linux-kernel - [PATCH RFC 4/4] virtio: use dependent_ptr

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Date:   Wed, 2 Jan 2019 15:58:03 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     Jason Wang <jasowang@...hat.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Will Deacon <will.deacon@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        David Howells <dhowells@...hat.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>, linux-arch@...r.kernel.org,
        netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: [PATCH RFC 4/4] virtio: use dependent_ptr_mb

Use dependent_ptr_mb which is - on some architectures -
more light-weight than an rmb.

Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
---
 drivers/virtio/virtio_ring.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 814b395007b2..2d320396eff8 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -702,6 +702,7 @@ void *virtqueue_get_buf_ctx(struct virtqueue *_vq, unsigned int *len,
 	void *ret;
 	unsigned int i;
 	u16 last_used;
+	bool more;
 
 	START_USE(vq);
 
@@ -710,14 +711,15 @@ void *virtqueue_get_buf_ctx(struct virtqueue *_vq, unsigned int *len,
 		return NULL;
 	}
 
-	if (!more_used(vq)) {
+	more = more_used(vq);
+	if (!more) {
 		pr_debug("No more buffers in queue\n");
 		END_USE(vq);
 		return NULL;
 	}
 
 	/* Only get used array entries after they have been exposed by host. */
-	virtio_rmb(vq->weak_barriers);
+	vq = dependent_ptr_mb(vq, more);
 
 	last_used = (vq->last_used_idx & (vq->vring.num - 1));
 	i = virtio32_to_cpu(_vq->vdev, vq->vring.used->ring[last_used].id);
-- 
MST