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From: paulmck@kernel.org
To: rcu@vger.kernel.org
Cc: linux-kernel@vger.kernel.org, kernel-team@fb.com,
	mingo@kernel.org, jiangshanlai@gmail.com,
	akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
	josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org,
	rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com,
	fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org,
	"Paul E . McKenney" <paulmck@kernel.org>
Subject: [PATCH tip/core/rcu 4/6] rcu/tree: segcblist: Remove redundant smp_mb()s
Date: Tue,  5 Jan 2021 17:26:15 -0800
Message-ID: <20210106012617.14122-4-paulmck@kernel.org> (raw)
In-Reply-To: <20210106012541.GA13972@paulmck-ThinkPad-P72>

From: "Joel Fernandes (Google)" <joel@joelfernandes.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@kernel.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.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


  parent reply index

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-06  1:25 [PATCH tip/core/rcu 0/6] Track callbacks on a per-segment basis Paul E. McKenney
2021-01-06  1:26 ` [PATCH tip/core/rcu 1/6] rcu/tree: Make rcu_do_batch count how many callbacks were executed paulmck
2021-01-06  1:26 ` [PATCH tip/core/rcu 2/6] rcu/segcblist: Add additional comments to explain smp_mb() paulmck
2021-01-06  1:26 ` [PATCH tip/core/rcu 3/6] rcu/segcblist: Add counters to segcblist datastructure paulmck
2021-01-06  1:26 ` paulmck [this message]
2021-01-06  1:26 ` [PATCH tip/core/rcu 5/6] rcu/trace: Add tracing for how segcb list changes paulmck
2021-01-06  1:26 ` [PATCH tip/core/rcu 6/6] rcu/segcblist: Add debug checks for segment lengths paulmck

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