Subject: Re: [RFC PATCH net-next 1/6] virtio: make sure used event never go backwards

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 15 Oct 2014 12:34:32 +0300

On Wed, Oct 15, 2014 at 03:25:25PM +0800, Jason Wang wrote:
> This patch checks the new event idx to make sure used event idx never
> goes back. This is used to synchronize the calls between
> virtqueue_enable_cb_delayed() and virtqueue_enable_cb().
> 
> Cc: Rusty Russell <[email-masked]>
> Cc: Michael S. Tsirkin <[email-masked]>
> Signed-off-by: Jason Wang <[email-masked]>

the implication being that moving event idx back might cause some race
condition?  If yes but please describe the race explicitly.
Is there a bug we need to fix on stable?
Please also explicitly describe a configuration that causes event idx
to go back.

All this info should go in the commit log.

> ---
>  drivers/virtio/virtio_ring.c |    7 +++++--
>  1 files changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 3b1f89b..1b3929f 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -559,14 +559,17 @@ unsigned virtqueue_enable_cb_prepare(struct virtqueue *_vq)
>  	u16 last_used_idx;
>  
>  	START_USE(vq);
> -
> +	last_used_idx = vq->last_used_idx;
>  	/* We optimistically turn back on interrupts, then check if there was
>  	 * more to do. */
>  	/* Depending on the VIRTIO_RING_F_EVENT_IDX feature, we need to
>  	 * either clear the flags bit or point the event index at the next
>  	 * entry. Always do both to keep code simple. */
>  	vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
> -	vring_used_event(&vq->vring) = last_used_idx = vq->last_used_idx;
> +	/* Make sure used event never go backwards */

s/go/goes/

> +	if (!vring_need_event(vring_used_event(&vq->vring),
> +			      vq->vring.avail->idx, last_used_idx))
> +		vring_used_event(&vq->vring) = last_used_idx;

The result will be that driver will *not* get an interrupt
on the next consumed buffer, which is likely not what driver
intended when it called virtqueue_enable_cb.

Instead, how about we simply document the requirement that drivers either
always call virtqueue_enable_cb_delayed or virtqueue_enable_cb
but not both?

>  	END_USE(vq);
>  	return last_used_idx;
>  }
> -- 
> 1.7.1