Ringbuffer同步问题分析&rculist测试

ringbuffer.c实现例子:

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include "ringbuffer.h"

void ringbuffer_init(struct ringbuffer *rb, uint8_t *pool, int16_t size)
{
    if(rb == NULL)
    {
        return;
    }

    rb->read_mirror = rb->read_index = 0;
    rb->write_mirror = rb->write_index = 0;

    rb->buffer_ptr = pool;
    rb->buffer_size = size;
    pthread_mutex_init(&rb->mutex_lock,NULL);

    return;
}


void ringbuffer_reset(struct ringbuffer *rb)
{
    if(rb == NULL)
    {
        return;
    }
    
    rb->read_mirror = 0;
    rb->read_index = 0;
    rb->write_mirror = 0;
    rb->write_index = 0;

    return;
}


int16_t ringbuffer_put(struct ringbuffer *rb, const uint8_t *ptr, int16_t length)
{
    if(rb == NULL || length == 0)
    {
        return 0;
    }
    
    int16_t size; 

    pthread_mutex_lock(&rb->mutex_lock);
    size = ringbuffer_space_len(rb);
    pthread_mutex_unlock(&rb->mutex_lock);

    if(size == 0)
	return 0;

    if (size < length)
    {
        length = size;
    }

    if (rb->buffer_size - rb->write_index > length)
    {
        memcpy(&rb->buffer_ptr[rb->write_index], ptr, length);
	rb->write_index += length; 
	return length;
    }

    memcpy(&rb->buffer_ptr[rb->write_index],&ptr[0],rb->buffer_size - rb->write_index);
    memcpy(&rb->buffer_ptr[0],&ptr[rb->buffer_size - rb->write_index],length - (rb->buffer_size - rb->write_index));

    pthread_mutex_lock(&rb->mutex_lock);
    rb->write_mirror = ~rb->write_mirror;
    rb->write_index = length - (rb->buffer_size - rb->write_index);
    pthread_mutex_unlock(&rb->mutex_lock);

    return length;
}

int16_t ringbuffer_get(struct ringbuffer *rb, uint8_t *ptr, int16_t length)
{
    if(rb == NULL || length == 0)
    {
        return 0;
    }
    
    int16_t size; 
    
    pthread_mutex_lock(&rb->mutex_lock);
    size = ringbuffer_data_len(rb);
    pthread_mutex_unlock(&rb->mutex_lock);

    if (size == 0) return 0;

    if (size < length)
    {
        length = size;
    }

    if (rb->buffer_size - rb->read_index > length)
    {
        memcpy(ptr, &rb->buffer_ptr[rb->read_index], length);
	rb->read_index += length;
	return length;
    }

    memcpy(&ptr[0],&rb->buffer_ptr[rb->read_index],rb->buffer_size - rb->read_index);
    memcpy(&ptr[rb->buffer_size - rb->read_index], &rb->buffer_ptr[0], length - (rb->buffer_size - rb->read_index));

    pthread_mutex_lock(&rb->mutex_lock);
    rb->read_mirror = ~rb->read_mirror; 
    rb->read_index = length - (rb->buffer_size - rb->read_index);
    pthread_mutex_unlock(&rb->mutex_lock);

    return length;
}

enum ringbuffer_state ringbuffer_status(struct ringbuffer *rb) 
{
    if (rb->read_index == rb->write_index)
    {
        if (rb->read_mirror == rb->write_mirror)
        {
            return RINGBUFFER_EMPTY;
        }
        else
        {
            return RINGBUFFER_FULL;
        }
    }

    return RINGBUFFER_HALFFULL;
}

int16_t ringbuffer_data_len(struct ringbuffer *rb)
{
    switch (ringbuffer_status(rb)) 
    {
        case RINGBUFFER_EMPTY:
            return 0;
        case RINGBUFFER_FULL:
	    return rb->buffer_size; 
        case RINGBUFFER_HALFFULL:
	default:
            if (rb->write_index > rb->read_index)
                return rb->write_index - rb->read_index;
	    else
                return rb->buffer_size - (rb->read_index - rb->write_index); 
    }
}

