On Mon, 2 Jul 2012 10:33:08 +0300, "Michael S. Tsirkin" [off-list ref] wrote:

In virtio balloon virtqueue_get_buf might now run concurrently with
virtqueue_kick.  I audited both and this seems safe in practice but
this is not guaranteed by the API.
Additionally, a spurious interrupt might in theory make
virtqueue_get_buf run in parallel with virtqueue_add_buf, which is racy.

While we might try to protect against spurious callbacks it's
easier to fix the driver: balloon seems to be the only one
(mis)using the API like this, so let's just fix balloon.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

I was thinking of a spinlock, but this is far more elegant.

And I added an explicit reference to the 'virtio: expose added
descriptors immediately.' commit in your commit msg.

Kudos!
Rusty.