Re: [PATCH net] virtio-net: don't re-enable refill work too early when NAPI is disabled

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Apr 29, 2025 at 07:31:04AM -0700, Jakub Kicinski wrote:
> Commit 4bc12818b363 ("virtio-net: disable delayed refill when pausing rx")
> fixed a deadlock between reconfig paths and refill work trying to disable
> the same NAPI instance. The refill work can't run in parallel with reconfig
> because trying to double-disable a NAPI instance causes a stall under the
> instance lock, which the reconfig path needs to re-enable the NAPI and
> therefore unblock the stalled thread.
> 
> There are two cases where we re-enable refill too early. One is in the
> virtnet_set_queues() handler. We call it when installing XDP:
> 
>    virtnet_rx_pause_all(vi);
>    ...
>    virtnet_napi_tx_disable(..);
>    ...
>    virtnet_set_queues(..);
>    ...
>    virtnet_rx_resume_all(..);
> 
> We want the work to be disabled until we call virtnet_rx_resume_all(),
> but virtnet_set_queues() kicks it before NAPIs were re-enabled.
> 
> The other case is a more trivial case of mis-ordering in
> __virtnet_rx_resume() found by code inspection.
> 
> Fixes: 4bc12818b363 ("virtio-net: disable delayed refill when pausing rx")
> Fixes: 413f0271f396 ("net: protect NAPI enablement with netdev_lock()")
> Signed-off-by: Jakub Kicinski <kuba@kernel.org>


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

> ---
> CC: mst@redhat.com
> CC: jasowang@redhat.com
> CC: xuanzhuo@linux.alibaba.com
> CC: eperezma@redhat.com
> CC: minhquangbui99@gmail.com
> CC: romieu@fr.zoreil.com
> CC: kuniyu@amazon.com
> CC: virtualization@lists.linux.dev
> ---
>  drivers/net/virtio_net.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 848fab51dfa1..4c904e176495 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -3383,12 +3383,15 @@ static void __virtnet_rx_resume(struct virtnet_info *vi,
>  				bool refill)
>  {
>  	bool running = netif_running(vi->dev);
> +	bool schedule_refill = false;
>  
>  	if (refill && !try_fill_recv(vi, rq, GFP_KERNEL))
> -		schedule_delayed_work(&vi->refill, 0);
> -
> +		schedule_refill = true;
>  	if (running)
>  		virtnet_napi_enable(rq);
> +
> +	if (schedule_refill)
> +		schedule_delayed_work(&vi->refill, 0);
>  }
>  
>  static void virtnet_rx_resume_all(struct virtnet_info *vi)
> @@ -3728,7 +3731,7 @@ static int virtnet_set_queues(struct virtnet_info *vi, u16 queue_pairs)
>  succ:
>  	vi->curr_queue_pairs = queue_pairs;
>  	/* virtnet_open() will refill when device is going to up. */
> -	if (dev->flags & IFF_UP)
> +	if (dev->flags & IFF_UP && vi->refill_enabled)
>  		schedule_delayed_work(&vi->refill, 0);
>  
>  	return 0;
> -- 
> 2.49.0