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From: "Bo Xu" <bo@cybershop.ca>
To: "Tomcat Developers List" <tomcat-dev@jakarta.apache.org>
References: <20011206002230.7918.qmail@nagoya.betaversion.org>
Subject: Re: DO NOT REPLY [Bug 4138]  -    HttpProcessor threads have
 inconsistent ClassLoader state
Date: Thu, 6 Dec 2001 14:50:15 -0500
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Dear TOMCAT developers, :-)


I forward the following email to you :-)

Bo
Dec.06, 2001

*****************************************************************

If you really want the developers to take a look at this, you should
probably post it to the tomcat-dev list. It's iffy whether or not they will
see it here.

Thanks,
--jeff

----- Original Message -----
From: "java programmer" <javadesigner@yahoo.com>
To: <tomcat-user@jakarta.apache.org>
Sent: Wednesday, December 05, 2001 10:44 PM
Subject: Double check idiom broken - Tomcat uses it ?


> Hi all:
>
> We all know that the lazy-double-check idiom doesn't
> apply to Java because of the Java Memory Model (JMM).
>
> That is to say, look at code such as:
>
> Example a)
> // Set by any other thread other than #1
> volatile boolean stop = false;
>
> // Thread #1 runs this as long as
> // stop is false. Only T1 will call this
> // method, so not synchronized. hence
> // broken due to staleness of 'stop'.
> // synch for _visbility_ ALSO.
> void foo() {
>  while (!stop ) {  //... }
> }
>
>
> Example b): The lazy double check idiom
> public static Foo haha = null;
> public static getFoo() {
> if (foo == null ) {
>   sychronized (Foo.class) {
>      if (foo == null )
>             foo = new Foo();
>   }
> }
> return foo;
> }
>
> Both examples are *guaranteed* to be incorrect.
> Note, this is the case, *even* though I am using
> 'volatile' for the stop variable. For more on the
> JMM, consult Item #48 in Effective Java (Josh Bloch),
> look at Bill Pughs' page at:
> http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html
> or check out Doug Lea's stuff.
> Well, here is the thing:
>
> Quite idly, and randomly, I was looking at:
>
> org.apache.jasper.servlet.JspServlet
>
> and I found:
>
> <snip>
> outDated = compiler.isOutDated();
>    if(!jsw.isInstantiated() || outDated ) {
>       synchronized(jsw){
>         outDated = compiler.compile();
>         if(!jsw.isInstantiated() || outDated) {
>         if( null ==ctxt.getServletClassName() ) {
> <snip>
>
> This is a complex use of double check type
> code and is really hard to analyse because references
> themselves and what they point to can have
> different levels of staleness (according to the JMM).
> So it's a turbo double idiom type usage, possibly
> incorrect.
>
> I just wanted to bring this to the attention of the
> development team and make sure that *someone* has
> really analysed this according to the JMM. (and
> any other code, similar to this).
>
> Best regards,
>
> javadesigner@yahoo.com



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