According to memory-barriers.txt, an smp memory barrier should always be paired with another smp memory barrier, 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 <[email protected]> --- Dave, I think this is needed in 2.6.34, I'll send a pull request after doing some more testing. Rusty, Juan, could you take a look as well please? Thanks! drivers/vhost/vhost.c | 5 ++++- 1 files changed, 4 insertions(+), 1 deletions(-) diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c index e69d238..14fa2f5 100644 --- a/drivers/vhost/vhost.c +++ b/drivers/vhost/vhost.c @@ -1035,7 +1035,10 @@ 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. */ + smp_mb(); + if (get_user(flags, &vq->avail->flags)) { vq_err(vq, "Failed to get flags"); return; -- 1.7.1.12.g42b7f _______________________________________________ Virtualization mailing list [email protected] https://lists.linux-foundation.org/mailman/listinfo/virtualization
