netdev - Re: [PATCH RFC 1/2] tun: don't require serialization lock on tx

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Message-ID: <20160413123828-mutt-send-email-mst@redhat.com>
Date:	Wed, 13 Apr 2016 12:41:45 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Paolo Abeni <pabeni@...hat.com>
Cc:	netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Greg Kurz <gkurz@...ux.vnet.ibm.com>,
	Jason Wang <jasowang@...hat.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [PATCH RFC 1/2] tun: don't require serialization lock on tx

On Wed, Apr 13, 2016 at 11:04:46AM +0200, Paolo Abeni wrote:
> The current tun_net_xmit() implementation don't need any external
> lock since it relay on rcu protection for the tun data structure
> and on socket queue lock for skb queuing.
> 
> This patch set the NETIF_F_LLTX feature bit in the tun device, so
> that on xmit, in absence of qdisc, no serialization lock is acquired
> by the caller.
> 
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> ---
>  drivers/net/tun.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index faf9297..42992dc 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -1796,7 +1796,7 @@ static int tun_set_iff(struct net *net, struct file *file, struct ifreq *ifr)
>  		dev->hw_features = NETIF_F_SG | NETIF_F_FRAGLIST |
>  				   TUN_USER_FEATURES | NETIF_F_HW_VLAN_CTAG_TX |
>  				   NETIF_F_HW_VLAN_STAG_TX;
> -		dev->features = dev->hw_features;
> +		dev->features = dev->hw_features | NETIF_F_LLTX;

the documentation says:
        NETIF_F_LLTX_BIT,               /* LockLess TX - deprecated.  Please */
                                        /* do not use LLTX in new drivers */


git log gives this explanation:
    > 1) unnecessary;
    > 2) causes problems with AF_PACKET seeing things twice.



>  		dev->vlan_features = dev->features &
>  				     ~(NETIF_F_HW_VLAN_CTAG_TX |
>  				       NETIF_F_HW_VLAN_STAG_TX);
> -- 
> 1.8.3.1