Houston Putman created SOLR-17720:
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Summary: 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
{{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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