kprobes: freelist.h removed (f46e8939) · Commits · github.com / kvm-riscv / linux

include/linux/freelist.h

deleted100644 → 0

+0 −129

Original line number	Diff line number	Diff line
		/* 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 */