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From "David Jencks (JIRA)" <j...@apache.org>
Subject [jira] Commented: (OPENJPA-826) moving some code outside of lock boundaries, that don't need to be locked
Date Sat, 13 Dec 2008 08:06:44 GMT

    [ https://issues.apache.org/jira/browse/OPENJPA-826?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12656263#action_12656263
] 

David Jencks commented on OPENJPA-826:
--------------------------------------

These changes look to me like they introduce serious thread safety issues.  For instance with
this snippet:

     public Class getResultType() {

+    	assertOpen();

+    	if (_resultClass != null || _compiled != null || _query == null

+    			|| _broker == null)

+    		return _resultClass;

+    	

         lock();

         try {

-            assertOpen();

-            if (_resultClass != null || _compiled != null || _query == null

-                || _broker == null)

-                return _resultClass;

-

             // check again after compilation; maybe encoded in string

             compileForCompilation();

             return _resultClass;

_resultClass must be volatile in order to assure that it is a completely constructed object,
and an arbitrarily large number of threads can get into the locked block and  call compileForCompilation
which I assume sets _resultClass.

You might be able to fix it with code something like this, assuming the assertions are thread
safe:

private volatile Class _resultClass;

...
     public Class getResultType() {
        assertOpen();
        if (_resultClass == null) {
            lock();
            if (_resultClass == null) {
                compileForCompilation();
            }
            unlock();
        }
        return _resultClass;
    }

(exception handling and some of the conditions left out).  This will not work unless the varlable
is volatile, as unlocked threads could then read a partially initialized object.  Google for
double-checked locking for more details.

> moving some code outside of lock boundaries, that don't need to be locked
> -------------------------------------------------------------------------
>
>                 Key: OPENJPA-826
>                 URL: https://issues.apache.org/jira/browse/OPENJPA-826
>             Project: OpenJPA
>          Issue Type: Improvement
>          Components: kernel
>    Affects Versions: 2.0.0
>            Reporter: Fernando
>         Attachments: locks-asserts.diff
>
>
> I am reviewing code because of slices concurrency/multithreaded issues, and I see that
some code in QueryImpl was too aggressive in their locks.  So I reviewed QueryImpl and BrokerImpl...
> Mainly: assertMethods do not need to be within the lock.
> Then I found a few methods that were doing precondition/shortcut checking, but all within
locks, so that seemed like a waste:
> 1) QueryImpl.setCandiateExtent; checks to see if value setting is same as current value
> 2) these places check to see if it needs to calculate the value of not:
> QueryImpl.isUnique, QueryImpl.setCandiateExtent, QueryImpl.getCandidateType, QueryImpl.getResultType,
QueryImpl.getParameterDeclaration

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