Tummy Bunny created HDFS-11820:
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Summary: Thread safety in logEdit?
Key: HDFS-11820
URL: https://issues.apache.org/jira/browse/HDFS-11820
Project: Hadoop HDFS
Issue Type: Bug
Components: hdfs
Reporter: Tummy Bunny
Hi there,
I am new to Hadoop and trying to understand how things work under the hood by
browsing through some of the codes.
I noticed a potential thread safety issue in in FSEditLog.java in version 2.7.1
where the following patterns is used (the current trunk also use the same
pattern):
1. Instance of FSEditLogOp is retrieved from cache for reuse
2. Set the attributes (e.g. path, timestamp, etc)
3. Invoke logEdit(op). This method has synchronized block in it, but also has
"wait" if auto-sync is scheduled.
Now, if I have two almost simultaneous rename operations, right after each is
about to write edit log:
Thread #1 acquired instance of RenameOp, set the attributes, and invoked
logEdit, then it waits because auto-sync is scheduled.
Thread #2 acquired same instance of RenameOp, set *different* attributes, and
also invoked logEdit.
The second renameOp could end up being logged twice because both renameOps are
actually the same instance.
The fix is to move synchronized(anyCachedOp) { ... } prior to calling logEdit
or clone the op (use the cached instance as template).
I could be wrong. Am I missing something?
Thanks,
Alexander Koentjara
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