HBASE-10087 Store should be locked during a memstore snapshot

git-svn-id: https://svn.apache.org/repos/asf/hbase/trunk@1560018 13f79535-47bb-0310-9956-ffa450edef68

hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/HStore.java

@@ -1921,6 +1921,10 @@ public class HStore implements Store {
       this.cacheFlushSeqNum = cacheFlushSeqNum;
     }
 
+    /**
+     * This is not thread safe. The caller should have a lock on the region or the store.
+     * If necessary, the lock can be added with the patch provided in HBASE-10087
+     */
     @Override
     public void prepare() {
       memstore.snapshot();