linux-kernel - [PATCH 1/8] rcu/exp: Remove full barrier upon main thread wakeup

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Date: Mon, 29 Jan 2024 15:23:39 -0800
From: Boqun Feng <boqun.feng@...il.com>
To: linux-kernel@...r.kernel.org,
	rcu@...r.kernel.org
Cc: Frederic Weisbecker <frederic@...nel.org>,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Neeraj Upadhyay <neeraj.iitr10@...il.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Josh Triplett <josh@...htriplett.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>
Subject: [PATCH 1/8] rcu/exp: Remove full barrier upon main thread wakeup

From: Frederic Weisbecker <frederic@...nel.org>

When an expedited grace period is ending, care must be taken so that all
the quiescent states propagated up to the root are correctly ordered
against the wake up of the main expedited grace period workqueue.

This ordering is already carried through the root rnp locking augmented
by an smp_mb__after_unlock_lock() barrier.

Therefore the explicit smp_mb() placed before the wake up is not needed
and can be removed.

Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tree_exp.h | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 2ac440bc7e10..014ddf672165 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -198,10 +198,9 @@ static void __rcu_report_exp_rnp(struct rcu_node *rnp,
 		}
 		if (rnp->parent == NULL) {
 			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
-			if (wake) {
-				smp_mb(); /* EGP done before wake_up(). */
+			if (wake)
 				swake_up_one_online(&rcu_state.expedited_wq);
-			}
+
 			break;
 		}
 		mask = rnp->grpmask;
-- 
2.43.0