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