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

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Message-ID: <CACGkMEs0LuLDdEphRcdmKthdJeNAJzHBqKTe8Euhm2adOS=k2w@mail.gmail.com>
Date: Wed, 30 Apr 2025 11:49:15 +0800
From: Jason Wang <jasowang@...hat.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: davem@...emloft.net, netdev@...r.kernel.org, edumazet@...gle.com, 
	pabeni@...hat.com, andrew+netdev@...n.ch, horms@...nel.org, mst@...hat.com, 
	xuanzhuo@...ux.alibaba.com, eperezma@...hat.com, minhquangbui99@...il.com, 
	romieu@...zoreil.com, kuniyu@...zon.com, virtualization@...ts.linux.dev
Subject: Re: [PATCH net] virtio-net: don't re-enable refill work too early
 when NAPI is disabled

On Tue, Apr 29, 2025 at 10:31 PM Jakub Kicinski <kuba@...nel.org> 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@...nel.org>
> ---
> CC: mst@...hat.com
> CC: jasowang@...hat.com
> CC: xuanzhuo@...ux.alibaba.com
> CC: eperezma@...hat.com
> CC: minhquangbui99@...il.com
> CC: romieu@...zoreil.com
> CC: kuniyu@...zon.com
> CC: virtualization@...ts.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);

This has the assumption that the toggle of the refill_enabled is under
RTNL. Though it's true now but it looks to me it's better to protect
it against refill_lock.

Thanks

>
>         return 0;
> --
> 2.49.0
>