Add implementation of exercise 3-48

Author: ramsayleung

README.org

@@ -25,7 +25,7 @@
   习题完成情况:
   - 章节一: 43/46
   - 章节二: 88/97
-  - 章节三: 47/82
+  - 章节三: 48/82
   - 章节四: TODO
   - 章节五: TODO
 * 运行

chapter3/exercise3-48.rkt

@@ -0,0 +1,65 @@
+#lang racket
+(require "mutex.rkt")
+
+;;; 设想 Peter 企图去交换 a1 和 a2，同时 Paul 并发地企图交换 a2 和 a1
+;;; 如果对账户编号，并使进程先试图获取编号较小的账户，那么意味着账户的
+;;; 顺序是确定的；即 Peter 和 Paul 都会先获取 a1 （假设 a1 编号更小），
+;;; 那么只有一个进程会获取成功，那么它就可以继续获取 a2, 从而避免死锁
+
+(define (make-account-and-serializer id balance)
+  (define (withdraw amount)
+    (if (>= balance amount)
+        (begin (set! balance (- balance amount))
+               balance)
+        "Insufficient funds"))
+  (define (deposit amount)
+    (set! balance (+ balance amount))
+    balance)
+
+  (let ((balance-serializer (make-serializer)))
+    (define (dispatch m)
+      (cond ((eq? m 'withdraw) withdraw)
+            ((eq? m 'deposit) deposit)
+            ((eq? m 'balance) balance)
+            ((eq? m 'serializer) balance-serializer)
+            ((eq? m 'id) id)
+            (else (error "Unknown request -- MAKE-ACCOUNT" m))))
+    dispatch))
+
+(define (exchange account1 account2)
+  (let ((difference (- (account1 'balance)
+                       (account2 'balance))))
+    ((account1 'withdraw) difference)
+    ((account2 'deposit) difference)))
+
+(define (serializer-exchange account1 account2)
+  (let* ((id1 (account1 'id))
+         (id2 (account2 'id))
+         (small-account (if (< id1 id2) account1 account2))
+         (large-account (if (< id1 id2) account2 account1))
+         (s-serializer (small-account 'serializer))
+         (l-serializer (large-account 'serializer)))
+    ;; always lock smaller Id first, then large Id
+    ((s-serializer (l-serializer exchange)) account1 account2)))
+
+(module+ test
+  (require rackunit)
+
+  (test-case "Test for no-dead lock version exchange"
+             (define account1 (make-account-and-serializer 1 40))
+             (define account2 (make-account-and-serializer 2 20))
+             (check-equal? (account1 'id) 1)
+             (check-equal? (account2 'id) 2)
+
+             (define t1 (thread (lambda ()(serializer-exchange account1 account2))))
+             (define t2 (thread (lambda ()(serializer-exchange account2 account1))))
+
+             ;; should avoid deadlock
+             (thread-wait t1)
+             (thread-wait t2)
+
+             ;; exchange twice means each account has the original balance
+             (check-equal? (account1 'balance) 40)
+             (check-equal? (account2 'balance) 20)
+             )
+  )

chapter3/mutex.rkt

@@ -17,4 +17,14 @@
 (define (clear! cell)
   (box-cas! cell #t #f))
 
-(provide make-mutex)
+(define (make-serializer)
+  (let ((mutex (make-mutex)))
+    (lambda (p)
+      (define (serializer-p . args)
+        (mutex 'acquire)
+        (let ((val (apply p args)))
+          (mutex 'release)
+          val))
+      serializer-p)))
+
+(provide make-mutex make-serializer)