ringbuffer.h

#ifndef __RING_BUFFER__
#define __RING_BUFFER__
#include <stdint.h>
#include <stddef.h>
#include <pthread.h>
#include <unistd.h>

struct ringbuffer
{
    uint8_t *buffer_ptr;

    uint16_t read_mirror : 1;
    uint16_t read_index : 15;

    uint16_t write_mirror : 1;
    uint16_t write_index : 15;

    int16_t buffer_size;
    pthread_mutex_t mutex_lock;
};

enum ringbuffer_state
{
    RINGBUFFER_EMPTY,
    RINGBUFFER_FULL,
    RINGBUFFER_HALFFULL,
    RINGBUFFER_INVALID,
};

void ringbuffer_init(struct ringbuffer *rb, uint8_t *pool, int16_t size);
void ringbuffer_reset(struct ringbuffer *rb);
int16_t ringbuffer_put(struct ringbuffer *rb, const uint8_t *ptr, int16_t length);
int16_t ringbuffer_get(struct ringbuffer *rb, uint8_t *ptr, int16_t length);
int16_t ringbuffer_data_len(struct ringbuffer *rb);

struct ringbuffer* ringbuffer_create(int16_t length);
void ringbuffer_destroy(struct ringbuffer *rb);

static inline int16_t ringbuffer_get_size(struct ringbuffer *rb)
{
   return rb->buffer_size;    
}

#define ringbuffer_space_len(rb) ((rb)->buffer_size - ringbuffer_data_len(rb))

#endif

main.c

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>
#include <string.h>
#include "ringbuffer.h"

pthread_mutex_t mutex=PTHREAD_MUTEX_INITIALIZER;

struct ringbuffer *rb;
void *writer_thread(void *args)
{
    uint8_t w = 0;
    while(1)
    {
        /*pthread_mutex_lock(&mutex);*/
        if(ringbuffer_put(rb, &w, 1) == 1)
	    w ++;
        /*pthread_mutex_unlock(&mutex);*/
    }

    return NULL;
}

void *reader_thread(void *args)
{
    uint8_t r;
    while(1)
    {
        /*pthread_mutex_lock(&mutex);*/
        if (ringbuffer_get(rb, &r, 1) == 1)
            printf("---%x---\n", r);
        /*pthread_mutex_unlock(&mutex);*/
    }

    return NULL;
}

int main(void)
{
    unsigned int rc;

#if 1
    uint8_t *buffer = (uint8_t *)malloc(1024 + sizeof(struct ringbuffer) + 16);

    if(buffer == NULL) return -1;
    if((unsigned long)buffer & 7 != 0)
    {
        printf("%s line %d not aligned.\n", __func__, __LINE__);
	return -1;
    }
   
    rb=(struct ringbuffer *)buffer;

    if(rb == NULL) return -1;
    ringbuffer_init(rb, buffer + sizeof(struct ringbuffer), 1024);
    printf("buffer = %p, sizeof(rb) = %ld.\n", buffer, sizeof(struct ringbuffer));

#else
    static struct ringbuffer rbb;

    uint8_t *buffer = (uint8_t *)malloc(1024);
    if(buffer == NULL) return -1;
    rb = &rbb;
    memset(rb, 0x00,sizeof(struct ringbuffer));
    memset(buffer, 0x00,1024);
    ringbuffer_init(rb, buffer, 1024);
#endif

    pthread_t writer;
    rc = pthread_create(&writer, NULL, writer_thread, NULL);
    if (rc)
    {
        printf("ERROR; return code is %d\n", rc);
        return EXIT_FAILURE;
    }

    pthread_t reader;
    rc = pthread_create(&reader, NULL, reader_thread, NULL);
    if (rc)
    {
        printf("ERROR; return code is %d\n", rc);
        return EXIT_FAILURE;
    }

    pthread_join(reader, NULL);
    pthread_join(writer, NULL);
    return EXIT_SUCCESS;
}

运行无问题,但是当把ringbuffer.c中的锁保护去掉后:

执行出错:

用GDB观察一下:

length竟然是负值.

所以源头是:

测试并计算并非原子的,所以这里肯定有问题,需要加锁保护,比如下图

在判断的时候write > read, 比如147行,但是当计算的时候,由于write是独立的线程,它已经将write递增并回绕到56.而且read由于在本线程,仍然保留了706,所以已经不符合判断时候的逻辑write>read.

