[jira] Commented: (JCR-2865) a dead lock in DefaultISMLocking

["Thomas Mueller (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/JCR-2865?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12989676#comment-12989676 ] 

Thomas Mueller commented on JCR-2865:
-------------------------------------

The obvious disadvantage is code duplication.

> a dead lock in DefaultISMLocking
> --------------------------------
>
>                 Key: JCR-2865
>                 URL: https://issues.apache.org/jira/browse/JCR-2865
>             Project: Jackrabbit Content Repository
>          Issue Type: Bug
>          Components: jackrabbit-core
>    Affects Versions: 2.2.0
>         Environment: winXP/JDK1.6
>            Reporter: codeparser
>            Assignee: Jukka Zitting
>             Fix For: 2.2.3
>
>         Attachments: trackReader.diff
>
>
> The jackrabbit 2.2 's org.apache.jackrabbit.core.state.DefaultISMLocking has a defect which will cause a dead lock in concurrent use cases.
> The use case is as follows:
> 1.	Thread A apply a read lock, now there is an active reader hold the read lock.
> 2.	Thread B apply a write lock, and then thread B will wait for thread A's reading end. You could see below code snippet from the Jackrabbit source. readerCount is the current active reader.
> writersWaiting++;
> while (writerId != null? !isSameThreadId(writerId, currentId) : readerCount > 0) {
>                                 wait();
> }
> 3.	Thread A apply another read lock, then it will wait too, since there is a writer is waiting.  Then a dead lock happens.
> while (writerId != null? (writerCount > 0 && !isSameThreadId(writerId, currentId)): writersWaiting > 0) {
>                                 wait();
> }
> Since the lock in DefaultISMLocking is global lock, so I think if a thread has already hold a reader lock, it could get the reader lock again. I create a fix with this idea.

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