On 14/09/2015 09:00, Mark Thomas wrote: > On 13/09/2015 23:11, Caldarale, Charles R wrote: >>> From: ma...@apache.org [mailto:ma...@apache.org] >>> Subject: svn commit: r1702821 - in /tomcat/tc8.0.x/trunk: >>> java/org/apache/tomcat/util/http/HttpMessages.java >>> webapps/docs/changelog.xml >> >>> --- tomcat/tc8.0.x/trunk/java/org/apache/tomcat/util/http/HttpMessages.java >>> (original) >>> +++ tomcat/tc8.0.x/trunk/java/org/apache/tomcat/util/http/HttpMessages.java >>> Sun Sep 13 20:36:40 2015 >>> @@ -69,26 +69,34 @@ public class HttpMessages { >>> // Does HTTP requires/allow international messages or >>> // are pre-defined? The user doesn't see them most of the time >>> switch( status ) { >>> - case 200: >>> - if(st_200 == null ) { >>> - st_200 = sm.getString("sc.200"); >>> + case 200: { >>> + String s = st_200; >>> + if(s == null ) { >>> + st_200 = s = sm.getString("sc.200"); >>> } >>> - return st_200; >> >> I'm not convinced this removes the data race, since there's still no >> guarantee that the storage writes that are part of the object initialization >> inside sm.getString() will be visible to another thread before the write to >> st_200. Shipilёv's blog has some interesting studies: >> http://shipilev.net/blog/2014/safe-public-construction/ > > OK. Let me have a read of that blog.
Thanks Chuck. That is the best explanation of all of this that I have seen so far. Mark > > At the moment we have two people who claim to be expert in this area > with different opinions on whether or not the above is safe. Personally, > my money is on Chuck being right. > > I don't know the JMM well enough to have a definitive opinion at this > point. I suspect that is going to have to change. This is why I *really* > want to see an explanation if why a data race occurs here - with > references to the JMM to back it up - not only to justify the work in > this area but to help those who are new to the JMM to understand exactly > why this is a problem. The more of us that understand this, the more > likely we are as a community to avoid issues like this in the future > and/or to spot them if one of us makes a mistake. > >> P.S. The formatting looks a little odd here, with no space after an "if" >> nor its opening parenthesis, but a space before the closing one. > > I spotted that and meant to fix it but got distracted and forgot. It > looks like I'm going to have to revist this code so I'll get that fixed > as well. > >> Also, the braces on the case statements aren't needed; using them results >> in having two closing braces in the same column at the end of the switch. > > The braces are required because of the repeat declarations of String s. > > Mark > > > --------------------------------------------------------------------- > To unsubscribe, e-mail: dev-unsubscr...@tomcat.apache.org > For additional commands, e-mail: dev-h...@tomcat.apache.org > --------------------------------------------------------------------- To unsubscribe, e-mail: dev-unsubscr...@tomcat.apache.org For additional commands, e-mail: dev-h...@tomcat.apache.org
