tree c436326c77afc9cd1c9e0315e313d6ba2502c160
parent de02d72bb3cc5b3d4c873db4ca8291723dd48479
author Michael S. Tsirkin <mst@redhat.com> 1273596257 +0300
committer Michael S. Tsirkin <mst@redhat.com> 1273676644 +0300

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 <mst@redhat.com>
Acked-by: Juan Quintela <quintela@redhat.com>