但是仍然按照错误的逻辑计算,所得得到了负值.

解决的办法就是将判断和计算上锁原子化.

for a single procducer and signle comsumer scenario, the ring buffer need not to be locked.

just use the correcdt memory barrier opertions to ganuantee the right result

you can find a implemention of this manner in linux kernel:

./linux-6.0.2/lib/kfifo.c

rculist

#define _GNU_SOURCE 
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <unistd.h>
#include <time.h>
#include "list.h"
 
#define  NUM_THREADS     200

int nano_delay(long delay)
{
    struct timespec req, rem;
    long nano_delay = delay;
    int ret = 0;
    while(nano_delay > 0)
    {
        rem.tv_sec = 0;
        rem.tv_nsec = 0;
        req.tv_sec = 0;
        req.tv_nsec = nano_delay;
        if(ret = (nanosleep(&req, &rem) == -1))
        {
            printf("nanosleep failed.\n");
        }
        nano_delay = rem.tv_nsec;
    }
    return ret;
}

struct list_head_obj list_array[100];
void *rcu_list_process(void *args)
{
    struct list_head_obj *pos, *n;
    while(1)
    {
    	list_for_each_entry_safe(pos, n, &list_array[0].list, list)
    	{
	    printf("%ld. pos->val = %d.\n", (unsigned long)args, pos->val);
	    nano_delay(3);
    	}
	//nano_delay(30000000);
    }
    return NULL;
}

int del_and_entry(int i)
{
    printf("----------------------------------------------------------------------------------------------------\n");
    list_del_rcu(&list_array[i].list);
    sched_yield();
    list_add_rcu(&list_array[i].list, &list_array[i-1].list);
    printf("----------------------------------------------------------------------------------------------------\n");
}
void *rcu_list_writer(void *args)
{
    int s = 0;
    while(1)
    {
        for(s = 1; s < 99; s ++)
	{
	    del_and_entry(s);
	}
    }
    return NULL;
}
int main(void)
{
    unsigned long t;
    int rc;
    pthread_t thread[NUM_THREADS];
    int i = 0;

    for(i = 0; i < 100; i ++)
    {
	INIT_LIST_HEAD(&list_array[i].list);
	list_array[i].val = i;
    }

    for(i = 1; i < 100; i ++)
    {
	list_add_tail(&list_array[i].list, &list_array[0].list)	;
    }

    pthread_t writer;
    rc = pthread_create(&writer, NULL, rcu_list_writer, NULL);
    if (rc)
    {
        printf("ERROR; return code is %d\n", rc);
        return EXIT_FAILURE;
    }

    for( t = 0; t < NUM_THREADS; t++)
    {
        printf("Creating thread %ld\n", t);
        rc = pthread_create(&thread[t], NULL, rcu_list_process, (void*)t);
        if (rc)
        {
            printf("ERROR; return code is %d\n", rc);
            return EXIT_FAILURE;
        }
    }

    for( t = 0; t < NUM_THREADS; t++)
        pthread_join(thread[t], NULL);
    return EXIT_SUCCESS;
}

list.h

/* SPDX-License-Identifier: GPL-2.0 */
#ifndef _LINUX_LIST_H
#define _LINUX_LIST_H
#include <stddef.h>
#include <stdint.h>

#define LIST_POISON1  ((void *) 0x00100100)
#define LIST_POISON2  ((void *) 0x00200200)

struct list_head {
	struct list_head *next, *prev;
};
struct list_head_obj {
	struct list_head list;
	int val;
};
/*
 * Simple doubly linked list implementation.
 *
 * Some of the internal functions ("__xxx") are useful when
 * manipulating whole lists rather than single entries, as
 * sometimes we already know the next/prev entries and we can
 * generate better code by using them directly rather than
 * using the generic single-entry routines.
 */

#define LIST_HEAD_INIT(name) { &(name), &(name) }

#define LIST_HEAD(name) \
	struct list_head name = LIST_HEAD_INIT(name)

static inline void INIT_LIST_HEAD(struct list_head *list)
{
	list->next = list;
	list->prev = list;
}

