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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Updated: (DERBY-3909) Race condition in NetXAResource.removeXaresFromSameRMchain()
Date Sat, 11 Oct 2008 23:34:44 GMT

     [ https://issues.apache.org/jira/browse/DERBY-3909?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Knut Anders Hatlen updated DERBY-3909:
--------------------------------------

    Attachment: d3909.diff

Here's a patch that attempts to fix the problem by checking the identity instead of the field
ignoreMe_. I haven't found any way to test it, since I don't have a test case that exposes
the problem, but I believe that the patch is an improvement. Derbyall and suites.All ran cleanly
with the patch. Reviews would be appreciated.

> Race condition in NetXAResource.removeXaresFromSameRMchain()
> ------------------------------------------------------------
>
>                 Key: DERBY-3909
>                 URL: https://issues.apache.org/jira/browse/DERBY-3909
>             Project: Derby
>          Issue Type: Bug
>          Components: Network Client
>    Affects Versions: 10.5.0.0
>            Reporter: Knut Anders Hatlen
>            Assignee: Knut Anders Hatlen
>         Attachments: d3909.diff
>
>
> NetXAResource.removeXaresFromSameRMchain() does the following to remove a NetXAResource
from what's logically a singly-linked list:
> 1) Mark the NetXAResource to remove with a flag (a field called ignoreMe_)
> 2) Synchronize on an object that protects the linked list
> 3) Follow the chain of next pointers in the linked list and remove the first flagged
object
> 4) Release synchronization lock obtained in (2)
> 5) Clear the flag set in (1)
> Now, say that two threads (T1 and T2) perform step 1 in parallel. T1 is granted the synchronization
lock in (2), and T2 must wait. T1 traverses the linked list, finds the object flagged by T2
and removes it. Further T1 releases the synchronization lock and clears the flag on the object
it had flagged. This is not the same object that it removed, so when T2 is granted the synchronization
lock, there is no object flagged for removal. As a result, only the object T2 attempted to
remove was in fact removed. The object that T1 flagged for removal is still in the linked
list.

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