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

[Frederic Weisbecker <frederic@kernel.org>]

Le Wed, Jun 05, 2024 at 11:44:42AM -0700, Paul E. McKenney a écrit :
> 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@kernel.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@kernel.org>
> > > Signed-off-by: Paul E. McKenney <paulmck@kernel.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.

Ok before I resend, how does the following comment look like?

/*
 * The first failing snapshot is already ordered against the accesses
 * performed by the remote CPU after it exiting idle.
 *
 * The second snapshot therefore only needs to order against accesses
 * performed by the remote CPU prior it entering idle and therefore can
 * solely on acquire semantics.
 */

Thanks.

> 
> 							Thanx, Paul
>