Re: [PATCH 2/3] rcu/exp: Remove confusing needless full barrier on task unblock

Frederic Weisbecker
2025-02-26T12:52:12Z

Le Tue, Feb 25, 2025 at 04:59:08PM -0500, Joel Fernandes a écrit :
> On Fri, Feb 14, 2025 at 12:25:58AM +0100, Frederic Weisbecker wrote:
> > A full memory barrier in the RCU-PREEMPT task unblock path advertizes
> > to order the context switch (or rather the accesses prior to
> > rcu_read_unlock()) with the expedited grace period fastpath.
> >
> > However the grace period can not complete without the rnp calling into
> > rcu_report_exp_rnp() with the node locked. This reports the quiescent
> > state in a fully ordered fashion against updater's accesses thanks to:
> >
> > 1) The READ-SIDE smp_mb__after_unlock_lock() barrier accross nodes
> > locking while propagating QS up to the root.
> >
> > 2) The UPDATE-SIDE smp_mb__after_unlock_lock() barrier while holding the
> > the root rnp to wait/check for the GP completion.
> >
> > 3) The (perhaps redundant given step 1) and 2)) smp_mb() in rcu_seq_end()
> > before the grace period completes.
> >
> > This makes the explicit barrier in this place superflous. Therefore
> > remove it as it is confusing.
> >
> > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> > ---
> > kernel/rcu/tree_plugin.h | 1 -
> > 1 file changed, 1 deletion(-)
> >
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index 3c0bbbbb686f..d51cc7a5dfc7 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -534,7 +534,6 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
> > WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq &&
> > (!empty_norm || rnp->qsmask));
> > empty_exp = sync_rcu_exp_done(rnp);
> > - smp_mb(); /* ensure expedited fastpath sees end of RCU c-s. */
>
> I was wondering though, this is a slow path and the smp_mb() has been there
> since 2009 or so. Not sure if it is super valuable to remove it at this
> point. But we/I should definitely understand it.

The point is indeed not to improve performance because this is a slowpath
(although...). The main goal is to maintain a clear picture of the ordering
without needless barriers that leave a taste of doubt to reviewers.

> I was wondering if you could also point to the fastpath that this is racing
> with, it is not immediately clear (to me) what this smp_mb() is pairing with
> :(

It is supposed to pair with the barrier in sync_exp_work_done() but then again
this is already enforced by the smp_mb__after_unlock_lock() chained through
rnp locking.

Thanks.

>
> thanks,
>
> - Joel
>
>
>
>
>
> > np = rcu_next_node_entry(t, rnp);
> > list_del_init(&t->rcu_node_entry);
> > t->rcu_blocked_node = NULL;
> > --
> > 2.46.0
> >