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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Updated: (DERBY-4080) Possible deadlock between locks and latches in BTreeController.compareRowsForInsert()
Date Wed, 04 Mar 2009 11:19:56 GMT

     [ https://issues.apache.org/jira/browse/DERBY-4080?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Knut Anders Hatlen updated DERBY-4080:

    Attachment: repro.sql

Here's a script that exposes the bug (seen in - (706043) and alpha - (749659)).

The script inserts a row at the end of one index page, with an uncommitted deleted duplicate
at the beginning of the next index page. The insert must wait for the uncommitted delete to
be committed. The transaction that holds the exclusive lock on the deleted duplicate then
tries to read a row on the previous index page, but cannot obtain the lock until the insert
operation times out.

A thread dump during the hang shows that this is a deadlock involving both locks and latches:

"Thread-2" prio=3 tid=0x08473800 nid=0x12 in Object.wait() [0xb60de000..0xb60debe0]
   java.lang.Thread.State: TIMED_WAITING (on object monitor)
	at java.lang.Object.wait(Native Method)
	- waiting on <0xf3c06298> (a org.apache.derby.impl.services.locks.ActiveLock)
	at org.apache.derby.impl.services.locks.ActiveLock.waitForGrant(Unknown Source)
	- locked <0xf3c06298> (a org.apache.derby.impl.services.locks.ActiveLock)
	at org.apache.derby.impl.services.locks.ConcurrentLockSet.lockObject(Unknown Source)
"main" prio=3 tid=0x0806f400 nid=0x2 in Object.wait() [0xfe34e000..0xfe34ed38]
   java.lang.Thread.State: WAITING (on object monitor)
	at java.lang.Object.wait(Native Method)
	- waiting on <0xf4235668> (a org.apache.derby.impl.store.raw.data.StoredPage)
	at java.lang.Object.wait(Object.java:485)
	at org.apache.derby.impl.store.raw.data.BasePage.setExclusive(Unknown Source)
	- locked <0xf4235668> (a org.apache.derby.impl.store.raw.data.StoredPage)
	at org.apache.derby.impl.store.raw.data.BaseContainer.latchPage(Unknown Source)

If the transaction that waits for the row lock had released all latches once it detected that
it would have to wait, there would not be a deadlock and both transactions would be able to
complete successfully. Once it has obtained the lock, it will release the latch on the page
that it "forgot" to unlatch and perform a rescan, so I would believe that it is fine to release
the latch earlier in this case.

> Possible deadlock between locks and latches in BTreeController.compareRowsForInsert()
> -------------------------------------------------------------------------------------
>                 Key: DERBY-4080
>                 URL: https://issues.apache.org/jira/browse/DERBY-4080
>             Project: Derby
>          Issue Type: Bug
>          Components: Store
>    Affects Versions:
>            Reporter: Knut Anders Hatlen
>         Attachments: repro.sql
> It looks like BTreeController.compareRowsForInsert(), which is used to check for duplicates
in a unique nullable index, can run into a deadlock which involves both locks and latches.
> Here's what I think can happen:
> comparePreviousRecord() (or compareNextRecord()) holds a latch on the index page where
a new row is about to be inserted, and needs to check if there's a duplicate on one of the
adjacent rows. Because the row is near a page boundary, this check moves to another index
page, while still holding the latch on the original index page. Then compareRowsForInsert()
is called, which tries to get an update lock on the existing row. If it has to wait for the
update lock, the latch on the current page is released, but the latch on the original index
page is kept. This means that the transaction is holding a latch while it is waiting for a
lock, which means that it is blocking all access to that page until it has been granted the
lock. If some other transaction that is holding a conflicting lock on the row later needs
to latch the index page, those two transactions run into a deadlock and the one that's waiting
for the lock will eventually time out (but it will not be reported as a dead
> If compareRowsForInsert() releases all latches when it needs to wait for a lock, the
deadlock is prevented, and both of the transactions may be able to complete without timing

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