网络驱动移植之sk_buff结构体及其相关操作函数(下)
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(1)、dev_alloc_skb
实际上,函数dev_alloc_skb最终是调用__alloc_skb函数来分配数据缓冲区和sk_buff结构体的,如下图:
从dev_alloc_skb到__alloc_skb所涉及的源代码如下:
- /* linux-2.6.38.8/net/core/skbuff.c */
- struct sk_buff *dev_alloc_skb(unsigned int length)
- {
- /*
- * There is more code here than it seems:
- * __dev_alloc_skb is an inline
- */
- return __dev_alloc_skb(length, GFP_ATOMIC);
- }
- /* linux-2.6.38.8/include/linux/skbuff.h */
- static inline struct sk_buff *__dev_alloc_skb(unsigned int length,
- gfp_t gfp_mask)
- {
- struct sk_buff *skb = alloc_skb(length + NET_SKB_PAD, gfp_mask);
- if (likely(skb))
- skb_reserve(skb, NET_SKB_PAD);
- return skb;
- }
- /* linux-2.6.38.8/include/linux/skbuff.h */
- static inline struct sk_buff *alloc_skb(unsigned int size,
- gfp_t priority)
- {
- return __alloc_skb(size, priority, 0, NUMA_NO_NODE);
- }
/* linux-2.6.38.8/net/core/skbuff.c */struct sk_buff *dev_alloc_skb(unsigned int length){/* * There is more code here than it seems: * __dev_alloc_skb is an inline */return __dev_alloc_skb(length, GFP_ATOMIC);}/* linux-2.6.38.8/include/linux/skbuff.h */static inline struct sk_buff *__dev_alloc_skb(unsigned int length, gfp_t gfp_mask){struct sk_buff *skb = alloc_skb(length + NET_SKB_PAD, gfp_mask);if (likely(skb))skb_reserve(skb, NET_SKB_PAD);return skb;}/* linux-2.6.38.8/include/linux/skbuff.h */static inline struct sk_buff *alloc_skb(unsigned int size,gfp_t priority){return __alloc_skb(size, priority, 0, NUMA_NO_NODE);}
其中,NET_SKB_PAD的值在ARM体系架构上为32。
接下来,在__alloc_skb函数中,首先通过kmem_cache_alloc_node函数(在未配置CONFIG_NUMA和CONFIG_SLOB的情况下,它的实现就是直接调用kmem_cache_alloc函数)从skbuff_head_cache高速缓存中申请一个sk_buff结构体对象。创建skbuff_head_cache高速缓存的源代码如下:
- /* linux-2.6.38.8/net/socket.c */
- static int __init sock_init(void)
- {
- ...
- /* Initialize skbuff SLAB cache */
- skb_init();
- ...
- }
- core_initcall(sock_init); /* early initcall */
- /* linux-2.6.38.8/net/core/skbuff.c */
- void __init skb_init(void)
- {
- skbuff_head_cache = kmem_cache_create("skbuff_head_cache",
- sizeof(struct sk_buff),
- 0,
- SLAB_HWCACHE_ALIGN|SLAB_PANIC,
- NULL);
- skbuff_fclone_cache = kmem_cache_create("skbuff_fclone_cache",
- (2*sizeof(struct sk_buff)) +
- sizeof(atomic_t),
- 0,
- SLAB_HWCACHE_ALIGN|SLAB_PANIC,
- NULL);
- }
/* linux-2.6.38.8/net/socket.c */static int __init sock_init(void){.../* Initialize skbuff SLAB cache */skb_init();...}core_initcall(sock_init);/* early initcall *//* linux-2.6.38.8/net/core/skbuff.c */void __init skb_init(void){skbuff_head_cache = kmem_cache_create("skbuff_head_cache", sizeof(struct sk_buff), 0, SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL);skbuff_fclone_cache = kmem_cache_create("skbuff_fclone_cache",(2*sizeof(struct sk_buff)) +sizeof(atomic_t),0,SLAB_HWCACHE_ALIGN|SLAB_PANIC,NULL);}
申请sk_buff结构体对象的代码如下:
- /* linux-2.6.38.8/net/core/skbuff.c */
- skb = kmem_cache_alloc_node(cache, gfp_mask & ~__GFP_DMA, node);
- if (!skb)
- goto out;
- prefetchw(skb);
/* linux-2.6.38.8/net/core/skbuff.c */skb = kmem_cache_alloc_node(cache, gfp_mask & ~__GFP_DMA, node);if (!skb)goto out;prefetchw(skb);
对于S3C2410,prefetchw函数的实现是使用GCC的内置函数__builtin_prefetch,定义如下:
- /* linux-2.6.38.8/include/linux/prefetch.h */
