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From Mike Matrigali <mikem_...@sbcglobal.net>
Subject Re: [jira] Updated: (DERBY-4080) Possible deadlock between locks and latches in BTreeController.compareRowsForInsert()
Date Wed, 04 Mar 2009 17:29:58 GMT
You seem to have found a number of problems with the new code that tries 
to implement the unique constraint with multiple nulls.  I definitely 
missed the deleted row case when reviewing that code, and had assumed it
just ever needed to search one row left and right.

I am now wondering if it would be better to make this code work the same
as the existing code in the regular unique case.  That code makes sure
there is always only ONE row with the given key (not including the row
id).  It manipulates the search params so that either that exact row
is found or it positions in the right place to do the insert.

It does have some tricky code for when it finds a matching row, 
especially if it is marked deleted, but that code has been working for
a long time now.

The approach would be something like:
o If there is a null in the key then just do the insert normally into 
the non-unique table, using all keys.
o If no null, then need to change the search params to only use leading
columns and to not allow exact match - this means changing the existing 
code which would use the
tables information which would say to use all columns.
o and then if match use the existing code for unique tables in this case.

Let me know what you think.  Is this a better approach or should we
patch up the new code?  Are you interested in doing this?  If not I
may have time to do this.


Knut Anders Hatlen (JIRA) wrote:
>      [ 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
> "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
>> 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 de
>  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

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