ramkrishna.s.vasudevan created HBASE-16643:

             Summary: Reverse scanner heap creation may not allow MSLAB closure 
due to inproper ref counting of segments
                 Key: HBASE-16643
                 URL: https://issues.apache.org/jira/browse/HBASE-16643
             Project: HBase
          Issue Type: Bug
            Reporter: ramkrishna.s.vasudevan
            Assignee: ramkrishna.s.vasudevan
            Priority: Critical

In the reverse scanner case,
While doing 'initBackwardHeapIfNeeded' in MemstoreScanner for setting the 
backward heap, we do a 
if ((backwardHeap == null) && (forwardHeap != null)) {
        forwardHeap = null;
        // before building the heap seek for the relevant key on the scanners,
        // for the heap to be built from the scanners correctly
        for (KeyValueScanner scan : scanners) {
          if (toLast) {
            res |= scan.seekToLastRow();
          } else {
            res |= scan.backwardSeek(cell);
forwardHeap.close(). This would internally decrement the MSLAB ref counter for 
the current active segment and snapshot segment.
When the scan is actually closed again we do close() and that will again 
decrement the count. Here chances are there that the count would go negative 
and hence the actual MSLAB closure that checks for refCount==0 will fail. Apart 
from this, when the refCount becomes 0 after the firstClose if any other thread 
requests to close the segment, then we will end up in corrupted segment.

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