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https://issues.apache.org/jira/browse/GROOVY-8067?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16088706#comment-16088706
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John Wagenleitner commented on GROOVY-8067:
-------------------------------------------
{quote}
So guess the question would be whether the responsibility lies with the caller
of {{ExpandoMetaClass#enableGlobally}} to check first to see if already enabled
or whether that method should not clear if already enabled.
{quote}
The enableGlobally call does already check and only clears if the
{{ExpandoMetaClassCreationHandle}} is not the current handle. In tracing the
calls to clearModifiedExpandos when running the sample project
grails-javamelody-issue the plugin triggers 2 calls to clearModifiedExpandos,
first call in {{ExpandoMetaClassCreationHandle#create}} (via
{{ExpandoMetaClass.enableGlobally}}) and second one in
{{MetaClassRegistryImpl#setMetaClassCreationHandle}} (via the before mentioned
create call). So even with the old {{ManagedLinkedList}} (in Groovy 2.4.8) the
plugin would have removed 2 modified expandos, those for
{{org.grails.plugins.codecs.URLCodec}} and
{{org.grails.encoder.impl.JavaScriptCodec}}.
> Possible deadlock when creating new ClassInfo entries in the cache
> ------------------------------------------------------------------
>
> Key: GROOVY-8067
> URL: https://issues.apache.org/jira/browse/GROOVY-8067
> Project: Groovy
> Issue Type: Bug
> Components: groovy-runtime
> Affects Versions: 2.4.8
> Reporter: John Wagenleitner
> Assignee: John Wagenleitner
> Priority: Critical
> Fix For: 2.4.9
>
> Attachments: ClassInfoDeadlockTest.java
>
>
> When running Groovy without {{-Dgroovy.use.classvalue=true}} the ClassInfo
> instances are cached in a {{ManagedConcurrentMap}} (MCM). New values are
> computed on demand and computation involves both a lock on a segment within
> the MCM and a lock on the {{GlobalClassSet}} (GCS) which is backed by a
> {{ManagedLinkedList}}. The problem is that both the ManagedConcurrentMap and
> the GlobalClassSet share the same ReferenceQueue.
> Assume there is an enqueued {{ClassInfo}} value that is stored in Segment2 of
> the MCM. Now assume that Thread1 and Thread2 both request
> {{ClassInfo.getClassInfo(..)}} for two different classes that do not
> currently exist in the cache. Assume that based on hashing Thread1 gets a
> lock on Segment1 and Thread2 gets a lock on Segment2. Assume that Thread1 is
> the first to call computeValue which in turn calls
> {{GlobalClassSet.add(..)}}. This call adds a new value to a
> {{ManagedLinkedList}}, and since it's managed the add operation will process
> the ReferenceQueue. So Thread1 will attempt to dequeue the ClassInfo and the
> finalizeReference method on it's entry will attempt to remove it from
> Segment2. Thread2 holds the lock for Segment2 and Thread2 is blocked and
> can't progress it's waiting on the the lock Thread1 holds the lock for the
> GlobalClassSet, so deadlock occurs.
> The attached test case includes a thread dump at the bottom.
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