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From "Daniel John Debrunner (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-2107) Move page latching out of the lock manager
Date Thu, 14 Dec 2006 21:33:22 GMT
    [ http://issues.apache.org/jira/browse/DERBY-2107?page=comments#action_12458609 ] 
Daniel John Debrunner commented on DERBY-2107:

I think the patch has a severe problem, here is code from RowLocking3.lockRecordForRead (one
of several instances):

        // Couldn't get the lock immediately. Release the latch while waiting.

            t.getCompatibilitySpace(), t, record, qualifier,


The api for Page.unlatch() states that the reference used to unlatch the page must be not
used (i.e. must be set to null) after calling unlatch().
This code uses the reference used to unlatch the page after the lock call *and* the caller
of this method will continue to use the same Page reference.
The reference must be set to null after an unlatch because once the page in unlatched there
is no guarantee that the page will remain in cache or that the reference will continue to
describe the same page.

So this code clearly uses a Page reference after it has been unlatched and thus can be referring
to a wrong page or an invlaid page.

I think it works with the current code because a a latch on the Page is a super-set of a lock
manager Latch, I'll add more on that in a separate comment.

> Move page latching out of the lock manager
> ------------------------------------------
>                 Key: DERBY-2107
>                 URL: http://issues.apache.org/jira/browse/DERBY-2107
>             Project: Derby
>          Issue Type: Improvement
>          Components: Store, Services, Performance
>    Affects Versions:
>            Reporter: Knut Anders Hatlen
>         Assigned To: Knut Anders Hatlen
>            Priority: Minor
>         Attachments: derby-2107-1a.diff, derby-2107-1a.stat, derby-2107-1b.diff
> Latching of pages could be done more efficiently locally in store than in the lock manager.
See the discussion here: http://thread.gmane.org/gmane.comp.apache.db.derby.devel/33135

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