Created
August 4, 2012 01:38
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lock-free queue
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(defpackage lock-free-queue | |
(:use :common-lisp) | |
(:export queue | |
make | |
enq | |
deq | |
empty-p | |
element-count | |
to-list)) | |
(in-package :lock-free-queue) | |
;; compare-and-swap: 成功した場合はTを、失敗した場合はNILを返す | |
(defmacro compare-and-swap (place old new) | |
`(eq (sb-ext:compare-and-swap ,place ,old ,new) ,old)) | |
;; キュー構造体 | |
(defstruct queue | |
(head nil :type list) | |
(tail nil :type list)) | |
;; リストへ変換/空判定/要素数取得 | |
(defun to-list (que) (copy-seq (cdr (queue-head que)))) | |
(defun empty-p (que) (endp (cdr (queue-head que)))) | |
(defun element-count (que) (length (cdr (queue-head que)))) | |
(defmethod print-object ((o queue) stream) | |
(print-unreadable-object (o stream :type t) | |
(format stream "~s ~s" :count (element-count o)))) | |
;; キューを生成 | |
(defun make (&optional initial-contents) | |
(let ((contents (cons :initial-head initial-contents))) | |
(make-queue :head contents | |
:tail (last contents)))) | |
;; キューの末尾に要素を追加する | |
;; => queue | |
(defun enq (x que) | |
(loop WITH new-elem = (list x) | |
FOR tail = (queue-tail que) | |
DO | |
(cond ((cdr tail) | |
(compare-and-swap (queue-tail que) tail (cdr tail))) ; tailの位置を調整 | |
((compare-and-swap (cdr tail) nil new-elem) | |
(return que))))) ; 追加成功 | |
;; キューの先頭から要素を取り出す | |
;; => (or (values 先頭要素 T) ; キューに要素がある場合 | |
;; (values NIL NIL)) ; キューが空の場合 | |
(defun deq (que) | |
(let* ((head (queue-head que)) | |
(next (cdr head))) | |
(cond ((null next) | |
(values nil nil)) ; 空 | |
((compare-and-swap (queue-head que) head next) | |
(values (car next) t)) ; 取得成功 | |
(t | |
(deq que))))) ; 他スレッドと競合(リトライ) |
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