java deadlock (not sure how to reproduce)

Hi, 

my colleague gives me an IoTDB instance which has no response for "select " SQL.

(but for Schema query sql, it works).

I used jstack to check the process and find a deadlock, you can get the detailed info in the attachment.

Found one Java-level deadlock:
=============================
"pool-2-IoTDB-JDBC-Client-thread-16":
waiting to lock monitor 0x00007fdab800ff68 (object 0x00000003c7163218, a org.apache.iotdb.db.query.control.FileReaderManager),
which is held by "pool-2-IoTDB-JDBC-Client-thread-3"
"pool-2-IoTDB-JDBC-Client-thread-3":
waiting for ownable synchronizer 0x00000003c6106f40, (a java.util.concurrent.locks.ReentrantReadWriteLock$NonfairSync),
which is held by "MergeThread-0"
"MergeThread-0":
waiting to lock monitor 0x00007fdab800ff68 (object 0x00000003c7163218, a org.apache.iotdb.db.query.control.FileReaderManager),
which is held by "pool-2-IoTDB-JDBC-Client-thread-3"