130 lines
3.8 KiB
C
130 lines
3.8 KiB
C
/* SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause */
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#ifndef FREELIST_H
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#define FREELIST_H
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#include <linux/atomic.h>
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/*
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* Copyright: cameron@moodycamel.com
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*
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* A simple CAS-based lock-free free list. Not the fastest thing in the world
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* under heavy contention, but simple and correct (assuming nodes are never
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* freed until after the free list is destroyed), and fairly speedy under low
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* contention.
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*
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* Adapted from: https://moodycamel.com/blog/2014/solving-the-aba-problem-for-lock-free-free-lists
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*/
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struct freelist_node {
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atomic_t refs;
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struct freelist_node *next;
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};
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struct freelist_head {
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struct freelist_node *head;
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};
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#define REFS_ON_FREELIST 0x80000000
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#define REFS_MASK 0x7FFFFFFF
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static inline void __freelist_add(struct freelist_node *node, struct freelist_head *list)
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{
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/*
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* Since the refcount is zero, and nobody can increase it once it's
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* zero (except us, and we run only one copy of this method per node at
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* a time, i.e. the single thread case), then we know we can safely
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* change the next pointer of the node; however, once the refcount is
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* back above zero, then other threads could increase it (happens under
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* heavy contention, when the refcount goes to zero in between a load
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* and a refcount increment of a node in try_get, then back up to
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* something non-zero, then the refcount increment is done by the other
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* thread) -- so if the CAS to add the node to the actual list fails,
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* decrese the refcount and leave the add operation to the next thread
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* who puts the refcount back to zero (which could be us, hence the
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* loop).
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*/
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struct freelist_node *head = READ_ONCE(list->head);
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for (;;) {
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WRITE_ONCE(node->next, head);
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atomic_set_release(&node->refs, 1);
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if (!try_cmpxchg_release(&list->head, &head, node)) {
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/*
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* Hmm, the add failed, but we can only try again when
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* the refcount goes back to zero.
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*/
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if (atomic_fetch_add_release(REFS_ON_FREELIST - 1, &node->refs) == 1)
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continue;
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}
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return;
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}
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}
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static inline void freelist_add(struct freelist_node *node, struct freelist_head *list)
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{
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/*
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* We know that the should-be-on-freelist bit is 0 at this point, so
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* it's safe to set it using a fetch_add.
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*/
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if (!atomic_fetch_add_release(REFS_ON_FREELIST, &node->refs)) {
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/*
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* Oh look! We were the last ones referencing this node, and we
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* know we want to add it to the free list, so let's do it!
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*/
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__freelist_add(node, list);
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}
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}
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static inline struct freelist_node *freelist_try_get(struct freelist_head *list)
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{
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struct freelist_node *prev, *next, *head = smp_load_acquire(&list->head);
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unsigned int refs;
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while (head) {
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prev = head;
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refs = atomic_read(&head->refs);
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if ((refs & REFS_MASK) == 0 ||
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!atomic_try_cmpxchg_acquire(&head->refs, &refs, refs+1)) {
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head = smp_load_acquire(&list->head);
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continue;
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}
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/*
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* Good, reference count has been incremented (it wasn't at
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* zero), which means we can read the next and not worry about
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* it changing between now and the time we do the CAS.
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*/
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next = READ_ONCE(head->next);
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if (try_cmpxchg_acquire(&list->head, &head, next)) {
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/*
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* Yay, got the node. This means it was on the list,
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* which means should-be-on-freelist must be false no
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* matter the refcount (because nobody else knows it's
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* been taken off yet, it can't have been put back on).
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*/
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WARN_ON_ONCE(atomic_read(&head->refs) & REFS_ON_FREELIST);
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/*
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* Decrease refcount twice, once for our ref, and once
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* for the list's ref.
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*/
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atomic_fetch_add(-2, &head->refs);
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return head;
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}
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/*
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* OK, the head must have changed on us, but we still need to decrement
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* the refcount we increased.
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*/
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refs = atomic_fetch_add(-1, &prev->refs);
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if (refs == REFS_ON_FREELIST + 1)
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__freelist_add(prev, list);
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}
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return NULL;
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}
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#endif /* FREELIST_H */
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