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From Daniel John Debrunner <...@apache.org>
Subject Re: [jira] Commented: (DERBY-2107) Move page latching out of the lock manager
Date Fri, 15 Dec 2006 15:33:41 GMT
Knut Anders Hatlen (JIRA) wrote:
>     [ http://issues.apache.org/jira/browse/DERBY-2107?page=comments#action_12458736 ]

>             
> Knut Anders Hatlen commented on DERBY-2107:
> -------------------------------------------
> 
> Thanks for your comments, Dan and Mike. It seems to me that all your
> comments are about the variants of LockingPolicy.lockRecordForRead and
> lockRecordForWrite that take a latch parameter. I share your concern
> for the correctness of this code, and that was actually what made me
> start the thread which is archived at
> <URL:http://thread.gmane.org/gmane.comp.apache.db.derby.devel/33135>.
> 
> But this brings us back to the original question in that thread: When
> is this code used? I believe it was concluded that those methods were
> only called when the locking policy was NoLocking, in which case they
> are no-ops. (With the exception of some unit tests which invoke the
> methods of the Page interface directly.)

I see:
    LockingPolicy.lockRecordForWrite(Latch ,...) used in one place in 
BasePage.
   LockingPolicy.lockRecordForWrite(Latch ,...) used in two places in 
BasePage.

 From a quick look, tracing back from those calls seems there are places 
where the code is used (excluding the test code) including non-btree code.

If the code is not required then yes we should just remove it, however 
I'm pretty sure that we (as closed source project) didn't originally add 
these calls for fun. I thought there was a definite requirement for 
releasing a latch while waiting for a lock and I don't believe anything 
has changed since then. I was surprised when looking yesterday that they 
didn't seem to be called (obviously) from the non-btree case, but I 
didn't do a complete search.

The case I'm thinking of is for a heap page from a heap scan:

    get heap page
    identify row that needs locking
    lock row
    process row
    release page

I would have thought that the lock page in this case must release the 
latch if it has to wait for the row lock.

I'll go back and re-read that thread and also look at the code and try 
and understand more about if these calls are required.

Thanks,
Dan.





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