list_nulls.h

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#ifndef _LINUX_LIST_NULLS_H
#define _LINUX_LIST_NULLS_H

/*
 * Special version of lists, where end of list is not a NULL pointer,
 * but a 'nulls' marker, which can have many different values.
 * (up to 2^31 different values guaranteed on all platforms)
 *
 * In the standard hlist, termination of a list is the NULL pointer.
 * In this special 'nulls' variant, we use the fact that objects stored in
 * a list are aligned on a word (4 or 8 bytes alignment).
 * We therefore use the last significant bit of 'ptr' :
 * Set to 1 : This is a 'nulls' end-of-list marker (ptr >> 1)
 * Set to 0 : This is a pointer to some object (ptr)
 */

struct hlist_nulls_head {
    struct hlist_nulls_node *first;
};

struct hlist_nulls_node {
    struct hlist_nulls_node *next, **pprev;
};
#define INIT_HLIST_NULLS_HEAD(ptr, nulls) \
    ((ptr)->first = (struct hlist_nulls_node *) (1UL | (((long)nulls) << 1)))

#define hlist_nulls_entry(ptr, type, member) container_of(ptr,type,member)

static inline int is_a_nulls(const struct hlist_nulls_node *ptr)
{
    return ((unsigned long)ptr & 1);
}

static inline unsigned long get_nulls_value(const struct hlist_nulls_node *ptr)
{
    return ((unsigned long)ptr) >> 1;
}

static inline int hlist_nulls_unhashed(const struct hlist_nulls_node *h)
{
    return !h->pprev;
}

static inline int hlist_nulls_empty(const struct hlist_nulls_head *h)
{
    return is_a_nulls(h->first);
}

static inline void hlist_nulls_add_head(struct hlist_nulls_node *n,
                    struct hlist_nulls_head *h)
{
    struct hlist_nulls_node *first = h->first;

    n->next = first;
    n->pprev = &h->first;
    h->first = n;
    if (!is_a_nulls(first))
        first->pprev = &n->next;
}

static inline void __hlist_nulls_del(struct hlist_nulls_node *n)
{
    struct hlist_nulls_node *next = n->next;
    struct hlist_nulls_node **pprev = n->pprev;
    *pprev = next;
    if (!is_a_nulls(next))
        next->pprev = pprev;
}

static inline void hlist_nulls_del(struct hlist_nulls_node *n)
{
    __hlist_nulls_del(n);
    n->pprev = LIST_POISON2;
}

#define hlist_nulls_for_each_entry(tpos, pos, head, member)            \
    for (pos = (head)->first;                          \
         (!is_a_nulls(pos)) &&                         \
        ({ tpos = hlist_nulls_entry(pos, typeof(*tpos), member); 1;}); \
         pos = pos->next)

#define hlist_nulls_for_each_entry_from(tpos, pos, member)  \
    for (; (!is_a_nulls(pos)) &&                \
        ({ tpos = hlist_nulls_entry(pos, typeof(*tpos), member); 1;}); \
         pos = pos->next)

#endif