/*
 * Insert a new entry between two known consecutive entries.
 *
 * This is only for internal list manipulation where we know
 * the prev/next entries already!
 */
static inline void __list_add(struct list_head *new,
			      struct list_head *prev,
			      struct list_head *next)
{
	next->prev = new;
	new->next = next;
	new->prev = prev;
	prev->next = new;
}

/**
 * list_add - add a new entry
 * @new: new entry to be added
 * @head: list head to add it after
 *
 * Insert a new entry after the specified head.
 * This is good for implementing stacks.
 */
static inline void list_add(struct list_head *new, struct list_head *head)
{
	__list_add(new, head, head->next);
}


/**
 * list_add_tail - add a new entry
 * @new: new entry to be added
 * @head: list head to add it before
 *
 * Insert a new entry before the specified head.
 * This is useful for implementing queues.
 */
static inline void list_add_tail(struct list_head *new, struct list_head *head)
{
	__list_add(new, head->prev, head);
}

/*
 * Delete a list entry by making the prev/next entries
 * point to each other.
 *
 * This is only for internal list manipulation where we know
 * the prev/next entries already!
 */
static inline void __list_del(struct list_head * prev, struct list_head * next)
{
	next->prev = prev;
	prev->next = next;
}

/**
 * list_del - deletes entry from list.
 * @entry: the element to delete from the list.
 * Note: list_empty() on entry does not return true after this, the entry is
 * in an undefined state.
 */
static inline void __list_del_entry(struct list_head *entry)
{
	__list_del(entry->prev, entry->next);
}

static inline void list_del(struct list_head *entry)
{
	__list_del_entry(entry);
	entry->next = LIST_POISON1;
	entry->prev = LIST_POISON2;
}

/**
 * list_replace - replace old entry by new one
 * @old : the element to be replaced
 * @new : the new element to insert
 *
 * If @old was empty, it will be overwritten.
 */
static inline void list_replace(struct list_head *old,
				struct list_head *new)
{
	new->next = old->next;
	new->next->prev = new;
	new->prev = old->prev;
	new->prev->next = new;
}

static inline void list_replace_init(struct list_head *old,
					struct list_head *new)
{
	list_replace(old, new);
	INIT_LIST_HEAD(old);
}

/**
 * list_swap - replace entry1 with entry2 and re-add entry1 at entry2's position
 * @entry1: the location to place entry2
 * @entry2: the location to place entry1
 */
static inline void list_swap(struct list_head *entry1,
			     struct list_head *entry2)
{
	struct list_head *pos = entry2->prev;

	list_del(entry2);
	list_replace(entry1, entry2);
	if (pos == entry1)
		pos = entry2;
	list_add(entry1, pos);
}

/**
 * list_del_init - deletes entry from list and reinitialize it.
 * @entry: the element to delete from the list.
 */
static inline void list_del_init(struct list_head *entry)
{
	__list_del_entry(entry);
	INIT_LIST_HEAD(entry);
}

/**
 * list_move - delete from one list and add as another's head
 * @list: the entry to move
 * @head: the head that will precede our entry
 */
static inline void list_move(struct list_head *list, struct list_head *head)
{
	__list_del_entry(list);
	list_add(list, head);
}

/**
 * list_move_tail - delete from one list and add as another's tail
 * @list: the entry to move
 * @head: the head that will follow our entry
 */
static inline void list_move_tail(struct list_head *list,
				  struct list_head *head)
{
	__list_del_entry(list);
	list_add_tail(list, head);
}

/**
 * list_bulk_move_tail - move a subsection of a list to its tail
 * @head: the head that will follow our entry
 * @first: first entry to move
 * @last: last entry to move, can be the same as first
 *
 * Move all entries between @first and including @last before @head.
 * All three entries must belong to the same linked list.
 */
static inline void list_bulk_move_tail(struct list_head *head,
				       struct list_head *first,
				       struct list_head *last)
{
	first->prev->next = last->next;
	last->next->prev = first->prev;

	head->prev->next = first;
	first->prev = head->prev;

	last->next = head;
	head->prev = last;
}

/**
 * list_is_first -- tests whether @list is the first entry in list @head
 * @list: the entry to test
 * @head: the head of the list
 */
static inline int list_is_first(const struct list_head *list,
					const struct list_head *head)
{
	return list->prev == head;
}

