linux-kernel - [PATCH v9 6/7] rcu/tree: segcblist: Remove redundant smp

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Date:   Tue,  3 Nov 2020 09:26:02 -0500
From:   "Joel Fernandes (Google)" <joel@...lfernandes.org>
To:     linux-kernel@...r.kernel.org
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Marco Elver <elver@...gle.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        "Uladzislau Rezki (Sony)" <urezki@...il.com>, fweisbec@...il.com,
        neeraj.iitr10@...il.com
Subject: [PATCH v9 6/7] rcu/tree: segcblist: Remove redundant smp_mb()s

This memory barrier is not needed as rcu_segcblist_add_len() already
includes a memory barrier *before* and *after* the length of the list is
updated.

Same reasoning for rcu_segcblist_enqueue().

Reviewed-by: Frederic Weisbecker <frederic@...nel.org>
Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.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 e9e72d72f7a6..d96272e8d604 100644
--- a/kernel/rcu/rcu_segcblist.c
+++ b/kernel/rcu/rcu_segcblist.c
@@ -268,7 +268,6 @@ 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 f6c6653b3ec2..fb2a5ac4a59c 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2525,7 +2525,6 @@ 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. */
-- 
2.29.1.341.ge80a0c044ae-goog