<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=utf-8"> <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN"> <HTML> <HEAD> <META NAME="Generator" CONTENT="MS Exchange Server version 6.0.6249.1"> <TITLE>Re: [plt-scheme] RE: Shared resources for servlets</TITLE> </HEAD> <BODY dir=ltr> <DIV>Well it's not perfect because it's just an abstraction. It doesn't deal with exceptions, for one.</DIV> <DIV>I have one questions though:</DIV> <DIV>Do the threads not wait to their turn and only change at (sleep)s and blocks?</DIV> <DIV>If so, all the race conditions would not happen</DIV> <BLOCKQUOTE dir=ltr style="MARGIN-RIGHT: 0px"> <DIV>-----Original Message----- From: Joe Marshall [mailto:jrm@ccs.neu.edu] Sent: ד 10/09/2003 20:21 To: Dor Kleiman; Keith Frost Cc: plt-scheme@list.cs.brown.edu Subject: Re: [plt-scheme] RE: Shared resources for servlets </DIV> Re: Shared resources for servletsFrom:  Dor Kleiman <dor@ntr.co.il>; To:  Keith Frost <keithf@amnis.com> > (module lock mzscheme >   (define read/write-object >     (class >       (define (read) 'todo) >       (define (write to-write) 'todo))) > >   (define lockable-object >     (class >       (define reading 0) >       (define writing? #f) > >       (init-field obj) > >       (define (open-read) >         (if writing? >             (begin (sleep) (open-read)) >             (set! reading (+ reading 1)))) Race condition here, writing? could be #F when conditional is tested, but changed to #T before reading is incremented. Additionally, reading is not atomically incremented, so an increment could be lost. >       (define (close-read) >         (set! reading (- reading 1))) Could lose the decrement on a race condition. >       (define (open-write) >         (if (or writing? (> reading 0)) >             (begin (sleep) (open-write)) >             (set! writing? #t))) Again, race condition in the conditional, two processes could both detect no writing? condition and each think that it can write. >       (define (close-write) >         (set! writing? #f)) > >       (define (read) >         ; returns current value. >         ; Copies it first so it doesn't change when writing later on >         (open-read) >         (let ([r (send obj read)]) >           (close-read) >           r))) Need a dynamic-wind here to ensure that the lock is freed even if the reader aborts. >       (define (write to-write) >         (open-write) >         (send obj write to-write) >         (close-write))))) Need a dynamic-wind here to ensure that the lock is freed even if the writer aborts. </BLOCKQUOTE> </BODY> </HTML>