NETIF_F_LLTX 的属性

在bond初始化的时候，我们可以看到如下属性：

/* don't acquire bond device's netif_tx_lock when transmitting */

    bond_dev->features |= NETIF_F_LLTX;

 

在lo口设置的时候，也可以看到这个属性：

static void loopback_setup(struct net_device *dev)
{
    dev->mtu        = 64 * 1024;
    dev->hard_header_len    = ETH_HLEN;    /* 14    */
    dev->addr_len        = ETH_ALEN;    /* 6    */
    dev->type        = ARPHRD_LOOPBACK;    /* 0x0001*/
    dev->flags        = IFF_LOOPBACK;
    dev->priv_flags        |= IFF_LIVE_ADDR_CHANGE | IFF_NO_QUEUE;
    netif_keep_dst(dev);
    dev->hw_features    = NETIF_F_GSO_SOFTWARE;
    dev->features         = NETIF_F_SG | NETIF_F_FRAGLIST
        | NETIF_F_GSO_SOFTWARE
        | NETIF_F_HW_CSUM
        | NETIF_F_RXCSUM
        | NETIF_F_SCTP_CRC
        | NETIF_F_HIGHDMA
        | NETIF_F_LLTX
        | NETIF_F_NETNS_LOCAL
        | NETIF_F_VLAN_CHALLENGED
        | NETIF_F_LOOPBACK;
    dev->ethtool_ops    = &loopback_ethtool_ops;
    dev->header_ops        = &eth_header_ops;
    dev->netdev_ops        = &loopback_ops;
    dev->destructor        = loopback_dev_free;
}

NETIF_F_LLTX 属性代表的是，我们在发包的时候，不需要获取锁。

而对于非虚拟设备，如实际网卡，则这个属性一般是不设置的，也就是需要获取TX的锁。

不过，也有注释说后面的驱动最好不要用这个特性：

* LLTX driver (deprecated for hardware drivers)

NETIF_F_LLTX should be set in drivers that implement their own locking in
transmit path or don't need locking at all (e.g. software tunnels).
In ndo_start_xmit, it is recommended to use a try_lock and return
NETDEV_TX_LOCKED when the spin lock fails.  The locking should also properly
protect against other callbacks (the rules you need to find out).

Don't use it for new drivers.