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