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Date:   Tue,  5 Jan 2021 17:26:15 -0800
From:   paulmck@...nel.org
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
        jiangshanlai@...il.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: [PATCH tip/core/rcu 4/6] rcu/tree: segcblist: Remove redundant smp_mb()s

From: "Joel Fernandes (Google)" <joel@...lfernandes.org>

The full memory barriers in rcu_segcblist_enqueue() and in rcu_do_batch()
are not needed because rcu_segcblist_add_len(), and thus also
rcu_segcblist_inc_len(), already includes a memory barrier *before*
and *after* the length of the list is updated.

This commit therefore removes these redundant smp_mb() invocations.

Reviewed-by: Frederic Weisbecker <frederic@...nel.org>
Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/rcu_segcblist.c | 1 -
 kernel/rcu/tree.c          | 1 -
 2 files changed, 2 deletions(-)

diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
index 0504fc83..5059b61 100644
--- a/kernel/rcu/rcu_segcblist.c
+++ b/kernel/rcu/rcu_segcblist.c
@@ -327,7 +327,6 @@ void rcu_segcblist_enqueue(struct rcu_segcblist *rsclp,
 			   struct rcu_head *rhp)
 {
 	rcu_segcblist_inc_len(rsclp);
-	smp_mb(); /* Ensure counts are updated before callback is enqueued. */
 	rcu_segcblist_inc_seglen(rsclp, RCU_NEXT_TAIL);
 	rhp->next = NULL;
 	WRITE_ONCE(*rsclp->tails[RCU_NEXT_TAIL], rhp);
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index cc6f379..b0fb654 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2523,7 +2523,6 @@ static void rcu_do_batch(struct rcu_data *rdp)
 
 	/* Update counts and requeue any remaining callbacks. */
 	rcu_segcblist_insert_done_cbs(&rdp->cblist, &rcl);
-	smp_mb(); /* List handling before counting for rcu_barrier(). */
 	rcu_segcblist_add_len(&rdp->cblist, -count);
 
 	/* Reinstate batch limit if we have worked down the excess. */
-- 
2.9.5

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