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https://issues.apache.org/jira/browse/DERBY-2991?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12660782#action_12660782
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Knut Anders Hatlen commented on DERBY-2991:
-------------------------------------------
I have taken another look at the previous key locking. If the latches
are released when we're searching backward for the previous key, it
seems like we'll rescan the tree, so it doesn't seem like we're
depending on the scan lock. Some relevant code fragments and comments:
* BTreeController.doIns():
while (true)
{
.
.
.
// Row locking - first lock row previous to row being inserted:
.
.
.
if (latch_released)
{
// Had to release latch in order to get the lock, probably
// because of a forward scanner, research tree, and try again.
targetleaf = null;
continue;
}
* B2IRowLocking3._lockScanRow():
if (pos.current_slot == 0)
{
// this call will take care of searching left in the btree
// to find the previous row to lock, 0 is the control row and
// not a valid thing to lock as a previous key.
// it is ok to call the non-scan as this is just a special
// case of a previous key lock call. The only scan code that
// will call this routine with slot == 0 will retry if this
// routine returns that a latch was released.
latch_released =
!lockNonScanPreviousRow(
* B2IRowLocking3.lockNonScanPreviousRow():
// RESOLVE RLL (mikem) - NO RECORD_ID PROTECTION IN EFFECT.
// caller must research, get new locks if this routine
// releases latches.
ret_status = this.searchLeftAndLockPreviousKey(
* B2IRowLocking3.searchLeftAndLockPreviousKey():
* If along the search a latch has to be waited on then latches are
* released and a wait is performed, and "false" status is returned to
* caller. In this case the routine can no longer be sure of it's current
* position and may have to retry the whole operation.
> Index split deadlock
> --------------------
>
> Key: DERBY-2991
> URL: https://issues.apache.org/jira/browse/DERBY-2991
> Project: Derby
> Issue Type: Bug
> Components: Store
> Affects Versions: 10.2.2.0, 10.3.1.4
> Environment: Windows XP, Java 6
> Reporter: Bogdan Calmac
> Assignee: Knut Anders Hatlen
> Attachments: d2991-preview-1a.diff, d2991-preview-1a.stat,
> d2991-preview-1b.diff, d2991-preview-1b.stat, d2991-preview-1c.diff,
> d2991-preview-1c.stat, d2991-preview-1d.diff, d2991-preview-1d.stat,
> derby.log, InsertSelectDeadlock.java, perftest.diff, Repro2991.java,
> stacktraces_during_deadlock.txt
>
>
> After doing dome research on the mailing list, it appears that the index
> split deadlock is a known behaviour, so I will start by describing the
> theoretical problem first and then follow with the details of my test case.
> If you have concurrent select and insert transactions on the same table, the
> observed locking behaviour is as follows:
> - the select transaction acquires an S lock on the root block of the index
> and then waits for an S lock on some uncommitted row of the insert transaction
> - the insert transaction acquires X locks on the inserted records and if it
> needs to do an index split creates a sub-transaction that tries to acquire an
> X lock on the root block of the index
> In summary: INDEX LOCK followed by ROW LOCK + ROW LOCK followed by INDEX LOCK
> = deadlock
> In the case of my project this is an important issue (lack of concurrency
> after being forced to use table level locking) and I would like to contribute
> to the project and fix this issue (if possible). I was wondering if someone
> that knows the code can give me a few pointers on the implications of this
> issue:
> - Is this a limitation of the top-down algorithm used?
> - Would fixing it require to use a bottom up algorithm for better
> concurrency (which is certainly non trivial)?
> - Trying to break the circular locking above, I would first question why
> does the select transaction need to acquire (and hold) a lock on the root
> block of the index. Would it be possible to ensure the consistency of the
> select without locking the index?
> -----
> The attached test (InsertSelectDeadlock.java) tries to simulate a typical
> data collection application, it consists of:
> - an insert thread that inserts records in batch
> - a select thread that 'processes' the records inserted by the other thread:
> 'select * from table where id > ?'
> The derby log provides detail about the deadlock trace and
> stacktraces_during_deadlock.txt shows that the inser thread is doing an index
> split.
> The test was run on 10.2.2.0 and 10.3.1.4 with identical behaviour.
> Thanks,
> Bogdan Calmac.
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