/**
 * list_is_last - tests whether @list is the last entry in list @head
 * @list: the entry to test
 * @head: the head of the list
 */
static inline int list_is_last(const struct list_head *list,
				const struct list_head *head)
{
	return list->next == head;
}

/**
 * list_empty - tests whether a list is empty
 * @head: the list to test.
 */
static inline int list_empty(const struct list_head *head)
{
	return head->next == head;
}

/**
 * list_empty_careful - tests whether a list is empty and not being modified
 * @head: the list to test
 *
 * Description:
 * tests whether a list is empty _and_ checks that no other CPU might be
 * in the process of modifying either member (next or prev)
 *
 * NOTE: using list_empty_careful() without synchronization
 * can only be safe if the only activity that can happen
 * to the list entry is list_del_init(). Eg. it cannot be used
 * if another CPU could re-list_add() it.
 */
static inline int list_empty_careful(const struct list_head *head)
{
	struct list_head *next = head->next;
	return (next == head) && (next == head->prev);
}

/**
 * list_rotate_left - rotate the list to the left
 * @head: the head of the list
 */
static inline void list_rotate_left(struct list_head *head)
{
	struct list_head *first;

	if (!list_empty(head)) {
		first = head->next;
		list_move_tail(first, head);
	}
}

/**
 * list_rotate_to_front() - Rotate list to specific item.
 * @list: The desired new front of the list.
 * @head: The head of the list.
 *
 * Rotates list so that @list becomes the new front of the list.
 */
static inline void list_rotate_to_front(struct list_head *list,
					struct list_head *head)
{
	/*
	 * Deletes the list head from the list denoted by @head and
	 * places it as the tail of @list, this effectively rotates the
	 * list so that @list is at the front.
	 */
	list_move_tail(head, list);
}

/**
 * list_is_singular - tests whether a list has just one entry.
 * @head: the list to test.
 */
static inline int list_is_singular(const struct list_head *head)
{
	return !list_empty(head) && (head->next == head->prev);
}

static inline void __list_cut_position(struct list_head *list,
		struct list_head *head, struct list_head *entry)
{
	struct list_head *new_first = entry->next;
	list->next = head->next;
	list->next->prev = list;
	list->prev = entry;
	entry->next = list;
	head->next = new_first;
	new_first->prev = head;
}

/**
 * list_cut_position - cut a list into two
 * @list: a new list to add all removed entries
 * @head: a list with entries
 * @entry: an entry within head, could be the head itself
 *	and if so we won't cut the list
 *
 * This helper moves the initial part of @head, up to and
 * including @entry, from @head to @list. You should
 * pass on @entry an element you know is on @head. @list
 * should be an empty list or a list you do not care about
 * losing its data.
 *
 */
static inline void list_cut_position(struct list_head *list,
		struct list_head *head, struct list_head *entry)
{
	if (list_empty(head))
		return;
	if (list_is_singular(head) &&
		(head->next != entry && head != entry))
		return;
	if (entry == head)
		INIT_LIST_HEAD(list);
	else
		__list_cut_position(list, head, entry);
}

/**
 * list_cut_before - cut a list into two, before given entry
 * @list: a new list to add all removed entries
 * @head: a list with entries
 * @entry: an entry within head, could be the head itself
 *
 * This helper moves the initial part of @head, up to but
 * excluding @entry, from @head to @list.  You should pass
 * in @entry an element you know is on @head.  @list should
 * be an empty list or a list you do not care about losing
 * its data.
 * If @entry == @head, all entries on @head are moved to
 * @list.
 */
static inline void list_cut_before(struct list_head *list,
				   struct list_head *head,
				   struct list_head *entry)
{
	if (head->next == entry) {
		INIT_LIST_HEAD(list);
		return;
	}
	list->next = head->next;
	list->next->prev = list;
	list->prev = entry->prev;
	list->prev->next = list;
	head->next = entry;
	entry->prev = head;
}

static inline void __list_splice(const struct list_head *list,
				 struct list_head *prev,
				 struct list_head *next)
{
	struct list_head *first = list->next;
	struct list_head *last = list->prev;

	first->prev = prev;
	prev->next = first;

	last->next = next;
	next->prev = last;
}

/**
 * list_splice - join two lists, this is designed for stacks
 * @list: the new list to add.
 * @head: the place to add it in the first list.
 */
static inline void list_splice(const struct list_head *list,
				struct list_head *head)
{
	if (!list_empty(list))
		__list_splice(list, head, head->next);
}

