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https://issues.apache.org/jira/browse/SOLR-17720?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17938704#comment-17938704
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David Smiley commented on SOLR-17720:
-------------------------------------
Fascinating. Did a test (which?) reveal this?
> Deadlock in CollectionPropertiesZkStateReader
> ---------------------------------------------
>
> Key: SOLR-17720
> URL: https://issues.apache.org/jira/browse/SOLR-17720
> Project: Solr
> Issue Type: Bug
> Components: SolrJ
> Affects Versions: 9.7
> Reporter: Houston Putman
> Priority: Blocker
> Fix For: 9.9
>
>
> {{CollectionPropertiesZkStateReader}} has multiple different mechanisms for
> synchronizing when modifying its concurrent data structures.
> # {{synchronized (getCollectionLock(collection))}}
> # {{collectionPropsObservers}} is a ConcurrentHashMap, and therefore locks
> on updating a single key within the map.
> Unfortunately this can cause a deadlock.
> In {{CollectionPropertiesZkStateReader.removeCollectionPropsWatcher()}},
> {{collectionPropsObservers.compute(collection, <function>)}} is used which
> will create a lock in {{collectionPropsObservers}} on the {{collection}} key.
> Within this locked {{<function>}} command, {{synchronized
> (getCollectionLock(collection))}} is called.
> In {{CollectionPropertiesZkStateReader.refreshAndWatch()}}, {{synchronized
> (getCollectionLock(coll))}} is used for the whole method. And within this
> synchronized block, {{collectionPropsObservers.remove(coll)}} is called
> (which will obviously get a lock on the {{coll}} key for
> {{collectionPropsObservers}}.
> So {{CollectionPropertiesZkStateReader.removeCollectionPropsWatcher()}} has
> the lock for {{collectionPropsObservers}} but is waiting on the lock for
> {{getCollectionLock(coll)}}. And
> {{CollectionPropertiesZkStateReader.refreshAndWatch()}} has the lock for
> {{getCollectionLock(coll)}} and is waiting on the lock for
> {{collectionPropsObservers}}. Hence deadlock.
> This code is quite complex, and I think it can really be simplified, but
> that's just a gut reaction. I think moving the {{synchronized
> (getCollectionLock(collection))}} block in {{removeCollectionPropsWatcher()}}
> outside of the {{compute()}} call would solve this one deadlock though.
> Hopefully we can really simplify this with Curator though.
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