On Thu, Jun 27, 2024 at 3:42 AM Frederic Weisbecker <frederic@xxxxxxxxxx> wrote:
>
> Le Wed, Jun 26, 2024 at 10:49:58PM +0530, Neeraj upadhyay a écrit :
> > On Wed, Jun 26, 2024 at 7:58 PM Frederic Weisbecker <frederic@xxxxxxxxxx> wrote:
> > >
> > > Le Wed, Jun 12, 2024 at 02:14:14PM +0530, Neeraj upadhyay a écrit :
> > > > 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?
> > >
> > > How about this?
> > >
> >
> > Looks good to me, thanks!
>
> And similar to the previous one, a last minute edition:
>
> /*
> * Full ordering between remote CPU's post idle accesses
> * and updater's accesses prior to current GP (and also
> * the started GP sequence number) is enforced by
> * rcu_seq_start() implicit barrier, relayed by kworkers
> * locking and even further by smp_mb__after_unlock_lock()
> * barriers chained all the way throughout the rnp locking
> * tree since sync_exp_reset_tree() and up to the current
> * leaf rnp locking.
> *
> * Ordering between remote CPU's pre idle accesses and
> * post grace period updater's accesses is enforced by the
> * below acquire semantic.
> */
>
> Still ok?
>
Yes, looks good, thanks.
Thanks
Neeraj
> Thanks.