/**
 * list_splice_tail - join two lists, each list being a queue
 * @list: the new list to add.
 * @head: the place to add it in the first list.
 */
static inline void list_splice_tail(struct list_head *list,
				struct list_head *head)
{
	if (!list_empty(list))
		__list_splice(list, head->prev, head);
}

/**
 * list_splice_init - join two lists and reinitialise the emptied list.
 * @list: the new list to add.
 * @head: the place to add it in the first list.
 *
 * The list at @list is reinitialised
 */
static inline void list_splice_init(struct list_head *list,
				    struct list_head *head)
{
	if (!list_empty(list)) {
		__list_splice(list, head, head->next);
		INIT_LIST_HEAD(list);
	}
}

/**
 * list_splice_tail_init - join two lists and reinitialise the emptied list
 * @list: the new list to add.
 * @head: the place to add it in the first list.
 *
 * Each of the lists is a queue.
 * The list at @list is reinitialised
 */
static inline void list_splice_tail_init(struct list_head *list,
					 struct list_head *head)
{
	if (!list_empty(list)) {
		__list_splice(list, head->prev, head);
		INIT_LIST_HEAD(list);
	}
}

#define container_of(ptr, type, member) ({          \
        const typeof( ((type *)0)->member ) *__mptr = (ptr);    \
        (type *)( (char *)__mptr - offsetof(type,member) );})

/**
 * list_entry - get the struct for this entry
 * @ptr:	the &struct list_head pointer.
 * @type:	the type of the struct this is embedded in.
 * @member:	the name of the list_head within the struct.
 */
#define list_entry(ptr, type, member) \
	container_of(ptr, type, member)

/**
 * list_first_entry - get the first element from a list
 * @ptr:	the list head to take the element from.
 * @type:	the type of the struct this is embedded in.
 * @member:	the name of the list_head within the struct.
 *
 * Note, that list is expected to be not empty.
 */
#define list_first_entry(ptr, type, member) \
	list_entry((ptr)->next, type, member)

/**
 * list_last_entry - get the last element from a list
 * @ptr:	the list head to take the element from.
 * @type:	the type of the struct this is embedded in.
 * @member:	the name of the list_head within the struct.
 *
 * Note, that list is expected to be not empty.
 */
#define list_last_entry(ptr, type, member) \
	list_entry((ptr)->prev, type, member)

/**
 * list_first_entry_or_null - get the first element from a list
 * @ptr:	the list head to take the element from.
 * @type:	the type of the struct this is embedded in.
 * @member:	the name of the list_head within the struct.
 *
 * Note that if the list is empty, it returns NULL.
 */
#define list_first_entry_or_null(ptr, type, member) ({ \
	struct list_head *head__ = (ptr); \
	struct list_head *pos__ = head__->next; \
	pos__ != head__ ? list_entry(pos__, type, member) : NULL; \
})

/**
 * list_next_entry - get the next element in list
 * @pos:	the type * to cursor
 * @member:	the name of the list_head within the struct.
 */
#define list_next_entry(pos, member) \
	list_entry((pos)->member.next, typeof(*(pos)), member)

/**
 * list_prev_entry - get the prev element in list
 * @pos:	the type * to cursor
 * @member:	the name of the list_head within the struct.
 */
#define list_prev_entry(pos, member) \
	list_entry((pos)->member.prev, typeof(*(pos)), member)

/**
 * list_for_each	-	iterate over a list
 * @pos:	the &struct list_head to use as a loop cursor.
 * @head:	the head for your list.
 */
#define list_for_each(pos, head) \
	for (pos = (head)->next; pos != (head); pos = pos->next)