- #ifndef ARCH_HAS_PREFETCHW
- #define prefetchw(x) __builtin_prefetch(x,1)
- #endif
/* linux-2.6.38.8/include/linux/prefetch.h */#ifndef ARCH_HAS_PREFETCHW#define prefetchw(x) __builtin_prefetch(x,1)#endif
__builtin_prefetch的函数原型为void __builtin_prefetch (const void *addr, ...),常用于最小化数据的存取时间。参数addr的值为将要预取的内存地址,另外,它还有两个可选的参数rw 和 locality,rw的值只能为常量0或者1,1用于写的预取,默认值0用于读的预取。关于它的详细使用说明请参考网址http://gcc.gnu.org/onlinedocs/gcc-4.6.2/gcc/Other-Builtins.html#Other-Builtins。
对于ARMv5,prefetchw函数使用另一种实现,而S3C2410是无法支持的。另外,对于S3C2410,__LINUX_ARM_ARCH__的值为4,在linux-2.6.38.8/arch/arm/Makefile文件中被声明。
__alloc_skb函数的另一个重要功能就是分配数据缓冲区,包括skb_shared_info结构体。先使用SKB_DATA_ALIGN宏以SMP_CACHE_BYTES(对于ARM体系架构,它的值为32)位对齐数据缓冲区(这里不包括skb_shared_info结构体)的大小,然后调用kmalloc_node_track_caller函数分配内存,代码如下:
- /* linux-2.6.38.8/net/core/skbuff.c */
- size = SKB_DATA_ALIGN(size);
- data = kmalloc_node_track_caller(size + sizeof(struct skb_shared_info),
- gfp_mask, node);
- if (!data)
- goto nodata;
- prefetchw(data + size);
/* linux-2.6.38.8/net/core/skbuff.c */size = SKB_DATA_ALIGN(size);data = kmalloc_node_track_caller(size + sizeof(struct skb_shared_info),gfp_mask, node);if (!data)goto nodata;prefetchw(data + size);
其中两个主要函数的实现如下:
- /* linux-2.6.38.8/include/linux/skbuff.h */
- #define SKB_DATA_ALIGN(X) (((X) + (SMP_CACHE_BYTES - 1)) & \
- ~(SMP_CACHE_BYTES - 1))
- /* linux-2.6.38.8/include/linux/slab.h */
- #define kmalloc_node_track_caller(size, flags, node) \
- kmalloc_track_caller(size, flags)
- #define kmalloc_track_caller(size, flags) \
- __kmalloc(size, flags)
- /* linux-2.6.38.8/mm/slab.c */
- void *__kmalloc(size_t size, gfp_t flags)
- {
- return __do_kmalloc(size, flags, NULL);
- }
/* linux-2.6.38.8/include/linux/skbuff.h */#define SKB_DATA_ALIGN(X)(((X) + (SMP_CACHE_BYTES - 1)) & \ ~(SMP_CACHE_BYTES - 1))/* linux-2.6.38.8/include/linux/slab.h */#define kmalloc_node_track_caller(size, flags, node) \kmalloc_track_caller(size, flags)#define kmalloc_track_caller(size, flags) \__kmalloc(size, flags)/* linux-2.6.38.8/mm/slab.c */void *__kmalloc(size_t size, gfp_t flags){return __do_kmalloc(size, flags, NULL);}
最后,__alloc_skb函数会完成对sk_buff和skb_shared_info两个结构体变量部分成员的初始化。
- /* linux-2.6.38.8/net/core/skbuff.c */
- memset(skb, 0, offsetof(struct sk_buff, tail));
- skb->truesize = size + sizeof(struct sk_buff);
- atomic_set(&skb->users, 1);
- skb->head = data;
- skb->data = data;
- skb_reset_tail_pointer(skb);
- skb->end = skb->tail + size;
- #ifdef NET_SKBUFF_DATA_USES_OFFSET
- skb->mac_header = ~0U;
- #endif
/* linux-2.6.38.8/net/core/skbuff.c */memset(skb, 0, offsetof(struct sk_buff, tail));skb->truesize = size + sizeof(struct sk_buff);atomic_set(&skb->users, 1);skb->head = data;skb->data = data;skb_reset_tail_pointer(skb);skb->end = skb->tail + size;#ifdef NET_SKBUFF_DATA_USES_OFFSETskb->mac_header = ~0U;#endif
其中,当NET_SKBUFF_DATA_USES_OFFSET未定义时,skb_reset_tail_pointer函数的定义如下:
- /* linux-2.6.38.8/include/linux/skbuff.h */
- static inline void skb_reset_tail_pointer(struct sk_buff *skb)
- {
- skb->tail = skb->data;
- }
/* linux-2.6.38.8/include/linux/skbuff.h */static inline void skb_reset_tail_pointer(struct sk_buff *skb){skb->tail = skb->data;}
- /* linux-2.6.38.8/net/core/skbuff.c */
- shinfo = skb_shinfo(skb);
- memset(shinfo, 0, offsetof(struct skb_shared_info, dataref));
