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

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Date: Wed, 26 Jun 2024 22:49:58 +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 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!


- Neeraj

>     /*
>      * Full ordering against accesses prior current GP and also against
>      * current 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.
>      */
>
> Thanks.
>
> >
> >
> > Thanks
> > Neeraj
> >
> > > +                        */
> > > +                       snap = ct_dynticks_cpu_acquire(cpu);
> > >                         if (rcu_dynticks_in_eqs(snap))
> > >                                 mask_ofl_test |= mask;
> > >                         else
> > > --
> > > 2.40.1
> > >
> > >