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From "Amar Kamat (JIRA)" <j...@apache.org>
Subject [jira] Commented: (MAPREDUCE-1342) Potential JT deadlock in faulty TT tracking
Date Wed, 06 Jan 2010 05:47:54 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-1342?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12796996#action_12796996
] 

Amar Kamat commented on MAPREDUCE-1342:
---------------------------------------

What if we move the code from JobTracker.blacklistedTaskTrackers() to FaultyTrackersInfo.
Something like
{code}
FaultyTrackersInfo {
  blacklistedTaskTrackers {
    synchronized (potentiallyFaultyTrackers) {
      synchronized (taskTrackers) {
        // code  that we have today JobTracker.blacklistedTaskTrackers()
        for (TaskTracker tt : taskTrackers.values()) {
          // ...
        }
      }
    }
  }
}
blacklistedTaskTrackers() {
return FaultyTrackersInfo.blacklistedTaskTrackers()
}
{code}

This kindof solves the lock reversal issue we are facing now and also makes more sense because
JobTracker.FaultyTrackersInfo is the right module to answer the blacklistedTaskTrackers()
query. Thoughts?

> Potential JT deadlock in faulty TT tracking
> -------------------------------------------
>
>                 Key: MAPREDUCE-1342
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-1342
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>          Components: jobtracker
>    Affects Versions: 0.22.0
>            Reporter: Todd Lipcon
>         Attachments: cycle0.png, mapreduce-1342-1.patch, mapreduce-1342-2.patch
>
>
> JT$FaultyTrackersInfo.incrementFaults first locks potentiallyFaultyTrackers, and then
calls blackListTracker, which calls removeHostCapacity, which locks JT.taskTrackers
> On the other hand, JT.blacklistedTaskTrackers() locks taskTrackers, then calls faultyTrackers.isBlacklisted()
which goes on to lock potentiallyFaultyTrackers.
> I haven't produced such a deadlock, but the lock ordering here is inverted and therefore
could deadlock.
> Not sure if this goes back to 0.21 or just in trunk.

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