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Posted to issues@hbase.apache.org by "chenglei (Jira)" <ji...@apache.org> on 2021/11/25 12:12:00 UTC
[jira] [Commented] (HBASE-26465) MemStoreLAB may be released early when its SegmentScanner is scanning
[ https://issues.apache.org/jira/browse/HBASE-26465?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17449155#comment-17449155 ]
chenglei commented on HBASE-26465:
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[~zhangduo],would you please help me have a review at your free time? Thank you very much.
> MemStoreLAB may be released early when its SegmentScanner is scanning
> ---------------------------------------------------------------------
>
> Key: HBASE-26465
> URL: https://issues.apache.org/jira/browse/HBASE-26465
> Project: HBase
> Issue Type: Bug
> Affects Versions: 2.5.0, 3.0.0-alpha-2
> Reporter: chenglei
> Assignee: chenglei
> Priority: Critical
> Fix For: 2.5.0, 3.0.0-alpha-2
>
>
> HBASE-26144 moved {{MemStore.clearSnapshot}} out of write lock of {{HStore#lock}}, but because {{MemStore.clearSnapshot}} closed {{DefaultMemStore#snapshot}} which may be used by {{DefaultMemStore#getScanners}}, when {{DefaultMemStore#getScanners}} and {{MemStore}} flushing execute conurrently, {{MemStoreLAB}} used by {{DefaultMemStore#snapshot}} may be released early when its {{SegmentScanner}} is scanning.
> Considering follow thread sequences:
> 1.The flush thread starts {{DefaultMemStore}} flushing after some cells have be added to {{DefaultMemStore}}.
> 2.The flush thread stopping before {{DefaultMemStore#clearSnapshot}} in following line 1238 in
> {{HStore#updateStorefiles}} after completed flushing memStore to hfile.
> {code:java}
> 1221 private boolean updateStorefiles(List<HStoreFile> sfs, long snapshotId) throws IOException {
> 1222 this.lock.writeLock().lock();
> 1223 try {
> 1224 this.storeEngine.getStoreFileManager().insertNewFiles(sfs);
> 1225 } finally {
> 1226 // We need the lock, as long as we are updating the storeFiles
> 1227 // or changing the memstore. Let us release it before calling
> 1228 // notifyChangeReadersObservers. See HBASE-4485 for a possible
> 1229 // deadlock scenario that could have happened if continue to hold
> 1230 // the lock.
> 1231 this.lock.writeLock().unlock();
> 1232 }
> 1233 // We do not need to call clearSnapshot method inside the write lock.
> 1234 // The clearSnapshot itself is thread safe, which can be called at the same time with other
> 1235 // memstore operations expect snapshot and clearSnapshot. And for these two methods, in HRegion
> 1236 // we can guarantee that there is only one onging flush, so they will be no race.
> 1237 if (snapshotId > 0) {
> 1238 this.memstore.clearSnapshot(snapshotId);
> 1239 }
> ......
> {code}
> 3.The scan thread starts and stopping after {{DefaultMemStore.snapshot.getAllSegments}} in following line 141 in
> {{DefaultMemStore#getScanners}},here the scan thread gets the {{DefaultMemStore#snapshot}} which is created by the flush thread.
> {code:java}
> 138 public List<KeyValueScanner> getScanners(long readPt) throws IOException {
> 139 List<KeyValueScanner> list = new ArrayList<>();
> 140 addToScanners(getActive(), readPt, list);
> 141 addToScanners(snapshot.getAllSegments(), readPt, list);
> 142 return list;
> 143 }
> {code}
> 4.The flush thread continues {{DefaultMemStore#clearSnapshot}} and close {{DefaultMemStore#snapshot}} in following line 253,because the reference count of the corresponding {{MemStoreLABImpl}} is decreased to 0, the Chunks in corresponding {{MemStoreLABImpl}} are recycled.
> {code:java}
> 240 public void clearSnapshot(long id) throws UnexpectedStateException {
> ......
> 248 Segment oldSnapshot = this.snapshot;
> 249 if (!this.snapshot.isEmpty()) {
> 250 this.snapshot = SegmentFactory.instance().createImmutableSegment(this.comparator);
> 251 }
> 252 this.snapshotId = NO_SNAPSHOT_ID;
> 253 oldSnapshot.close();
> 254 }
> {code}
> 5.The scan thread continues {{DefaultMemStore#getScanners}}, and create a {{SegmentScanner}} for this {{DefaultMemStore#snapshot}}, and invokes {{MemStoreLABImpl.incScannerCount}} in line 281 to increase the reference count , but here {{MemStoreLABImpl.incScannerCount}} does not check that the reference count is already 0 and {{Chunks}} are already recycled by step 4, so these {{Chunks}} may be overwritten by other write threads when the {{SegmentScanner}} is scanning, which may cause serious problem.
> {code:java}
> 280 public void incScannerCount() {
> 281 this.openScannerCount.incrementAndGet();
> 282 }
> {code}
> I simulate above case in my PR, and my Fix is as following:
> 1.Moving {{MemStore.clearSnapshot}} back under write lock of {{HStore#lock}}, because {{DefaultMemStore#getScanners}} is protected under the read lock of {{HStore#lock}}, so {{DefaultMemStore#getScanners}} and
> {{DefaultMemStore#clearSnapshot}} could not execute concurrently.
> 2. Introducing {{RefCnt}} into {{MemStoreLABImpl}} to replace its flawed reference count implementation, so checking and increasing or decreasing is done in atomicity and if there is illegal state in reference count, it would throw exception rather than using corrupt data, but this modification is not included in PR now because I find that there is another bug in flushing would disrupt the reference count in {{MemStoreLABImpl}} , I would Introduce {{RefCnt}} after fixing this another bug.
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