CME in StoreScanner causes region server crash

RS crashed with ConcurrentModificationException on our 1.3 cluster, stack trace below. toffer and I checked and there is a race condition between flush and scanner close. When StoreScanner.updateReaders() is updating the scanners after a newly flushed file (in this trace below a region close during a split), the client's scanner could be closing thus causing CME.

Its rare, but since it crashes the region server, needs to be fixed.

FATAL regionserver.HRegionServer [regionserver/<rs>] : ABORTING region server <rs>: Replay of WAL required. Forcing server shutdown
org.apache.hadoop.hbase.DroppedSnapshotException: region: <regionname>
at org.apache.hadoop.hbase.regionserver.HRegion.internalFlushCacheAndCommit(HRegion.java:2579)
at org.apache.hadoop.hbase.regionserver.HRegion.internalFlushcache(HRegion.java:2255)
at org.apache.hadoop.hbase.regionserver.HRegion.internalFlushcache(HRegion.java:2217)
at org.apache.hadoop.hbase.regionserver.HRegion.internalFlushcache(HRegion.java:2207)
at org.apache.hadoop.hbase.regionserver.HRegion.doClose(HRegion.java:1501)
at org.apache.hadoop.hbase.regionserver.HRegion.close(HRegion.java:1420)
at org.apache.hadoop.hbase.regionserver.SplitTransactionImpl.stepsBeforePONR(SplitTransactionImpl.java:398)
at org.apache.hadoop.hbase.regionserver.SplitTransactionImpl.createDaughters(SplitTransactionImpl.java:278)
at org.apache.hadoop.hbase.regionserver.SplitTransactionImpl.execute(SplitTransactionImpl.java:566)
at org.apache.hadoop.hbase.regionserver.SplitRequest.doSplitting(SplitRequest.java:82)
at org.apache.hadoop.hbase.regionserver.SplitRequest.run(SplitRequest.java:154)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
Caused by: java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
at java.util.ArrayList$Itr.next(ArrayList.java:851)
at org.apache.hadoop.hbase.regionserver.StoreScanner.clearAndClose(StoreScanner.java:797)
at org.apache.hadoop.hbase.regionserver.StoreScanner.updateReaders(StoreScanner.java:825)
at org.apache.hadoop.hbase.regionserver.HStore.notifyChangedReadersObservers(HStore.java:1155)

PS: ignore the line no in the above stack trace, method calls should help understand whats happening.