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

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Date: Thu, 27 Jun 2024 08:03:10 +0530
From: Neeraj upadhyay <neeraj.iitr10@...il.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org, linux-kernel@...r.kernel.org, 
	kernel-team@...a.com, rostedt@...dmis.org
Subject: Re: [PATCH rcu 3/6] rcu/exp: Remove superfluous full memory barrier
 upon first EQS snapshot

On Thu, Jun 27, 2024 at 3:42 AM Frederic Weisbecker <frederic@...nel.org> wrote:
>
> Le Wed, Jun 26, 2024 at 10:49:58PM +0530, Neeraj upadhyay a écrit :
> > On Wed, Jun 26, 2024 at 7:58 PM Frederic Weisbecker <frederic@...nel.org> wrote:
> > >
> > > Le Wed, Jun 12, 2024 at 02:14:14PM +0530, Neeraj upadhyay a écrit :
> > > > 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?
> > >
> > > How about this?
> > >
> >
> > Looks good to me, thanks!
>
> And similar to the previous one, a last minute edition:
>
>                         /*
>                          * Full ordering between remote CPU's post idle accesses
>                          * and updater's accesses prior to current GP (and also
>                          * the started GP sequence number) is enforced by
>                          * rcu_seq_start() implicit barrier, relayed by kworkers
>                          * locking and even further by smp_mb__after_unlock_lock()
>                          * barriers chained all the way throughout the rnp locking
>                          * tree since sync_exp_reset_tree() and up to the current
>                          * leaf rnp locking.
>                          *
>                          * Ordering between remote CPU's pre idle accesses and
>                          * post grace period updater's accesses is enforced by the
>                          * below acquire semantic.
>                          */
>
> Still ok?
>

Yes, looks good, thanks.


Thanks
Neeraj

> Thanks.