mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-22 00:40:03 +00:00
130 lines
3.8 KiB
C
130 lines
3.8 KiB
C
![]() |
/* SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause */
|
||
|
#ifndef FREELIST_H
|
||
|
#define FREELIST_H
|
||
|
|
||
|
#include <linux/atomic.h>
|
||
|
|
||
|
/*
|
||
|
* Copyright: cameron@moodycamel.com
|
||
|
*
|
||
|
* 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 */
|