netdev - Re: [PATCH net] net: openvswitch: Avoid needlessly taking the RTNL on vport destroy

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <B299AD16-8511-41B7-A36A-25B911AEEBF4@redhat.com>
Date: Wed, 10 Dec 2025 14:28:36 +0100
From: Eelco Chaudron <echaudro@...hat.com>
To: Toke Høiland-Jørgensen <toke@...hat.com>
Cc: Aaron Conole <aconole@...hat.com>, Ilya Maximets <i.maximets@....org>,
 Alexei Starovoitov <ast@...nel.org>, Jesse Gross <jesse@...ira.com>,
 Adrian Moreno <amorenoz@...hat.com>, "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>,
 netdev@...r.kernel.org, dev@...nvswitch.org
Subject: Re: [PATCH net] net: openvswitch: Avoid needlessly taking the RTNL on
 vport destroy

On 10 Dec 2025, at 13:59, Toke Høiland-Jørgensen wrote:

> The openvswitch teardown code will immediately call
> ovs_netdev_detach_dev() in response to a NETDEV_UNREGISTER notification.
> It will then start the dp_notify_work workqueue, which will later end up
> calling the vport destroy() callback. This callback takes the RTNL to do
> another ovs_netdev_detach_port(), which in this case is unnecessary.
> This causes extra pressure on the RTNL, in some cases leading to
> "unregister_netdevice: waiting for XX to become free" warnings on
> teardown.
>
> We can straight-forwardly avoid the extra RTNL lock acquisition by
> checking the device flags before taking the lock, and skip the locking
> altogether if the IFF_OVS_DATAPATH flag has already been unset.
>
> Fixes: b07c26511e94 ("openvswitch: fix vport-netdev unregister")
> Tested-by: Adrian Moreno <amorenoz@...hat.com>
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> ---
>  net/openvswitch/vport-netdev.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/net/openvswitch/vport-netdev.c b/net/openvswitch/vport-netdev.c
> index 91a11067e458..519f038526f9 100644
> --- a/net/openvswitch/vport-netdev.c
> +++ b/net/openvswitch/vport-netdev.c
> @@ -160,10 +160,13 @@ void ovs_netdev_detach_dev(struct vport *vport)
>
>  static void netdev_destroy(struct vport *vport)
>  {
> -	rtnl_lock();
> -	if (netif_is_ovs_port(vport->dev))
> -		ovs_netdev_detach_dev(vport);
> -	rtnl_unlock();
> +	if (netif_is_ovs_port(vport->dev)) {

Hi Toke,

Thanks for digging into this!

The patch looks technically correct to me, but maybe we should add a comment here explaining why we can do it this way, i.e., why we can call netif_is_ovs_port() without the lock.
For example:

/* We can avoid taking the rtnl lock as the IFF_OVS_DATAPATH flag is set/cleared in either netdev_create()/netdev_destroy(), which are both called under the global ovs_lock(). */

Additionally, I think the second netif_is_ovs_port() under the rtnl lock is not required due to the above.

> +		rtnl_lock();
> +		/* check again while holding the lock */
> +		if (netif_is_ovs_port(vport->dev))
> +			ovs_netdev_detach_dev(vport);
> +		rtnl_unlock();
> +	}
>
>  	call_rcu(&vport->rcu, vport_netdev_free);
>  }
> -- 
> 2.52.0