019 at 03:40:40PM -0400, Joel Fernandes wrote:
> On Fri, Jul 12, 2019 at 10:46:30AM -0700, Paul E. McKenney wrote:
> > On Fri, Jul 12, 2019 at 01:06:31PM -0400, Joel Fernandes wrote:
> > > On Fri, Jul 12, 2019 at 09:45:31AM -0700, Paul E. McKenney wrote:
> > > > 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)?
> > > 
> > > Hmm, I was borrowing the strategy from rcu_read_lock_bh_held() which does not
> > > check for a lock held in this map.
> > > 
> > > Honestly, even  lock_is_held(&rcu_sched_lock_map) seems unnecessary per-se
> > > since !preemptible() will catch that? rcu_read_lock_sched() disables
> > > preemption already, so lockdep's opinion of the matter seems redundant there.
> > 
> > Good point!  At least as long as the lockdep splats list RCU-bh among
> > the locks held, which they did last I checked.
> > 
> > Of course, you could make the same argument for getting rid of
> > rcu_sched_lock_map.  Does it make sense to have the one without
> > the other?
> 
> It probably makes it inconsistent in the least. I will add the check for
> the rcu_bh_lock_map in a separate patch, if that's Ok with you - since I also
> want to update the rcu_read_lock_bh_held() logic in the same patch.
> 
> That rcu_read_lock_bh_held() could also just return !preemptible as Peter
> suggested for the bh case.

Although that seems reasonable, please check the call sites.

> > > Sorry I already sent out patches again before seeing your comment but I can
> > > rework and resend them based on any other suggestions.
> > 
> > Not a problem!
> 
> Thanks. Depending on whether there is any other feedback, I will work on the
> bh_ stuff as a separate patch on top of this series, or work it into the next
> series revision if I'm reposting. Hopefully that sounds Ok to you.

Agreed -- let's separate concerns.  And promote bisectability.

							Thanx, Paul