关于内核 sk_buff clone

lfwalkman

看协议栈代码的时候，看到 skb_cloned和skb_header_cloned两个函数实现，好像把skb_shinfo(skb)->dataref分作了两部分，
一部分用来高16位记录skb->data的负载的引用计数，低16位记录了整个skb->data的引用计数。
请问
1.这两部分有什么区别？在什么情况下使用？
2.为什么skb_cloned，skb_header_cloned，skb_header_release 的实现对skb_shinfo(skb)->dataref 的处理有什么意义？
/* We divide dataref into two halves.  The higher 16 bits hold references
* to the payload part of skb->data.  The lower 16 bits hold references to
* the entire skb->data.  A clone of a headerless skb holds the length of
* the header in skb->hdr_len.
*
* All users must obey the rule that the skb->data reference count must be
* greater than or equal to the payload reference count.
*
* Holding a reference to the payload part means that the user does not
* care about modifications to the header part of skb->data.
*/
#define SKB_DATAREF_SHIFT 16
#define SKB_DATAREF_MASK ((1 << SKB_DATAREF_SHIFT) - 1)

/**
*   skb_cloned - is the buffer a clone
*   @skb: buffer to check
*
*   Returns true if the buffer was generated with skb_clone() and is
*   one of multiple shared copies of the buffer. Cloned buffers are
*   shared data so must not be written to under normal circumstances.
*/
static inline int skb_cloned(const struct sk_buff *skb)
{
   return skb->cloned &&
          (atomic_read(&skb_shinfo(skb)->dataref) & SKB_DATAREF_MASK) != 1;
}

/**
*   skb_header_cloned - is the header a clone
*   @skb: buffer to check
*
*   Returns true if modifying the header part of the buffer requires
*   the data to be copied.
*/
static inline int skb_header_cloned(const struct sk_buff *skb)
{
   int dataref;
   if (!skb->cloned)
      return 0;
   dataref = atomic_read(&skb_shinfo(skb)->dataref);
   dataref = (dataref & SKB_DATAREF_MASK) - (dataref >> SKB_DATAREF_SHIFT);
   return dataref != 1;
}
/**
*   skb_header_release - release reference to header
*   @skb: buffer to operate on
*
*   Drop a reference to the header part of the buffer.  This is done
*   by acquiring a payload reference.  You must not read from the header
*   part of skb->data after this.
*/
static inline void skb_header_release(struct sk_buff *skb)
{
   BUG_ON(skb->nohdr);
   skb->nohdr = 1;
   atomic_add(1 << SKB_DATAREF_SHIFT, &skb_shinfo(skb)->dataref);
}

lfwalkman

自己顶

lfwalkman

今天继续查看源码，和TSO有关。
有没有感兴趣的朋友，一起研究。

lfwalkman

这个是tso的一部份。