linux-kernel - [PATCH 6/6] rcu/exp: Remove redundant full memory barrier at the end of GP

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Date: Wed, 15 May 2024 14:53:32 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Frederic Weisbecker <frederic@...nel.org>,
	Valentin Schneider <vschneid@...hat.com>,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Neeraj Upadhyay <neeraj.upadhyay@....com>,
	Uladzislau Rezki <urezki@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>,
	rcu <rcu@...r.kernel.org>
Subject: [PATCH 6/6] rcu/exp: Remove redundant full memory barrier at the end of GP

A full memory barrier is necessary at the end of the expedited grace
period to order:

1) The grace period completion (pictured by the GP sequence
   number) with all preceding accesses. This pairs with rcu_seq_end()
   performed by the concurrent kworker.

2) The grace period completion and subsequent post-GP update side
   accesses. Pairs again against rcu_seq_end().

This full barrier is already provided by the final sync_exp_work_done()
test, making the subsequent explicit one redundant. Remove it and
improve comments.

Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
---
 kernel/rcu/tree_exp.h | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index bec24ea6777e..721cb93b1fec 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -265,7 +265,12 @@ static bool sync_exp_work_done(unsigned long s)
 {
 	if (rcu_exp_gp_seq_done(s)) {
 		trace_rcu_exp_grace_period(rcu_state.name, s, TPS("done"));
-		smp_mb(); /* Ensure test happens before caller kfree(). */
+		/*
+		 * Order GP completion with preceding accesses. Order also GP
+		 * completion with post GP update side accesses. Pairs with
+		 * rcu_seq_end().
+		 */
+		smp_mb();
 		return true;
 	}
 	return false;
@@ -959,7 +964,6 @@ void synchronize_rcu_expedited(void)
 	rnp = rcu_get_root();
 	wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
 		   sync_exp_work_done(s));
-	smp_mb(); /* Work actions happen before return. */
 
 	/* Let the next expedited grace period start. */
 	mutex_unlock(&rcu_state.exp_mutex);
-- 
2.44.0