- atomic_set(&shinfo->dataref, 1);
- kmemcheck_annotate_variable(shinfo->destructor_arg);
/* linux-2.6.38.8/net/core/skbuff.c */shinfo = skb_shinfo(skb);memset(shinfo, 0, offsetof(struct skb_shared_info, dataref));atomic_set(&shinfo->dataref, 1);kmemcheck_annotate_variable(shinfo->destructor_arg);
其中,skb_shinfo函数的定义如下:
- /* linux-2.6.38.8/include/linux/skbuff.h */
- #define skb_shinfo(SKB) ((struct skb_shared_info *)(skb_end_pointer(SKB)))
/* linux-2.6.38.8/include/linux/skbuff.h */#define skb_shinfo(SKB)((struct skb_shared_info *)(skb_end_pointer(SKB)))
当NET_SKBUFF_DATA_USES_OFFSET未定义时,skb_end_pointer函数的定义如下:
- /* linux-2.6.38.8/include/linux/skbuff.h */
- static inline unsigned char *skb_end_pointer(const struct sk_buff *skb)
- {
- return skb->end;
- }
/* linux-2.6.38.8/include/linux/skbuff.h */static inline unsigned char *skb_end_pointer(const struct sk_buff *skb){return skb->end;}
__alloc_skb函数完成的工作大致如下图(图片来自《Understanding Linux Network Internals》):
另外,当NET_SKBUFF_DATA_USES_OFFSET未定义时,sk_buff_data_t的声明如下:
- /* linux-2.6.38.8/include/linux/skbuff.h */
- typedef unsigned char *sk_buff_data_t;
/* linux-2.6.38.8/include/linux/skbuff.h */typedef unsigned char *sk_buff_data_t;
(2)、skb_reserve
skb_reserve函数用于在缓冲区的头部预留一些空间,其定义如下:
- /* linux-2.6.38.8/include/linux/skbuff.h */
- static inline void skb_reserve(struct sk_buff *skb, int len)
- {
- skb->data += len;
- skb->tail += len;
- }
/* linux-2.6.38.8/include/linux/skbuff.h */static inline void skb_reserve(struct sk_buff *skb, int len){skb->data += len;skb->tail += len;}
skb_reserve函数只是简单地更新data和tail两个指针而已,如下图(图片来自《Understanding LinuxNetwork Internals》):
(3)、skb_put
skb_put函数会把一个数据块添加到缓冲区的尾端。
- /* linux-2.6.38.8/net/core/skbuff.c */
- unsigned char *skb_put(struct sk_buff *skb, unsigned int len)
- {
- unsigned char *tmp = skb_tail_pointer(skb);
- SKB_LINEAR_ASSERT(skb);
- skb->tail += len;
- skb->len += len;
- if (unlikely(skb->tail > skb->end))
- skb_over_panic(skb, len, __builtin_return_address(0));
- return tmp;
- }
/* linux-2.6.38.8/net/core/skbuff.c */unsigned char *skb_put(struct sk_buff *skb, unsigned int len){unsigned char *tmp = skb_tail_pointer(skb);SKB_LINEAR_ASSERT(skb);skb->tail += len;skb->len += len;if (unlikely(skb->tail > skb->end))skb_over_panic(skb, len, __builtin_return_address(0));return tmp;}
其中,skb_tail_pointer函数在NET_SKBUFF_DATA_USES_OFFSET未定义时,其定义如下:
- /* linux-2.6.38.8/include/linux/skbuff.h */
- static inline unsigned char *skb_tail_pointer(const struct sk_buff *skb)
- {
- return skb->tail;
- }
/* linux-2.6.38.8/include/linux/skbuff.h */static inline unsigned char *skb_tail_pointer(const struct sk_buff *skb){return skb->tail;}
对于ARM体系结构,在CONFIG_BUG和CONFIG_DEBUG_BUGVERBOSE都配置的情况下,SKB_LINEAR_ASSERT的定义如下:
- /* linux-2.6.38.8/include/linux/skbuff.h */
- #define SKB_LINEAR_ASSERT(skb) BUG_ON(skb_is_nonlinear(skb))
- /* linux-2.6.38.8/include/asm-generic/bug.h */
- #ifndef HAVE_ARCH_BUG_ON
- #define BUG_ON(condition) do { if (unlikely(condition)) BUG(); } while(0)
- #endif
- /* linux-2.6.38.8/arch/arm/include/asm/bug.h */
- extern void __bug(const char *file, int line) __attribute__((noreturn));
- #define BUG() __bug(__FILE__, __LINE__) /* give file/line information */
- /* linux-2.6.38.8/arch/arm/kernel/traps.c */
