freelist: Implement lockless freelist

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.
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: NMasami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: NIngo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/159870622579.1229682.16729440870040944993.stgit@devnote2

include/linux/freelist.h

+/* 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 */