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@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > ---
> > 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
