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Date: Mon, 16 Oct 2023 22:23:14 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: "wuqiang.matt" <wuqiang.matt@...edance.com>
Cc: linux-trace-kernel@...r.kernel.org, davem@...emloft.net,
anil.s.keshavamurthy@...el.com, naveen.n.rao@...ux.ibm.com,
rostedt@...dmis.org, peterz@...radead.org,
akpm@...ux-foundation.org, sander@...nheule.net,
ebiggers@...gle.com, dan.j.williams@...el.com, jpoimboe@...nel.org,
linux-kernel@...r.kernel.org, lkp@...el.com, mattwu@....com
Subject: Re: [PATCH v10 4/5] kprobes: freelist.h removed
Hi Peter,
This freelist has been introduced by you, is it OK to remove this because no
other user exists?
Thank you,
On Sun, 15 Oct 2023 13:32:50 +0800
"wuqiang.matt" <wuqiang.matt@...edance.com> wrote:
> This patch will remove freelist.h from kernel source tree, since the
> only use cases (kretprobe and rethook) are converted to objpool.
>
> Signed-off-by: wuqiang.matt <wuqiang.matt@...edance.com>
> ---
> include/linux/freelist.h | 129 ---------------------------------------
> 1 file changed, 129 deletions(-)
> delete mode 100644 include/linux/freelist.h
>
> diff --git a/include/linux/freelist.h b/include/linux/freelist.h
> deleted file mode 100644
> index fc1842b96469..000000000000
> --- a/include/linux/freelist.h
> +++ /dev/null
> @@ -1,129 +0,0 @@
> -/* SPDX-License-Identifier: GPL-2.0-only OR BSD-2-Clause */
> -#ifndef FREELIST_H
> -#define FREELIST_H
> -
> -#include <linux/atomic.h>
> -
> -/*
> - * Copyright: cameron@...dycamel.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 */
> --
> 2.40.1
>
--
Masami Hiramatsu (Google) <mhiramat@...nel.org>
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