我正在读双倍链表的linux kernel implementation页.我不懂宏WRITE_ONCE(x, val)的用法.它在编译器中的定义如下.h:

#define WRITE_ONCE(x, val) x=(val)

它在文件中使用了七次，例如

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;
    WRITE_ONCE(prev->next, new);
}

我已经读到它是用来避免比赛条件的.

我有两个问题:

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;
}

2/如何知道何时应该使用它？例如，它用于__lst_add()，但不用于__lst_splice():

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;
}

编辑:

list: Use WRITE_ONCE() when initializing list_head structures
Code that does lockless emptiness testing of non-RCU lists is relying on INIT_LIST_HEAD() to write the list head's ->next pointer atomically, particularly when INIT_LIST_HEAD() is invoked from list_del_init(). This commit therefore adds WRITE_ONCE() to this function's pointer stores that could affect the head's ->next pointer.