yikf opened a new pull request #3042:
URL: https://github.com/apache/hadoop/pull/3042
### What changes were proposed in this pull request?
In `StatisticsDataReferenceCleaner`, Cleaner thread will be blocked if we
remove reference from ReferenceQueue unless the queue.enqueue` called but no
now.
As shown below, We call ReferenceQueue.remove() now while cleanUp, Call
chain as follow：
`StatisticsDataReferenceCleaner#queue.remove() -> ReferenceQueue.remove(0)
-> lock.wait(0)`
lock.wait(0) will waitting perpetual unless lock.notify/notifyAll be called,
But, lock.notifyAll is called when queue.enqueue only, so Cleaner thread will
be blocked.
**ThreadDump:**
```
"Reference Handler" #2 daemon prio=10 os_prio=0 tid=0x00007f7afc088800
nid=0x2119 in Object.wait() [0x00007f7b00230000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x00000000c00c2f58> (a java.lang.ref.Reference$Lock)
at java.lang.Object.wait(Object.java:502)
at java.lang.ref.Reference.tryHandlePending(Reference.java:191)
- locked <0x00000000c00c2f58> (a java.lang.ref.Reference$Lock)
at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:153)
```
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