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https://issues.apache.org/jira/browse/DERBY-4080?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Knut Anders Hatlen updated DERBY-4080:
--------------------------------------
Attachment: repro.sql
Here's a script that exposes the bug (seen in 10.4.2.1 - (706043) and 10.5.0.0
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: 10.4.2.0
> 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
lock).
> 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 out.
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