linux-kernel - [PATCH] ptr_ring: Remove now-redundant smp_read_barrier

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Date:   Fri, 16 Feb 2018 12:06:13 +0100
From:   Andrea Parri <parri.andrea@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Andrea Parri <parri.andrea@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        John Fastabend <john.fastabend@...il.com>,
        Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org
Subject: [PATCH] ptr_ring: Remove now-redundant smp_read_barrier_depends()

Because READ_ONCE() now implies smp_read_barrier_depends(), the
smp_read_barrier_depends() in __ptr_ring_consume() is redundant;
this commit removes it and updates the comments.

Signed-off-by: Andrea Parri <parri.andrea@...il.com>
Cc: "David S. Miller" <davem@...emloft.net>
Cc: "Michael S. Tsirkin" <mst@...hat.com>
Cc: Jason Wang <jasowang@...hat.com>
Cc: John Fastabend <john.fastabend@...il.com>
Cc: Eric Dumazet <edumazet@...gle.com>
Cc: <linux-kernel@...r.kernel.org>
Cc: <netdev@...r.kernel.org>
---
 include/linux/ptr_ring.h | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h
index b884b7794187e..ddfed1dce9369 100644
--- a/include/linux/ptr_ring.h
+++ b/include/linux/ptr_ring.h
@@ -296,13 +296,14 @@ static inline void *__ptr_ring_consume(struct ptr_ring *r)
 {
 	void *ptr;
 
+	/* The READ_ONCE in __ptr_ring_peek guarantees that anyone
+	 * accessing data through the pointer is up to date. Pairs
+	 * with smp_wmb in __ptr_ring_produce.
+	 */
 	ptr = __ptr_ring_peek(r);
 	if (ptr)
 		__ptr_ring_discard_one(r);
 
-	/* Make sure anyone accessing data through the pointer is up to date. */
-	/* Pairs with smp_wmb in __ptr_ring_produce. */
-	smp_read_barrier_depends();
 	return ptr;
 }
 
-- 
2.7.4