An example of using memory barriers in a device driver

As one example, take the Realtek 8139 "fast Ethernet" network driver. In order to transmit a network packet via DMA, it must first set up a DMA (transmit) descriptor object. For this particular hardware (NIC chip), the DMA descriptor object is defined as follows:

//​ drivers/net/ethernet/realtek/8139cp.c
struct cp_desc {
    __le32 opts1;
    __le32 opts2;
    __le64 addr;
};

The DMA descriptor object, christened struct cp_desc, has three "words." Each of them has to be initialized. Now, to ensure that the descriptor is correctly interpreted by the DMA controller, it's often critical that the writes to the DMA descriptor are seen in the same order as the driver author intends. To guarantee this, memory barriers are used. In fact, the relevant kernel documentation – the Dynamic DMA mapping Guide (https://www.kernel.org/doc/Documentation/DMA-API-HOWTO.txt) – tells us to ensure that...