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Date:   Mon, 16 Oct 2023 22:23:14 +0900
From:   Masami Hiramatsu (Google) <mhiramat@kernel.org>
To:     "wuqiang.matt" <wuqiang.matt@bytedance.com>
Cc:     linux-trace-kernel@vger.kernel.org, davem@davemloft.net,
        anil.s.keshavamurthy@intel.com, naveen.n.rao@linux.ibm.com,
        rostedt@goodmis.org, peterz@infradead.org,
        akpm@linux-foundation.org, sander@svanheule.net,
        ebiggers@google.com, dan.j.williams@intel.com, jpoimboe@kernel.org,
        linux-kernel@vger.kernel.org, lkp@intel.com, mattwu@163.com
Subject: Re: [PATCH v10 4/5] kprobes: freelist.h removed
Message-Id: <20231016222314.291e370a53a1f1922f8182c3@kernel.org>
In-Reply-To: <20231015053251.707442-5-wuqiang.matt@bytedance.com>
References: <20231015053251.707442-1-wuqiang.matt@bytedance.com>
        <20231015053251.707442-5-wuqiang.matt@bytedance.com>
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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@bytedance.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@bytedance.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@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 */
> -- 
> 2.40.1
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>