Job history server scans can become blocked on a single, slow entry

HistoryFileInfo firstValue = cache.get(key);
    synchronized(firstValue) {  ---------------> Synchronized is not needed here
              if (firstValue.isMovePending()) {
                if(firstValue.didMoveFail() && 
                    firstValue.jobIndexInfo.getFinishTime() <= cutoff) {
                  cache.remove(key);
                  //Now lets try to delete it
                  try {
                    firstValue.delete();
                  } catch (IOException e) {
                    LOG.error("Error while trying to delete history files" +
                    " that could not be moved to done.", e);
                  }
                } else {
                  LOG.warn("Waiting to remove " + key
                      + " from JobListCache because it is not in done yet.");
                }
              } else {
                cache.remove(key);
              }
            }

Note: stacktrace is from hadoop-2.4.0 version and the problem exists in latest hadoop as well

"2144820863@qtp-313351300-38156" daemon prio=10 tid=0x0000000001e13800 nid=0xf133 waiting for monitor entry [0x00007f7c1d8dd000]
   java.lang.Thread.State: BLOCKED (on object monitor)
        at org.apache.hadoop.mapreduce.v2.hs.HistoryFileManager$JobListCache.addIfAbsent(HistoryFileManager.java:226)
        - waiting to lock <0x000000040145c4d8> (a org.apache.hadoop.mapreduce.v2.hs.HistoryFileManager$HistoryFileInfo)
        at org.apache.hadoop.mapreduce.v2.hs.HistoryFileManager.scanIntermediateDirectory(HistoryFileManager.java:825)
        at org.apache.hadoop.mapreduce.v2.hs.HistoryFileManager.access$200(HistoryFileManager.java:82)
        at org.apache.hadoop.mapreduce.v2.hs.HistoryFileManager$UserLogDir.scanIfNeeded(HistoryFileManager.java:280)
        - locked <0x0000000400375388> (a org.apache.hadoop.mapreduce.v2.hs.HistoryFileManager$UserLogDir)
        at org.apache.hadoop.mapreduce.v2.hs.HistoryFileManager.scanIntermediateDirectory(HistoryFileManager.java:792)
        at org.apache.hadoop.mapreduce.v2.hs.HistoryFileManager.getAllFileInfo(HistoryFileManager.java:920)
        at org.apache.hadoop.mapreduce.v2.hs.CachedHistoryStorage.getAllPartialJobs(CachedHistoryStorage.java:156)
        at org.apache.hadoop.mapreduce.v2.hs.JobHistory.getAllJobs(JobHistory.java:235)