[jira] [Created] (HDFS-11507) NetworkTopology#chooseRandom may run into a dead loop due to race condition

["Chen Liang (JIRA)" <ji...@apache.org>]

Chen Liang created HDFS-11507:
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             Summary: NetworkTopology#chooseRandom may run into a dead loop due to race condition
                 Key: HDFS-11507
                 URL: https://issues.apache.org/jira/browse/HDFS-11507
             Project: Hadoop HDFS
          Issue Type: Bug
          Components: namenode
            Reporter: Chen Liang
            Assignee: Chen Liang


{{NetworkTopology#chooseRandom()}} works as:
1. counts the number of available nodes as {{availableNodes}},
2. checks how many nodes are excluded, deduct from {{availableNodes}}
3. if {{availableNodes}} still > 0, then there are nodes available.
4. keep looping to find that node

But now imagine, in the meantime, the actually available nodes got removed in step 3 or step 4, and all remaining nodes are excluded nodes. Then, although there are no more nodes actually available, the code would still run as {{availableNodes}} > 0, and then it would keep getting excluded node and loop forever, as 
{{if (excludedNodes == null || !excludedNodes.contains(ret))}} 
will always be false.

We may fix this by expanding the while loop to also include the {{availableNodes}} calculation. Such that we re-calculate {{availableNodes}} every time it fails to find an available node.



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