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https://issues.apache.org/jira/browse/HBASE-16643?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15520629#comment-15520629
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Anoop Sam John commented on HBASE-16643:
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bq.It appears to me that you assume first action for forward scan is
seek/reseek() and first action for reverse scan is
backwardSeek/seekToPreviousRow(). This helps you to avoid using two heaps
variants.
Ya this is what Ram checked and confirmed.. I dont think any logic abt first
calling seek / backwardSeek has changed recently. This is the core of the read
flow and very rarely change. Ram pls double confirm.
> Reverse scanner heap creation may not allow MSLAB closure due to improper 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
> Fix For: 2.0.0
>
> Attachments: HBASE-16643.patch, HBASE-16643_1.patch,
> HBASE-16643_2.patch, HBASE-16643_3.patch, HBASE-16643_4.patch,
> HBASE-16643_5.patch, HBASE-16643_6.patch
>
>
> In the reverse scanner case,
> While doing 'initBackwardHeapIfNeeded' in MemstoreScanner for setting the
> backward heap, we do a
> {code}
> if ((backwardHeap == null) && (forwardHeap != null)) {
> forwardHeap.close();
> 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);
> }
> }
> {code}
> 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 because the segment could be put back to the MSLAB pool.
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