From: Frederic Weisbecker <[email protected]>
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 <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
---
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().
+ */
+ snap = ct_dynticks_cpu_acquire(cpu);
if (rcu_dynticks_in_eqs(snap))
mask_ofl_test |= mask;
else
--
2.40.1
On Wed, Jun 5, 2024 at 3:58 AM Paul E. McKenney <[email protected]> wrote:
>
> From: Frederic Weisbecker <[email protected]>
>
> 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 <[email protected]>
> Signed-off-by: Paul E. McKenney <[email protected]>
> ---
> 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?
Thanks
Neeraj
> + */
> + snap = ct_dynticks_cpu_acquire(cpu);
> if (rcu_dynticks_in_eqs(snap))
> mask_ofl_test |= mask;
> else
> --
> 2.40.1
>
>
On Wed, Jun 12, 2024 at 02:14:14PM +0530, Neeraj upadhyay wrote:
> On Wed, Jun 5, 2024 at 3:58 AM Paul E. McKenney <[email protected]> wrote:
> >
> > From: Frederic Weisbecker <[email protected]>
> >
> > 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 <[email protected]>
> > Signed-off-by: Paul E. McKenney <[email protected]>
> > ---
> > 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?
It might well be in both cases. Could you and Frederic propose
agreed-upon appropriate changes (including the null change, if
appropriate)?
Thanx, Paul
> Thanks
> Neeraj
>
> > + */
> > + snap = ct_dynticks_cpu_acquire(cpu);
> > if (rcu_dynticks_in_eqs(snap))
> > mask_ofl_test |= mask;
> > else
> > --
> > 2.40.1
> >
> >