linux-kernel - Re: [PATCH rcu 3/6] rcu/exp: Remove superfluous full memory barrier upon first EQS snapshot

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Message-ID: <5bef03bd-84c4-4e9b-b045-ea023f91d772@paulmck-laptop>
Date: Wed, 12 Jun 2024 07:45:38 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Neeraj upadhyay <neeraj.iitr10@...il.com>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
	rostedt@...dmis.org, Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH rcu 3/6] rcu/exp: Remove superfluous full memory barrier
 upon first EQS snapshot

On Wed, Jun 12, 2024 at 02:14:14PM +0530, Neeraj upadhyay wrote:
> On Wed, Jun 5, 2024 at 3:58 AM Paul E. McKenney <paulmck@...nel.org> wrote:
> >
> > From: Frederic Weisbecker <frederic@...nel.org>
> >
> > When the grace period kthread checks the extended quiescent state
> > counter of a CPU, full ordering is necessary to ensure that either:
> >
> > * If the GP kthread observes the remote target in an extended quiescent
> >   state, then that target must observe all accesses prior to the current
> >   grace period, including the current grace period sequence number, once
> >   it exits that extended quiescent state.
> >
> > or:
> >
> > * If the GP kthread observes the remote target NOT in an extended
> >   quiescent state, then the target further entering in an extended
> >   quiescent state must observe all accesses prior to the current
> >   grace period, including the current grace period sequence number, once
> >   it enters that extended quiescent state.
> >
> > This ordering is enforced through a full memory barrier placed right
> > before taking the first EQS snapshot. However this is superfluous
> > because the snapshot is taken while holding the target's rnp lock which
> > provides the necessary ordering through its chain of
> > smp_mb__after_unlock_lock().
> >
> > Remove the needless explicit barrier before the snapshot and put a
> > comment about the implicit barrier newly relied upon here.
> >
> > Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > ---
> >  kernel/rcu/tree_exp.h | 8 +++++++-
> >  1 file changed, 7 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> > index 8a1d9c8bd9f74..bec24ea6777e8 100644
> > --- a/kernel/rcu/tree_exp.h
> > +++ b/kernel/rcu/tree_exp.h
> > @@ -357,7 +357,13 @@ static void __sync_rcu_exp_select_node_cpus(struct rcu_exp_work *rewp)
> >                     !(rnp->qsmaskinitnext & mask)) {
> >                         mask_ofl_test |= mask;
> >                 } else {
> > -                       snap = rcu_dynticks_snap(cpu);
> > +                       /*
> > +                        * Full ordering against accesses prior current GP and
> > +                        * also against current GP sequence number is enforced
> > +                        * by current rnp locking with chained
> > +                        * smp_mb__after_unlock_lock().
> 
> Again, worth mentioning the chaining sites sync_exp_reset_tree() and
> this function?

It might well be in both cases.  Could you and Frederic propose
agreed-upon appropriate changes (including the null change, if
appropriate)?

							Thanx, Paul

> Thanks
> Neeraj
> 
> > +                        */
> > +                       snap = ct_dynticks_cpu_acquire(cpu);
> >                         if (rcu_dynticks_in_eqs(snap))
> >                                 mask_ofl_test |= mask;
> >                         else
> > --
> > 2.40.1
> >
> >