netdev - Re: [PATCH Repost to netdev] virtio_net: Fix recursive call to cpus_read

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Message-ID: <20201223091344.1363c061@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Wed, 23 Dec 2020 09:13:44 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Jeff Dike <jdike@...mai.com>
Cc:     <netdev@...r.kernel.org>,
        <virtualization@...ts.linux-foundation.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>, <stable@...r.kernel.org>
Subject: Re: [PATCH Repost to netdev] virtio_net: Fix recursive call to
 cpus_read_lock()

On Tue, 22 Dec 2020 21:54:21 -0500 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.

Fixes: 47be24796c13 ("virtio-net: fix the set affinity bug when CPU IDs are not consecutive")

> Signed-off-by: Jeff Dike <jdike@...mai.com>
> Acked-by: Jason Wang <jasowang@...hat.com>
> Acked-by: Michael S. Tsirkin <mst@...hat.com>
> Cc: stable@...r.kernel.org

Queued for stable.

> 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){

Added missing space here.

> +		put_online_cpus();
> +		goto err;
>  	}

And applied, thanks!