klist.c

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/*
 * klist.c - Routines for manipulating klists.
 *
 * Copyright (C) 2005 Patrick Mochel
 *
 * This file is released under the GPL v2.
 *
 * This klist interface provides a couple of structures that wrap around
 * struct list_head to provide explicit list "head" (struct klist) and list
 * "node" (struct klist_node) objects. For struct klist, a spinlock is
 * included that protects access to the actual list itself. struct
 * klist_node provides a pointer to the klist that owns it and a kref
 * reference count that indicates the number of current users of that node
 * in the list.
 *
 * The entire point is to provide an interface for iterating over a list
 * that is safe and allows for modification of the list during the
 * iteration (e.g. insertion and removal), including modification of the
 * current node on the list.
 *
 * It works using a 3rd object type - struct klist_iter - that is declared
 * and initialized before an iteration. klist_next() is used to acquire the
 * next element in the list. It returns NULL if there are no more items.
 * Internally, that routine takes the klist's lock, decrements the
 * reference count of the previous klist_node and increments the count of
 * the next klist_node. It then drops the lock and returns.
 *
 * There are primitives for adding and removing nodes to/from a klist.
 * When deleting, klist_del() will simply decrement the reference count.
 * Only when the count goes to 0 is the node removed from the list.
 * klist_remove() will try to delete the node from the list and block until
 * it is actually removed. This is useful for objects (like devices) that
 * have been removed from the system and must be freed (but must wait until
 * all accessors have finished).
 */

#include <linux/klist.h>
#include <linux/export.h>
#include <linux/sched.h>

/*
 * Use the lowest bit of n_klist to mark deleted nodes and exclude
 * dead ones from iteration.
 */
#define KNODE_DEAD      1LU
#define KNODE_KLIST_MASK    ~KNODE_DEAD

static struct klist *knode_klist(struct klist_node *knode)
{
    return (struct klist *)
        ((unsigned long)knode->n_klist & KNODE_KLIST_MASK);
}

static bool knode_dead(struct klist_node *knode)
{
    return (unsigned long)knode->n_klist & KNODE_DEAD;
}

static void knode_set_klist(struct klist_node *knode, struct klist *klist)
{
    knode->n_klist = klist;
    /* no knode deserves to start its life dead */
    WARN_ON(knode_dead(knode));
}

static void knode_kill(struct klist_node *knode)
{
    /* and no knode should die twice ever either, see we're very humane */
    WARN_ON(knode_dead(knode));
    *(unsigned long *)&knode->n_klist |= KNODE_DEAD;
}

void klist_init(struct klist *k, void (*get)(struct klist_node *),
        void (*put)(struct klist_node *))
{
    INIT_LIST_HEAD(&k->k_list);
    spin_lock_init(&k->k_lock);
    k->get = get;
    k->put = put;
}
EXPORT_SYMBOL_GPL(klist_init);

static void add_head(struct klist *k, struct klist_node *n)
{
    spin_lock(&k->k_lock);
    list_add(&n->n_node, &k->k_list);
    spin_unlock(&k->k_lock);
}

static void add_tail(struct klist *k, struct klist_node *n)
{
    spin_lock(&k->k_lock);
    list_add_tail(&n->n_node, &k->k_list);
    spin_unlock(&k->k_lock);
}

static void klist_node_init(struct klist *k, struct klist_node *n)
{
    INIT_LIST_HEAD(&n->n_node);
    kref_init(&n->n_ref);
    knode_set_klist(n, k);
    if (k->get)
        k->get(n);
}

void klist_add_head(struct klist_node *n, struct klist *k)
{
    klist_node_init(k, n);
    add_head(k, n);
}
EXPORT_SYMBOL_GPL(klist_add_head);

void klist_add_tail(struct klist_node *n, struct klist *k)
{
    klist_node_init(k, n);
    add_tail(k, n);
}
EXPORT_SYMBOL_GPL(klist_add_tail);

void klist_add_after(struct klist_node *n, struct klist_node *pos)
{
    struct klist *k = knode_klist(pos);

    klist_node_init(k, n);
    spin_lock(&k->k_lock);
    list_add(&n->n_node, &pos->n_node);
    spin_unlock(&k->k_lock);
}
EXPORT_SYMBOL_GPL(klist_add_after);

void klist_add_before(struct klist_node *n, struct klist_node *pos)
{
    struct klist *k = knode_klist(pos);

