Re: [PATCH 3/4] rcu/nocb: Recheck lazy callbacks under the ->nocb_lock from shrinker

[Frederic Weisbecker]

On Wed, Mar 29, 2023 at 01:54:20PM -0700, Paul E. McKenney wrote:
> On Wed, Mar 29, 2023 at 06:02:02PM +0200, Frederic Weisbecker wrote:
> > The ->lazy_len is only checked locklessly. Recheck again under the
> > ->nocb_lock to avoid spending more time on flushing/waking if not
> > necessary. The ->lazy_len can still increment concurrently (from 1 to
> > infinity) but under the ->nocb_lock we at least know for sure if there
> > are lazy callbacks at all (->lazy_len > 0).
> > 
> > Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> > ---
> >  kernel/rcu/tree_nocb.h | 16 ++++++++++++----
> >  1 file changed, 12 insertions(+), 4 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> > index c321fce2af8e..dfa9c10d6727 100644
> > --- a/kernel/rcu/tree_nocb.h
> > +++ b/kernel/rcu/tree_nocb.h
> > @@ -1358,12 +1358,20 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
> >  		if (!rcu_rdp_is_offloaded(rdp))
> >  			continue;
> >  
> > +		if (!READ_ONCE(rdp->lazy_len))
> > +			continue;
> 
> Do you depend on the ordering of the above read of ->lazy_len against
> anything in the following, aside from the re-read of ->lazy_len?  (Same
> variable, both READ_ONCE() or stronger, so you do get that ordering.)
> 
> If you do need that ordering, the above READ_ONCE() needs to instead
> be smp_load_acquire() or similar.  If you don't need that ordering,
> what you have is good.

No ordering dependency intended here. The early ->lazy_len read is really just
an optimization here to avoid locking if it *seems* there is nothing to do with
this rdp. But what follows doesn't depend on that read.

Thanks.