/**
 * list_for_each_prev	-	iterate over a list backwards
 * @pos:	the &struct list_head to use as a loop cursor.
 * @head:	the head for your list.
 */
#define list_for_each_prev(pos, head) \
	for (pos = (head)->prev; pos != (head); pos = pos->prev)

/**
 * list_for_each_safe - iterate over a list safe against removal of list entry
 * @pos:	the &struct list_head to use as a loop cursor.
 * @n:		another &struct list_head to use as temporary storage
 * @head:	the head for your list.
 */
#define list_for_each_safe(pos, n, head) \
	for (pos = (head)->next, n = pos->next; pos != (head); \
		pos = n, n = pos->next)

/**
 * list_for_each_prev_safe - iterate over a list backwards safe against removal of list entry
 * @pos:	the &struct list_head to use as a loop cursor.
 * @n:		another &struct list_head to use as temporary storage
 * @head:	the head for your list.
 */
#define list_for_each_prev_safe(pos, n, head) \
	for (pos = (head)->prev, n = pos->prev; \
	     pos != (head); \
	     pos = n, n = pos->prev)

/**
 * list_for_each_entry	-	iterate over list of given type
 * @pos:	the type * to use as a loop cursor.
 * @head:	the head for your list.
 * @member:	the name of the list_head within the struct.
 */
#define list_for_each_entry(pos, head, member)				\
	for (pos = list_first_entry(head, typeof(*pos), member);	\
	     &pos->member != (head);					\
	     pos = list_next_entry(pos, member))

/**
 * list_for_each_entry_reverse - iterate backwards over list of given type.
 * @pos:	the type * to use as a loop cursor.
 * @head:	the head for your list.
 * @member:	the name of the list_head within the struct.
 */
#define list_for_each_entry_reverse(pos, head, member)			\
	for (pos = list_last_entry(head, typeof(*pos), member);		\
	     &pos->member != (head); 					\
	     pos = list_prev_entry(pos, member))

/**
 * list_prepare_entry - prepare a pos entry for use in list_for_each_entry_continue()
 * @pos:	the type * to use as a start point
 * @head:	the head of the list
 * @member:	the name of the list_head within the struct.
 *
 * Prepares a pos entry for use as a start point in list_for_each_entry_continue().
 */
#define list_prepare_entry(pos, head, member) \
	((pos) ? : list_entry(head, typeof(*pos), member))

/**
 * list_for_each_entry_continue - continue iteration over list of given type
 * @pos:	the type * to use as a loop cursor.
 * @head:	the head for your list.
 * @member:	the name of the list_head within the struct.
 *
 * Continue to iterate over list of given type, continuing after
 * the current position.
 */
#define list_for_each_entry_continue(pos, head, member) 		\
	for (pos = list_next_entry(pos, member);			\
	     &pos->member != (head);					\
	     pos = list_next_entry(pos, member))

/**
 * list_for_each_entry_continue_reverse - iterate backwards from the given point
 * @pos:	the type * to use as a loop cursor.
 * @head:	the head for your list.
 * @member:	the name of the list_head within the struct.
 *
 * Start to iterate over list of given type backwards, continuing after
 * the current position.
 */
#define list_for_each_entry_continue_reverse(pos, head, member)		\
	for (pos = list_prev_entry(pos, member);			\
	     &pos->member != (head);					\
	     pos = list_prev_entry(pos, member))

/**
 * list_for_each_entry_from - iterate over list of given type from the current point
 * @pos:	the type * to use as a loop cursor.
 * @head:	the head for your list.
 * @member:	the name of the list_head within the struct.
 *
 * Iterate over list of given type, continuing from current position.
 */
#define list_for_each_entry_from(pos, head, member) 			\
	for (; &pos->member != (head);					\
	     pos = list_next_entry(pos, member))

/**
 * list_for_each_entry_from_reverse - iterate backwards over list of given type
 *                                    from the current point
 * @pos:	the type * to use as a loop cursor.
 * @head:	the head for your list.
 * @member:	the name of the list_head within the struct.
 *
 * Iterate backwards over list of given type, continuing from current position.
 */
#define list_for_each_entry_from_reverse(pos, head, member)		\
	for (; &pos->member != (head);					\
	     pos = list_prev_entry(pos, member))

