From 06b1297017415ae6a07a0e97ad7d8e90b2d95823 Mon Sep 17 00:00:00 2001
From: Tom Lendacky <tahm@linux.vnet.ibm.com>
Date: Mon, 8 Feb 2010 10:10:01 -0600
Subject: [PATCH] virtio-net: fix network stall under load
Fix a race condition where qemu finds that there are not enough virtio
ring buffers available and the guest make more buffers available before
qemu can enable notifications.
Signed-off-by: Tom Lendacky <toml@us.ibm.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
---
hw/virtio-net.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/hw/virtio-net.c b/hw/virtio-net.c
index 6e48997c0e..5c0093e879 100644
--- a/hw/virtio-net.c
+++ b/hw/virtio-net.c
@@ -379,7 +379,15 @@ static int virtio_net_has_buffers(VirtIONet *n, int bufsize)
(n->mergeable_rx_bufs &&
!virtqueue_avail_bytes(n->rx_vq, bufsize, 0))) {
virtio_queue_set_notification(n->rx_vq, 1);
- return 0;
+
+ /* To avoid a race condition where the guest has made some buffers
+ * available after the above check but before notification was
+ * enabled, check for available buffers again.
+ */
+ if (virtio_queue_empty(n->rx_vq) ||
+ (n->mergeable_rx_bufs &&
+ !virtqueue_avail_bytes(n->rx_vq, bufsize, 0)))
+ return 0;
}
virtio_queue_set_notification(n->rx_vq, 0);
--
2.30.2