- void __attribute__((noreturn)) __bug(const char *file, int line)
- {
- printk(KERN_CRIT"kernel BUG at %s:%d!\n", file, line);
- *(int *)0 = 0;
- /* Avoid "noreturn function does return" */
- for (;;);
- }
/* linux-2.6.38.8/include/linux/skbuff.h */#define SKB_LINEAR_ASSERT(skb) BUG_ON(skb_is_nonlinear(skb))/* linux-2.6.38.8/include/asm-generic/bug.h */#ifndef HAVE_ARCH_BUG_ON#define BUG_ON(condition) do { if (unlikely(condition)) BUG(); } while(0)#endif/* linux-2.6.38.8/arch/arm/include/asm/bug.h */extern void __bug(const char *file, int line) __attribute__((noreturn));#define BUG()__bug(__FILE__, __LINE__) /* give file/line information *//* linux-2.6.38.8/arch/arm/kernel/traps.c */void __attribute__((noreturn)) __bug(const char *file, int line){printk(KERN_CRIT"kernel BUG at %s:%d!\n", file, line);*(int *)0 = 0;/* Avoid "noreturn function does return" */for (;;);}
当函数skb_is_nonlinear返回非零值(也就是skb->data_len的值不为0)时,SKB_LINEAR_ASSERT将产生一个oops消息。skb_is_nonlinear的定义如下:
- /* linux-2.6.38.8/include/linux/skbuff.h */
- static inline int skb_is_nonlinear(const struct sk_buff *skb)
- {
- return skb->data_len;
- }
/* linux-2.6.38.8/include/linux/skbuff.h */static inline int skb_is_nonlinear(const struct sk_buff *skb){return skb->data_len;}
skb_put函数其实也没有真的把数据添加到缓冲区中,而只是简单地更新了skb->tail和skb->len的值,如下图(图片来自《Understanding Linux Network Internals》):
(4)、skb_push
skb_push函数会把一个数据块添加到缓冲区的开端。
- /* linux-2.6.38.8/net/core/skbuff.c */
- unsigned char *skb_push(struct sk_buff *skb, unsigned int len)
- {
- skb->data -= len;
- skb->len += len;
- if (unlikely(skb->data<skb->head))
- skb_under_panic(skb, len, __builtin_return_address(0));
- return skb->data;
- }
/* linux-2.6.38.8/net/core/skbuff.c */unsigned char *skb_push(struct sk_buff *skb, unsigned int len){skb->data -= len;skb->len += len;if (unlikely(skb->data<skb->head))skb_under_panic(skb, len, __builtin_return_address(0));return skb->data;}
skb_push函数其实也没有真的把数据添加到缓冲区中,而只是简单地更新了skb->data和skb->len的值,如下图(图片来自《Understanding Linux Network Internals》):
(5)、skb_pull
skb_pull函数会把一个数据块从缓冲区中的顶端删除。
- /* linux-2.6.38.8/net/core/skbuff.c */
- unsigned char *skb_pull(struct sk_buff *skb, unsigned int len)
- {
- return skb_pull_inline(skb, len);
- }
- /* linux-2.6.38.8/include/linux/skbuff.h */
- static inline unsigned char *skb_pull_inline(struct sk_buff *skb, unsigned int len)
- {
- return unlikely(len > skb->len) ? NULL : __skb_pull(skb, len);
- }
- static inline unsigned char *__skb_pull(struct sk_buff *skb, unsigned int len)
- {
- skb->len -= len;
- BUG_ON(skb->len < skb->data_len);
- return skb->data += len;
- }
/* linux-2.6.38.8/net/core/skbuff.c */unsigned char *skb_pull(struct sk_buff *skb, unsigned int len){return skb_pull_inline(skb, len);}/* linux-2.6.38.8/include/linux/skbuff.h */static inline unsigned char *skb_pull_inline(struct sk_buff *skb, unsigned int len){return unlikely(len > skb->len) ? NULL : __skb_pull(skb, len);}static inline unsigned char *__skb_pull(struct sk_buff *skb, unsigned int len){skb->len -= len;BUG_ON(skb->len < skb->data_len);return skb->data += len;}
skb_pull函数其实也没有真的把数据从缓冲区中删除,而只是简单地更新了skb->data和skb->len的值,如下图(图片来自《Understanding Linux Network Internals》):
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