linux netfilter hook中自己构造skbuff，发送全新数据包

lyl19

没有吧，我认为转发的skb最终的释放不是在dev_queue_xmit中，而是在硬件发送完成，触发的软中断中

lyl19

net_tx_action works in a similar way. It can be triggered with raise_softirq_irqoff(NET_TX_SOFTIRQ) by devices in two different contexts, to accomplish two main tasks:

By netif_wake_queue when transmission is enabled on a device. In this case, it makes sure that frames waiting to be sent are actually sent when all the needed conditions are met (for instance, when the device has enough memory).

By dev_kfree_skb_irq when a transmission has completed and the device driver signals with the former routine that the associated buffer can be released. In this case, it deallocates the sk_buff structures associated with successfully transmitted buffers.

The reason for the second task is as follows. We know that when code from the device driver runs in interrupt context, it needs to be as quick as possible. Releasing a buffer can take time, so it is deferred by asking the net_tx_action softirq to take care of it. Instead of using dev_kfree_skb, device drivers use dev_kfree_skb_irq. While the former deallocates the sk_buff (which actually consists of the buffer going back into a per-CPU cache), the latter simply adds the pointer to the buffer being released to the completion_queue list of the softnet_data structure associated with the CPU and lets net_tx_action do the real job later.