1 ;;;; Lock-free FIFO queues, from "An Optimistic Approach to Lock-Free FIFO
2 ;;;; Queues" by Edya Ladan-Mozes and Nir Shavit.
4 ;;;; Written by Nikodemus Siivola for SBCL.
6 ;;;; This software is part of the SBCL system. See the README file for
9 ;;;; This software is derived from the CMU CL system, which was written at
10 ;;;; Carnegie Mellon University and released into the public domain. The
11 ;;;; software is in the public domain and is provided with absolutely no
12 ;;;; warranty. See the COPYING and CREDITS files for more information.
14 (in-package :sb-concurrency)
16 (defconstant +dummy+ '.dummy.)
18 (declaim (inline make-node))
21 (prev nil :type (or null node))
22 (next nil :type (or null node)))
24 (declaim (inline %make-queue))
25 (defstruct (queue (:constructor %make-queue (head tail name))
28 "Lock-free thread safe queue."
29 (head (error "No HEAD.") :type node)
30 (tail (error "No TAIL.") :type node)
33 (setf (documentation 'queuep 'function)
34 "Returns true if argument is a QUEUE, NIL otherwise."
35 (documentation 'queue-name 'function)
36 "Name of a QUEUE. Can be assingned to using SETF. Queue names
37 can be arbitrary printable objects, and need not be unique.")
39 (defun make-queue (&key name initial-contents)
40 "Returns a new QUEUE with NAME and contents of the INITIAL-CONTENTS
42 (let* ((dummy (make-node :value +dummy+))
43 (queue (%make-queue dummy dummy name)))
46 (declare (dynamic-extent #'enc-1))
47 (map nil #'enc-1 initial-contents))
50 (defun enqueue (value queue)
51 "Adds VALUE to the end of QUEUE. Returns VALUE."
52 (let ((node (make-node :value value)))
53 (loop for tail = (queue-tail queue)
54 do (setf (node-next node) tail)
55 (when (eq tail (sb-ext:compare-and-swap (queue-tail queue) tail node))
56 (setf (node-prev tail) node)
59 (defun dequeue (queue)
60 "Retrieves the oldest value in QUEUE and returns it as the primary value,
61 and T as secondary value. If the queue is empty, returns NIL as both primary
65 (let* ((head (queue-head queue))
66 (tail (queue-tail queue))
67 (first-node-prev (node-prev head))
68 (val (node-value head)))
70 (when (eq head (queue-head queue))
71 (cond ((not (eq val +dummy+))
73 (let ((dummy (make-node :value +dummy+ :next tail)))
74 (when (eq tail (sb-ext:compare-and-swap (queue-tail queue)
76 (setf (node-prev head) dummy))
78 (when (null first-node-prev)
79 (fixList queue tail head)
81 (when (eq head (sb-ext:compare-and-swap (queue-head queue)
82 head first-node-prev))
83 ;; This assignment is not present in the paper, but is
84 ;; equivalent to the free(head.ptr) call there: it unlinks
85 ;; the HEAD from the queue -- the code in the paper leaves
86 ;; the dangling pointer in place.
87 (setf (node-next first-node-prev) nil)
88 (return-from dequeue (values val t))))
90 (return-from dequeue (values nil nil)))
91 ((null first-node-prev)
92 (fixList queue tail head)
95 (sb-ext:compare-and-swap (queue-head queue)
96 head first-node-prev)))))
99 (defun fixlist (queue tail head)
100 (let ((current tail))
101 (loop while (and (eq head (queue-head queue)) (not (eq current head)))
102 do (let ((next (node-next current)))
104 (return-from fixlist nil))
105 (let ((nextNodePrev (node-prev next)))
106 (when (not (eq nextNodePrev current))
107 (setf (node-prev next) current))
108 (setf current next))))))
110 (defun list-queue-contents (queue)
111 "Returns the contents of QUEUE as a list without removing them from the
112 QUEUE. Mainly useful for manual examination of queue state."
114 (labels ((walk (node)
115 ;; Since NEXT pointers are always right, traversing from tail
117 (let ((value (node-value node))
118 (next (node-next node)))
119 (unless (eq +dummy+ value)
123 (walk (queue-tail queue)))
126 (defun queue-count (queue)
127 "Returns the number of objects in QUEUE. Mainly useful for manual
128 examination of queue state, and in PRINT-OBJECT methods: inefficient as it
129 walks the entire queue."
131 (declare (unsigned-byte n))
132 (labels ((walk (node)
133 (let ((value (node-value node))
134 (next (node-next node)))
135 (unless (eq +dummy+ value)
139 (walk (queue-tail queue))
142 (defun queue-empty-p (queue)
143 "Returns T if QUEUE is empty, NIL otherwise."
144 (let* ((head (queue-head queue))
145 (tail (queue-tail queue))
146 (val (node-value head)))
147 (and (eq head tail) (eq val +dummy+))))