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From Mike Matrigali <mikem_...@sbcglobal.net>
Subject Re: [jira] Commented: (DERBY-145) RAFContainer readPage method is not thread safe
Date Mon, 14 Feb 2005 18:55:32 GMT
I agree with dan (and from your reply below - it looks like you
also agree).  The synchronization looks right to me.

When reading the raw store page code, remember that most of the
synchonization at the page level is by getting a latch on the
page for the duration of the page operation - thus you won't see
a lot of sync blocks.  As dan points out the sync blocks you
see are going to be protecting one page from another page while
reading from the file shared by both.

Could you mark the jira entry not a bug (or whatever the jira equivalent
is) - or let me know if you
don't have permission to do so.

/mikem

RPost wrote:
> You are correct. Brain fart! Fix - new supply of Beano.
> ----- Original Message -----
> From: "Daniel John Debrunner (JIRA)" <derby-dev@db.apache.org>
> To: <rp0428@pacbell.net>
> Sent: Saturday, February 12, 2005 9:46 PM
> Subject: [jira] Commented: (DERBY-145) RAFContainer readPage method is not
> thread safe
> 
> 
> 
>>     [
> 
> http://issues.apache.org/jira/browse/DERBY-145?page=comments#action_59105 ]
> 
>>Daniel John Debrunner commented on DERBY-145:
>>---------------------------------------------
>>
>>readPage() is correctly thread safe. The calculation of pageOffset is a
> 
> local variable and references a local variable pageNumber and a constant
> pageSize (for the lifetime of the object), thus it does not need to be
> synchronized. The synchronized block is there to prevent multiple callers of
> readPage from modifying the file descriptor fileData's position used to read
> the page from file. I.e if multiple callers were allowed into that small
> synchronized section then the offset could change between setting it and
> reading the data, leading to the wrong page being read in. Comments could
> probably be added to the code.
> 
>>>RAFContainer readPage method is not thread safe
>>>-----------------------------------------------
>>>
>>>         Key: DERBY-145
>>>         URL: http://issues.apache.org/jira/browse/DERBY-145
>>>     Project: Derby
>>>        Type: Bug
>>>  Components: Store
>>>    Versions: 10.0.2.1
>>> Environment: N/A
>>>    Reporter: Rick Post
>>>    Priority: Minor
>>
>>>readPage method comment says 'thread safe and has a synchronized block.
>>>But 'pageOffset' computation occurs outside the sync block as does
> 
> decryption.
> 
>>>This allows the (remote?) possibility of part of the operation(s) being
> 
> performed using the wrong pageNumber or pageData.
> 
>>>Fix - synchronize the method rather than the block
>>>/**
>>>Read a page into the supplied array.
>>><BR> MT - thread safe
>>>@exception IOException exception reading page
>>>@exception StandardException Standard Cloudscape error policy
>>>*/
>>>protected void readPage(long pageNumber, byte[] pageData)
>>>throws IOException, StandardException
>>>{
>>>if (SanityManager.DEBUG) {
>>>SanityManager.ASSERT(!getCommittedDropState());
>>>}
>>>long pageOffset = pageNumber * pageSize;
>>>synchronized (this) {
>>>fileData.seek(pageOffset);
>>>fileData.readFully(pageData, 0, pageSize);
>>>}
>>>if (dataFactory.databaseEncrypted() &&
>>>pageNumber != FIRST_ALLOC_PAGE_NUMBER)
>>>{
>>>decryptPage(pageData, pageSize);
>>>}
>>>}
>>
>>--
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>>
> 
> 
> 
> 


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