Understanding Linux Network Internals

Old Interface Between Device Drivers and Kernel: First Part of netif_rx

The netif_rx function, defined in net/core/dev.c, is normally called by device drivers when new input frames are waiting to be processed;^[*] its job is to schedule the softirq that runs shortly to dequeue and handle the frames. Figure 10-3 shows what it checks for and the flow of its events. The figure is practically longer than the code, but it is useful to help understand how netif_rx reacts to its context.

netif_rx is usually called by a driver while in interrupt context, but there are exceptions, notably when the function is called by the loopback device. For this reason, netif_rx disables interrupts on the local CPU when it starts, and re-enables them when it finishes.^[†]

When looking at the code, one should keep in mind that different CPUs can run netif_rx concurrently. This is not a problem, since each CPU is associated with a private softnet_data structure that maintains state information. Among other things, the CPU's softnet_data structure includes a private input queue (see the section "softnet_data Structure" in Chapter 9).

Figure 10-3. netif_rx function

This is the function's prototype:

int netif_rx(struct sk_buff *skb)

Its only input parameter is the buffer received by the device, and the output value is an indication of the congestion level (you can find details in the section "Congestion Management