Subject: question about napi_disable (was Re: [PATCH] virtio_net: set/cancel work on ndo_open/ndo_stop)

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 4 Apr 2012 12:32:29 +0300

On Thu, Dec 29, 2011 at 09:12:38PM +1030, Rusty Russell wrote:
> Michael S. Tsirkin noticed that we could run the refill work after
> ndo_close, which can re-enable napi - we don't disable it until
> virtnet_remove.  This is clearly wrong, so move the workqueue control
> to ndo_open and ndo_stop (aka. virtnet_open and virtnet_close).
> 
> One subtle point: virtnet_probe() could simply fail if it couldn't
> allocate a receive buffer, but that's less polite in virtnet_open() so
> we schedule a refill as we do in the normal receive path if we run out
> of memory.
> 
> Signed-off-by: Rusty Russell <[email-masked]>

Doh.
napi_disable does not prevent the following
napi_schedule, does it?

Can someone confirm that I am not seeing things please?

And this means this hack does not work:
try_fill_recv can still run in parallel with
napi, corrupting the vq.

I suspect we need to resurrect a patch that used a
dedicated flag to avoid this race.

Comments?

> ---
>  drivers/net/virtio_net.c |   17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -439,7 +439,13 @@ static int add_recvbuf_mergeable(struct 
>  	return err;
>  }
>  
> -/* Returns false if we couldn't fill entirely (OOM). */
> +/*
> + * Returns false if we couldn't fill entirely (OOM).
> + *
> + * Normally run in the receive path, but can also be run from ndo_open
> + * before we're receiving packets, or from refill_work which is
> + * careful to disable receiving (using napi_disable).
> + */
>  static bool try_fill_recv(struct virtnet_info *vi, gfp_t gfp)
>  {
>  	int err;
> @@ -719,6 +725,10 @@ static int virtnet_open(struct net_devic
>  {
>  	struct virtnet_info *vi = netdev_priv(dev);
>  
> +	/* Make sure we have some buffers: if oom use wq. */
> +	if (!try_fill_recv(vi, GFP_KERNEL))
> +		schedule_delayed_work(&vi->refill, 0);
> +
>  	virtnet_napi_enable(vi);
>  	return 0;
>  }
> @@ -772,6 +782,8 @@ static int virtnet_close(struct net_devi
>  {
>  	struct virtnet_info *vi = netdev_priv(dev);
>  
> +	/* Make sure refill_work doesn't re-enable napi! */
> +	cancel_delayed_work_sync(&vi->refill);
>  	napi_disable(&vi->napi);
>  
>  	return 0;
> @@ -1082,7 +1094,6 @@ static int virtnet_probe(struct virtio_d
>  
>  unregister:
>  	unregister_netdev(dev);
> -	cancel_delayed_work_sync(&vi->refill);
>  free_vqs:
>  	vdev->config->del_vqs(vdev);
>  free_stats:
> @@ -1121,9 +1132,7 @@ static void __devexit virtnet_remove(str
>  	/* Stop all the virtqueues. */
>  	vdev->config->reset(vdev);
>  
> -
>  	unregister_netdev(vi->dev);
> -	cancel_delayed_work_sync(&vi->refill);
>  
>  	/* Free unused buffers in both send and recv, if any. */
>  	free_unused_bufs(vi);