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@gmail.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: <linux-kernel@vger.kernel.org>
Cc: <netdev@vger.kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

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;
 }