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From bugzi...@apache.org
Subject DO NOT REPLY [Bug 37356] - Tomcat does not invalidate sessions after session-timeout period has passed.
Date Fri, 10 Mar 2006 16:17:33 GMT
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http://issues.apache.org/bugzilla/show_bug.cgi?id=37356





------- Additional Comments From Lothsahn@yahoo.com  2006-03-10 16:17 -------
(In reply to comment #24)
> Proposed patch:
> 
> in org.apache.catalina.session.StandardSession
>  line 284 (tomcat 5.0.28)
> 
>     /**
>      * The access count for this session.
>      */
>     protected transient int accessCount = 0;
> 
> has to be replaced by 
> 
>     /**
>      * The access count for this session.
>      */
>     protected volatile transient int accessCount = 0;
> 
> Explanation:
> The expression accessCount++ is not thread-safe because each thread may hold a
> local copy of the variable which does not have to be synchronized with main
> memory immediately.
> The volatile keyword forces this synchronization.
> 
> See the discussion on 
> http://forum.java.sun.com/thread.jspa?threadID=604831&start=30&tstart=0
> 
> [quote]
> 2) Local vs. main mem: Threads have--or can have--their own local copies of
> shared variables. When T1 writes a value to a variable, if the variable is not
> volatile and there's no syncing, that new value may live only in T1's local copy
> and may never get written to main mem, which means other threads may never see
> that new value. Entering and leaving sync blocks forces a reconciliation between
> the thread's local memory and main mem. Declaring a variable volatile forces
> every read and write of that variable to go to main mem.
> [end quote]
> 
> Another solution would be to synchronize the code segments.
> 
> I have to apologize for my claim that ++ and -- were atomic for int datatypes. 
> This seems to be only true for single-CPU systems (without hyperThreading).

Volatile may not be the best idea.  From the article:
http://cephas.net/blog/2003/02/17/using_the_volatile_keyword_in_java.html

"Careful, volatile is ignored or at least not implemented properly on many
common JVM's, including (last time I checked) Sun's JVM 1.3.1 for Windows. There
was an article on DDJ that could demonstrate this even on single-processor
machines..."

And from the article: 
http://www.javaperformancetuning.com/tips/volatile.shtml

"Note however that volatile has been incompletely implemented in most JVMs.
Using volatile may not help to achieve the results you desire (yes this is a JVM
bug, but its been low priority until recently). "


Our environment:
We are only seeing this issue for sure on one machine, which is a 4 way (with
HT--8 virtual CPU's) box.  We see the issue pretty frequently occur there... 
maybe 20-100 times a day for a total of 2,000-4,000 total sessions.

Also, this environment is NOT 64-bit.  These are 32-bit HT Xeons.  They are
running the Sun 1.4.2_06 or the 1.4.2_08 JDK...  So we know this issue occurs on
32 bit systems as well.


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