linux-kernel - Re: [PATCH rcu 1/6] rcu: Remove full ordering on second EQS snapshot

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Date: Wed, 5 Jun 2024 11:44:42 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
	rostedt@...dmis.org
Subject: Re: [PATCH rcu 1/6] rcu: Remove full ordering on second EQS snapshot

On Wed, Jun 05, 2024 at 02:21:13PM +0200, Frederic Weisbecker wrote:
> Le Tue, Jun 04, 2024 at 03:26:47PM -0700, Paul E. McKenney a écrit :
> > 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. Also the GP kthread must
> >   observe all accesses performed by the target prior it entering in
> >   EQS.
> > 
> > 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. Also the GP kthread later
> >   observing that EQS must also observe all accesses performed by the
> >   target prior it entering in EQS.
> > 
> > This ordering is explicitly performed both on the first EQS snapshot
> > and on the second one as well through the combination of a preceding
> > full barrier followed by an acquire read. However the second snapshot's
> > full memory barrier is redundant and not needed to enforce the above
> > guarantees:
> > 
> >     GP kthread                  Remote target
> >     ----                        -----
> >     // Access prior GP
> >     WRITE_ONCE(A, 1)
> >     // first snapshot
> >     smp_mb()
> >     x = smp_load_acquire(EQS)
> >                                // Access prior GP
> >                                WRITE_ONCE(B, 1)
> >                                // EQS enter
> >                                // implied full barrier by atomic_add_return()
> >                                atomic_add_return(RCU_DYNTICKS_IDX, EQS)
> >                                // implied full barrier by atomic_add_return()
> >                                READ_ONCE(A)
> >     // second snapshot
> >     y = smp_load_acquire(EQS)
> >     z = READ_ONCE(B)
> > 
> > If the GP kthread above fails to observe the remote target in EQS
> > (x not in EQS), the remote target will observe A == 1 after further
> > entering in EQS. Then the second snapshot taken by the GP kthread only
> > need to be an acquire read in order to observe z == 1.
> > 
> > Therefore remove the needless full memory barrier on second snapshot.
> > 
> > Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > ---
> >  kernel/rcu/tree.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 28c7031711a3f..f07b8bff4621b 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -321,7 +321,7 @@ static bool rcu_dynticks_in_eqs(int snap)
> >   */
> >  static bool rcu_dynticks_in_eqs_since(struct rcu_data *rdp, int snap)
> >  {
> > -	return snap != rcu_dynticks_snap(rdp->cpu);
> > +	return snap != ct_dynticks_cpu_acquire(rdp->cpu);
> 
> I guess I'm going to add a comment here to elaborate on the fact
> it relies on the ordering enforced before the first snapshot. Would
> you prefer a delta patch or an updated patch?

Either works, just tell me which you are doing when you submit the patch.
Either way, I will arrange for there to be a single combined commit.

							Thanx, Paul