Finish 3.49

ex-3_46-49.scm

@@ -124,9 +124,9 @@
 (s 'release)
 (assert (s 'remaining) 3)
 
-(display "\nex-3.48\n")
+(display "\nex-3.48 - exchange without deadlocks\n")
 
-(define (make-account-and-serializer balance)
+(define (make-account-and-serializer balance number)
   (define (withdraw amount)
     (if (>= balance amount)
         (begin (set! balance (- balance amount))
@@ -141,6 +141,7 @@
             ((eq? m 'deposit) deposit)
             ((eq? m 'balance) balance)
             ((eq? m 'serializer) balance-serializer)
+            ((eq? m 'number) number)
             (else (error "Unknown request -- MAKE-ACCOUNT"
                          m))))
     dispatch))
@@ -159,12 +160,24 @@
 (define (serialized-exchange account1 account2)
   (let ((serializer1 (account1 'serializer))
         (serializer2 (account2 'serializer)))
-    ((serializer1 (serializer2 exchange))
-     account1
-     account2)))
+    (if (< (account1 'number) (account2 'number))
+      ((serializer1 (serializer2 exchange)) account1 account2)
+      ((serializer2 (serializer1 exchange)) account1 account2))))
 
-(define a (make-account-and-serializer 100))
-(define b (make-account-and-serializer 40))
+(define a (make-account-and-serializer 100 1))
+(define b (make-account-and-serializer 40 2))
 (serialized-exchange a b)
 (assert (a 'balance) 40)
 
+; Assuming number(a) = 1 and number(b) = 2 the new procedure forces the lock
+; for a to be taken before the one for b. This means the process that gets a
+; first is able to complete because the second process cannot lock b.
+
+(display "\nex-3.49\n")
+(display "[answered]")
+
+; If we have a procedure that takes and account and then executes exchanges for
+; a list of other accounts related to that account. In a situation where two
+; accounts reference each other we might again run into a deadlock. That is a bad
+; example, but I cannot think of a better one right now.
+

ex-3_50-xx.scm

@@ -0,0 +1,5 @@
+(load "util.scm")
+
+(display "\nex-3.50\n")
+
+