netdev - Re: [PATCH RFC v7 02/14] fixup! vhost: option to fetch descriptors through an independent struct

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Date:   Wed, 10 Jun 2020 16:00:01 +0200
From:   Eugenio Pérez <eperezma@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>, linux-kernel@...r.kernel.org
Cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH RFC v7 02/14] fixup! vhost: option to fetch descriptors
 through an independent struct

On Wed, 2020-06-10 at 07:35 -0400, Michael S. Tsirkin wrote:
> ---
>  drivers/vhost/vhost.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 180b7b58c76b..11433d709651 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -2614,7 +2614,7 @@ int vhost_get_vq_desc_batch(struct vhost_virtqueue *vq,
>  err_fetch:
>  	vq->ndescs = 0;
>  
> -	return ret;
> +	return ret ? ret : vq->num;
>  }
>  EXPORT_SYMBOL_GPL(vhost_get_vq_desc_batch);
>  

I'm able to lock the vhost_get_vq_desc running virtio_test with no arguments against this patch. It does not happen if
it returns vq->num early, appended below.

Let me know if you prefer your conditional at the end of the function and I will investigate the cause.

Thanks!

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index 28f324fd77df..4d198994e7be 100644
@@ -2350,7 +2345,9 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
 	int ret = fetch_descs(vq);
 	int i;
 
-	if (ret <= 0)
+	if (ret == 0)
+		return vq->num;
+	else if (ret < 0)
 		goto err;
 
 	/* Now convert to IOV */