Re: RFD: virtio balloon API use (was Re: [PATCH 5 of 5] virtio: expose added descriptors immediately)

From: Rusty Russell
Date: Sun Jul 01 2012 - 21:53:25 EST

On Sun, 1 Jul 2012 12:20:51 +0300, "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:
> On Thu, Nov 03, 2011 at 06:12:53PM +1030, Rusty Russell wrote:
> > A virtio driver does virtqueue_add_buf() multiple times before finally
> > calling virtqueue_kick(); previously we only exposed the added buffers
> > in the virtqueue_kick() call. This means we don't need a memory
> > barrier in virtqueue_add_buf(), but it reduces concurrency as the
> > device (ie. host) can't see the buffers until the kick.
> >
> > Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>
> Looking at recent mm compaction patches made me look at locking
> in balloon closely. And I noticed the referenced patch (commit
> ee7cd8981e15bcb365fc762afe3fc47b8242f630 upstream) interacts strangely
> with virtio balloon; balloon currently does:
> static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
> {
> struct scatterlist sg;
> sg_init_one(&sg, vb->pfns, sizeof(vb->pfns[0]) * vb->num_pfns);
> init_completion(&vb->acked);
> /* We should always be able to add one buffer to an empty queue. */
> if (virtqueue_add_buf(vq, &sg, 1, 0, vb, GFP_KERNEL) < 0)
> BUG();
> virtqueue_kick(vq);
> /* When host has read buffer, this completes via balloon_ack */
> wait_for_completion(&vb->acked);
> }
> While vq callback does:
> static void balloon_ack(struct virtqueue *vq)
> {
> struct virtio_balloon *vb;
> unsigned int len;
> vb = virtqueue_get_buf(vq, &len);
> if (vb)
> complete(&vb->acked);
> }
> So virtqueue_get_buf might now run concurrently with virtqueue_kick.
> I audited both and this seems safe in practice but I think

Good spotting!

Agreed. Because there's only add_buf, we get away with it: the add_buf
must be almost finished by the time get_buf runs because the device has
seen the buffer.

> we need to either declare this legal at the API level
> or add locking in driver.

I wonder if we should just lock in the balloon driver, rather than
document this corner case and set a bad example. Are there other
drivers which take the same shortcut?

> Further, is there a guarantee that we never get
> spurious callbacks? We currently check ring not empty
> but esp for non shared MSI this might not be needed.

Yes, I think this saves us. A spurious interrupt won't trigger
a spurious callback.

> If a spurious callback triggers, virtqueue_get_buf can run
> concurrently with virtqueue_add_buf which is known to be racy.
> Again I think this is currently safe as no spurious callbacks in
> practice but should we guarantee no spurious callbacks at the API level
> or add locking in driver?

I think we should guarantee it, but is there a hole in the current

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