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
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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