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Date:   Tue, 22 Dec 2020 11:49:04 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Jeff Dike <jdike@...mai.com>, "Michael S. Tsirkin" <mst@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH] virtio_net: Fix recursive call to cpus_read_lock()


On 2020/12/22 上午11:36, Jeff Dike wrote:
> virtnet_set_channels can recursively call cpus_read_lock if CONFIG_XPS
> and CONFIG_HOTPLUG are enabled.
>
> The path is:
>      virtnet_set_channels - calls get_online_cpus(), which is a trivial
> wrapper around cpus_read_lock()
>      netif_set_real_num_tx_queues
>      netif_reset_xps_queues_gt
>      netif_reset_xps_queues - calls cpus_read_lock()
>
> This call chain and potential deadlock happens when the number of TX
> queues is reduced.
>
> This commit the removes netif_set_real_num_[tr]x_queues calls from
> inside the get/put_online_cpus section, as they don't require that it
> be held.
>
> Signed-off-by: Jeff Dike <jdike@...mai.com>
> ---


Adding netdev.

The patch can go with -net and is needed for -stable.

Acked-by: Jason Wang <jasowang@...hat.com>


>   drivers/net/virtio_net.c | 12 +++++++-----
>   1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 052975ea0af4..e02c7e0f1cf9 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -2093,14 +2093,16 @@ static int virtnet_set_channels(struct net_device *dev,
>   
>   	get_online_cpus();
>   	err = _virtnet_set_queues(vi, queue_pairs);
> -	if (!err) {
> -		netif_set_real_num_tx_queues(dev, queue_pairs);
> -		netif_set_real_num_rx_queues(dev, queue_pairs);
> -
> -		virtnet_set_affinity(vi);
> +	if (err){
> +		put_online_cpus();
> +		goto err;
>   	}
> +	virtnet_set_affinity(vi);
>   	put_online_cpus();
>   
> +	netif_set_real_num_tx_queues(dev, queue_pairs);
> +	netif_set_real_num_rx_queues(dev, queue_pairs);
> + err:
>   	return err;
>   }
>   

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