LXR linux/include/linux/freelist.h

   1/* SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause */
   2#ifndef FREELIST_H
   3#define FREELIST_H
   4
   5#include <linux/atomic.h>
   6
   7/*
   8 * Copyright: cameron@moodycamel.com
   9 *
  10 * A simple CAS-based lock-free free list. Not the fastest thing in the world
  11 * under heavy contention, but simple and correct (assuming nodes are never
  12 * freed until after the free list is destroyed), and fairly speedy under low
  13 * contention.
  14 *
  15 * Adapted from: https://moodycamel.com/blog/2014/solving-the-aba-problem-for-lock-free-free-lists
  16 */
  17
  18struct freelist_node {
  19        atomic_t                refs;
  20        struct freelist_node    *next;
  21};
  22
  23struct freelist_head {
  24        struct freelist_node    *head;
  25};
  26
  27#define REFS_ON_FREELIST 0x80000000
  28#define REFS_MASK        0x7FFFFFFF
  29
  30static inline void __freelist_add(struct freelist_node *node, struct freelist_head *list)
  31{
  32        /*
  33         * Since the refcount is zero, and nobody can increase it once it's
  34         * zero (except us, and we run only one copy of this method per node at
  35         * a time, i.e. the single thread case), then we know we can safely
  36         * change the next pointer of the node; however, once the refcount is
  37         * back above zero, then other threads could increase it (happens under
  38         * heavy contention, when the refcount goes to zero in between a load
  39         * and a refcount increment of a node in try_get, then back up to
  40         * something non-zero, then the refcount increment is done by the other
  41         * thread) -- so if the CAS to add the node to the actual list fails,
  42         * decrese the refcount and leave the add operation to the next thread
  43         * who puts the refcount back to zero (which could be us, hence the
  44         * loop).
  45         */
  46        struct freelist_node *head = READ_ONCE(list->head);
  47
  48        for (;;) {
  49                WRITE_ONCE(node->next, head);
  50                atomic_set_release(&node->refs, 1);
  51
  52                if (!try_cmpxchg_release(&list->head, &head, node)) {
  53                        /*
  54                         * Hmm, the add failed, but we can only try again when
  55                         * the refcount goes back to zero.
  56                         */
  57                        if (atomic_fetch_add_release(REFS_ON_FREELIST - 1, &node->refs) == 1)
  58                                continue;
  59                }
  60                return;
  61        }
  62}
  63
  64static inline void freelist_add(struct freelist_node *node, struct freelist_head *list)
  65{
  66        /*
  67         * We know that the should-be-on-freelist bit is 0 at this point, so
  68         * it's safe to set it using a fetch_add.
  69         */
  70        if (!atomic_fetch_add_release(REFS_ON_FREELIST, &node->refs)) {
  71                /*
  72                 * Oh look! We were the last ones referencing this node, and we
  73                 * know we want to add it to the free list, so let's do it!
  74                 */
  75                __freelist_add(node, list);
  76        }
  77}
  78
  79static inline struct freelist_node *freelist_try_get(struct freelist_head *list)
  80{
  81        struct freelist_node *prev, *next, *head = smp_load_acquire(&list->head);
  82        unsigned int refs;
  83
  84        while (head) {
  85                prev = head;
  86                refs = atomic_read(&head->refs);
  87                if ((refs & REFS_MASK) == 0 ||
  88                    !atomic_try_cmpxchg_acquire(&head->refs, &refs, refs+1)) {
  89                        head = smp_load_acquire(&list->head);
  90                        continue;
  91                }
  92
  93                /*
  94                 * Good, reference count has been incremented (it wasn't at
  95                 * zero), which means we can read the next and not worry about
  96                 * it changing between now and the time we do the CAS.
  97                 */
  98                next = READ_ONCE(head->next);
  99                if (try_cmpxchg_acquire(&list->head, &head, next)) {
 100                        /*
 101                         * Yay, got the node. This means it was on the list,
 102                         * which means should-be-on-freelist must be false no
 103                         * matter the refcount (because nobody else knows it's
 104                         * been taken off yet, it can't have been put back on).
 105                         */
 106                        WARN_ON_ONCE(atomic_read(&head->refs) & REFS_ON_FREELIST);
 107
 108                        /*
 109                         * Decrease refcount twice, once for our ref, and once
 110                         * for the list's ref.
 111                         */
 112                        atomic_fetch_add(-2, &head->refs);
 113
 114                        return head;
 115                }
 116
 117                /*
 118                 * OK, the head must have changed on us, but we still need to decrement
 119                 * the refcount we increased.
 120                 */
 121                refs = atomic_fetch_add(-1, &prev->refs);
 122                if (refs == REFS_ON_FREELIST + 1)
 123                        __freelist_add(prev, list);
 124        }
 125
 126        return NULL;
 127}
 128
 129#endif /* FREELIST_H */
 130