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Message-ID: <6fa10edf-a04c-4a1d-9e11-fbed6332bff0@paulmck-laptop>
Date: Tue, 18 Mar 2025 10:18:12 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Boqun Feng <boqun.feng@...il.com>,
Joel Fernandes <joelagnelf@...dia.com>,
Neeraj Upadhyay <neeraj.upadhyay@....com>,
Uladzislau Rezki <urezki@...il.com>,
Zqiang <qiang.zhang1211@...il.com>, rcu <rcu@...r.kernel.org>
Subject: Re: [PATCH 2/5] rcu/exp: Remove confusing needless full barrier on
task unblock
On Fri, Mar 14, 2025 at 03:36:39PM +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@...nel.org>
Still cannot see a problem with this, but still a bit nervous.
Acked-by: Paul E. McKenney <paulmck@...nel.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. */
> np = rcu_next_node_entry(t, rnp);
> list_del_init(&t->rcu_node_entry);
> t->rcu_blocked_node = NULL;
> --
> 2.48.1
>
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