From 0d4993563bde70bc7c7718686bc5c5c089733001 Mon Sep 17 00:00:00 2001 From: "Michael S. Tsirkin" Date: Tue, 11 May 2010 19:44:17 +0300 Subject: [PATCH] vhost: fix barrier pairing According to memory-barriers.txt, an smp memory barrier in guest should always be paired with an smp memory barrier in host, and I quote "a lack of appropriate pairing is almost certainly an error". In case of vhost, failure to flush out used index update before looking at the interrupt disable flag could result in missed interrupts, resulting in networking hang under stress. This might happen when flags read bypasses used index write. So we see interrupts disabled and do not interrupt, at the same time guest writes flags value to enable interrupt, reads an old used index value, thinks that used ring is empty and waits for interrupt. Note: the barrier we pair with here is in drivers/virtio/virtio_ring.c, function vring_enable_cb. Signed-off-by: Michael S. Tsirkin Acked-by: Juan Quintela --- drivers/vhost/vhost.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c index e69d238c5af0..49fa953aaf6e 100644 --- a/drivers/vhost/vhost.c +++ b/drivers/vhost/vhost.c @@ -1035,7 +1035,12 @@ int vhost_add_used(struct vhost_virtqueue *vq, unsigned int head, int len) /* This actually signals the guest, using eventfd. */ void vhost_signal(struct vhost_dev *dev, struct vhost_virtqueue *vq) { - __u16 flags = 0; + __u16 flags; + /* Flush out used index updates. This is paired + * with the barrier that the Guest executes when enabling + * interrupts. */ + smp_mb(); + if (get_user(flags, &vq->avail->flags)) { vq_err(vq, "Failed to get flags"); return; -- 2.20.1