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

[Jason Wang]

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@xxxxxxxxxxxxxxx>
Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
---
 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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