[PATCH v4 22/23] freelist: Lock less freelist

Masami Hiramatsu
2020-08-28T12:31:40Z

From: Peter Zijlstra <peterz@infradead.org>

Cc: cameron@moodycamel.com
Cc: oleg@redhat.com
Cc: will@kernel.org
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
include/linux/freelist.h | 129 ++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 129 insertions(+)
create mode 100644 include/linux/freelist.h

diff --git a/include/linux/freelist.h b/include/linux/freelist.h
new file mode 100644
index 000000000000..fc1842b96469
--- /dev/null
+++ b/include/linux/freelist.h
@@ -0,0 +1,129 @@
+/* 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 */