netdev - [RFC PATCH net-next 1/6] virtio: make sure used event never go backwards

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Date:	Wed, 15 Oct 2014 15:25:25 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	rusty@...tcorp.com.au, mst@...hat.com,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Cc:	davem@...emloft.net, eric.dumazet@...il.com,
	Jason Wang <jasowang@...hat.com>
Subject: [RFC PATCH net-next 1/6] virtio: make sure used event never go backwards

This patch checks the new event idx to make sure used event idx never
goes back. This is used to synchronize the calls between
virtqueue_enable_cb_delayed() and virtqueue_enable_cb().

Cc: Rusty Russell <rusty@...tcorp.com.au>
Cc: Michael S. Tsirkin <mst@...hat.com>
Signed-off-by: Jason Wang <jasowang@...hat.com>
---
 drivers/virtio/virtio_ring.c |    7 +++++--
 1 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 3b1f89b..1b3929f 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -559,14 +559,17 @@ unsigned virtqueue_enable_cb_prepare(struct virtqueue *_vq)
 	u16 last_used_idx;
 
 	START_USE(vq);
-
+	last_used_idx = vq->last_used_idx;
 	/* We optimistically turn back on interrupts, then check if there was
 	 * more to do. */
 	/* Depending on the VIRTIO_RING_F_EVENT_IDX feature, we need to
 	 * either clear the flags bit or point the event index at the next
 	 * entry. Always do both to keep code simple. */
 	vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
-	vring_used_event(&vq->vring) = last_used_idx = vq->last_used_idx;
+	/* Make sure used event never go backwards */
+	if (!vring_need_event(vring_used_event(&vq->vring),
+			      vq->vring.avail->idx, last_used_idx))
+		vring_used_event(&vq->vring) = last_used_idx;
 	END_USE(vq);
 	return last_used_idx;
 }
-- 
1.7.1

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