    klist_node_init(k, n);
    spin_lock(&k->k_lock);
    list_add_tail(&n->n_node, &pos->n_node);
    spin_unlock(&k->k_lock);
}
EXPORT_SYMBOL_GPL(klist_add_before);

struct klist_waiter {
    struct list_head list;
    struct klist_node *node;
    struct task_struct *process;
    int woken;
};

static DEFINE_SPINLOCK(klist_remove_lock);
static LIST_HEAD(klist_remove_waiters);

static void klist_release(struct kref *kref)
{
    struct klist_waiter *waiter, *tmp;
    struct klist_node *n = container_of(kref, struct klist_node, n_ref);

    WARN_ON(!knode_dead(n));
    list_del(&n->n_node);
    spin_lock(&klist_remove_lock);
    list_for_each_entry_safe(waiter, tmp, &klist_remove_waiters, list) {
        if (waiter->node != n)
            continue;

        waiter->woken = 1;
        mb();
        wake_up_process(waiter->process);
        list_del(&waiter->list);
    }
    spin_unlock(&klist_remove_lock);
    knode_set_klist(n, NULL);
}

static int klist_dec_and_del(struct klist_node *n)
{
    return kref_put(&n->n_ref, klist_release);
}

static void klist_put(struct klist_node *n, bool kill)
{
    struct klist *k = knode_klist(n);
    void (*put)(struct klist_node *) = k->put;

    spin_lock(&k->k_lock);
    if (kill)
        knode_kill(n);
    if (!klist_dec_and_del(n))
        put = NULL;
    spin_unlock(&k->k_lock);
    if (put)
        put(n);
}

void klist_del(struct klist_node *n)
{
    klist_put(n, true);
}
EXPORT_SYMBOL_GPL(klist_del);

void klist_remove(struct klist_node *n)
{
    struct klist_waiter waiter;

    waiter.node = n;
    waiter.process = current;
    waiter.woken = 0;
    spin_lock(&klist_remove_lock);
    list_add(&waiter.list, &klist_remove_waiters);
    spin_unlock(&klist_remove_lock);

    klist_del(n);

    for (;;) {
        set_current_state(TASK_UNINTERRUPTIBLE);
        if (waiter.woken)
            break;
        schedule();
    }
    __set_current_state(TASK_RUNNING);
}
EXPORT_SYMBOL_GPL(klist_remove);

int klist_node_attached(struct klist_node *n)
{
    return (n->n_klist != NULL);
}
EXPORT_SYMBOL_GPL(klist_node_attached);

void klist_iter_init_node(struct klist *k, struct klist_iter *i,
              struct klist_node *n)
{
    i->i_klist = k;
    i->i_cur = n;
    if (n)
        kref_get(&n->n_ref);
}
EXPORT_SYMBOL_GPL(klist_iter_init_node);

void klist_iter_init(struct klist *k, struct klist_iter *i)
{
    klist_iter_init_node(k, i, NULL);
}
EXPORT_SYMBOL_GPL(klist_iter_init);

void klist_iter_exit(struct klist_iter *i)
{
    if (i->i_cur) {
        klist_put(i->i_cur, false);
        i->i_cur = NULL;
    }
}
EXPORT_SYMBOL_GPL(klist_iter_exit);

static struct klist_node *to_klist_node(struct list_head *n)
{
    return container_of(n, struct klist_node, n_node);
}

struct klist_node *klist_next(struct klist_iter *i)
{
    void (*put)(struct klist_node *) = i->i_klist->put;
    struct klist_node *last = i->i_cur;
    struct klist_node *next;

    spin_lock(&i->i_klist->k_lock);

    if (last) {
        next = to_klist_node(last->n_node.next);
        if (!klist_dec_and_del(last))
            put = NULL;
    } else
        next = to_klist_node(i->i_klist->k_list.next);

    i->i_cur = NULL;
    while (next != to_klist_node(&i->i_klist->k_list)) {
        if (likely(!knode_dead(next))) {
            kref_get(&next->n_ref);
            i->i_cur = next;
            break;
        }
        next = to_klist_node(next->n_node.next);
    }

    spin_unlock(&i->i_klist->k_lock);

    if (put && last)
        put(last);
    return i->i_cur;
}
EXPORT_SYMBOL_GPL(klist_next);