/**
 * list_for_each_entry_safe - iterate over list of given type safe against removal of list entry
 * @pos:	the type * to use as a loop cursor.
 * @n:		another type * to use as temporary storage
 * @head:	the head for your list.
 * @member:	the name of the list_head within the struct.
 */
#define list_for_each_entry_safe(pos, n, head, member)			\
	for (pos = list_first_entry(head, typeof(*pos), member),	\
		n = list_next_entry(pos, member);			\
	     &pos->member != (head); 					\
	     pos = n, n = list_next_entry(n, member))

/**
 * list_for_each_entry_safe_continue - continue list iteration safe against removal
 * @pos:	the type * to use as a loop cursor.
 * @n:		another type * to use as temporary storage
 * @head:	the head for your list.
 * @member:	the name of the list_head within the struct.
 *
 * Iterate over list of given type, continuing after current point,
 * safe against removal of list entry.
 */
#define list_for_each_entry_safe_continue(pos, n, head, member) 		\
	for (pos = list_next_entry(pos, member), 				\
		n = list_next_entry(pos, member);				\
	     &pos->member != (head);						\
	     pos = n, n = list_next_entry(n, member))

/**
 * list_for_each_entry_safe_from - iterate over list from current point safe against removal
 * @pos:	the type * to use as a loop cursor.
 * @n:		another type * to use as temporary storage
 * @head:	the head for your list.
 * @member:	the name of the list_head within the struct.
 *
 * Iterate over list of given type from current point, safe against
 * removal of list entry.
 */
#define list_for_each_entry_safe_from(pos, n, head, member) 			\
	for (n = list_next_entry(pos, member);					\
	     &pos->member != (head);						\
	     pos = n, n = list_next_entry(n, member))

/**
 * list_for_each_entry_safe_reverse - iterate backwards over list safe against removal
 * @pos:	the type * to use as a loop cursor.
 * @n:		another type * to use as temporary storage
 * @head:	the head for your list.
 * @member:	the name of the list_head within the struct.
 *
 * Iterate backwards over list of given type, safe against removal
 * of list entry.
 */
#define list_for_each_entry_safe_reverse(pos, n, head, member)		\
	for (pos = list_last_entry(head, typeof(*pos), member),		\
		n = list_prev_entry(pos, member);			\
	     &pos->member != (head); 					\
	     pos = n, n = list_prev_entry(n, member))

/**
 * list_safe_reset_next - reset a stale list_for_each_entry_safe loop
 * @pos:	the loop cursor used in the list_for_each_entry_safe loop
 * @n:		temporary storage used in list_for_each_entry_safe
 * @member:	the name of the list_head within the struct.
 *
 * list_safe_reset_next is not safe to use in general if the list may be
 * modified concurrently (eg. the lock is dropped in the loop body). An
 * exception to this is if the cursor element (pos) is pinned in the list,
 * and list_safe_reset_next is called after re-taking the lock and before
 * completing the current iteration of the loop body.
 */
#define list_safe_reset_next(pos, n, member)				\
	n = list_next_entry(pos, member)

static inline void list_del_rcu(struct list_head *entry)
{
	__list_del_entry(entry);
	entry->prev = LIST_POISON2;
}

static inline void __list_add_rcu(struct list_head *new,
		struct list_head *prev, struct list_head *next)
{
	new->next = next;
	new->prev = prev;
	prev->next = new;
	next->prev = new;
}

/**
 * list_add_rcu - add a new entry to rcu-protected list
 * @new: new entry to be added
 * @head: list head to add it after
 *
 * Insert a new entry after the specified head.
 * This is good for implementing stacks.
 *
 * The caller must take whatever precautions are necessary
 * (such as holding appropriate locks) to avoid racing
 * with another list-mutation primitive, such as list_add_rcu()
 * or list_del_rcu(), running on this same list.
 * However, it is perfectly legal to run concurrently with
 * the _rcu list-traversal primitives, such as
 * list_for_each_entry_rcu().
 */
static inline void list_add_rcu(struct list_head *new, struct list_head *head)
{
	__list_add_rcu(new, head, head->next);
}

#endif

list_add和list_add_rcu操作的不同

结束!