Microsoft KB Archive/247871

= FIX: DTC commit/abort operation may leave suspended worker thread with waittype 0x0201 =

Article ID: 247871

Article Last Modified on 1/10/2006

-

APPLIES TO


 * Microsoft SQL Server 7.0 Standard Edition

-



This article was previously published under Q247871



BUG #: 57127 (SQLBUG_70)



SYMPTOMS
In the process of completing a Microsoft Distributed Transaction Coordinator (DTC) transaction abort or commit operation the SQL Server may experience a race condition between two or more server process ids (SPIDs) enlisted in the same DTC transaction.

The race condition occurs after the proper abort or commit of the DTC transaction.

The database maintains the proper and consistent state but one or more of the SPIDs involved in the original DTC transaction can maintain the waittype of 0x0201 (XCB).



CAUSE
SQL Server is not properly protecting the waiting status flag in the shared transaction control block (XCB) between multiple worker threads running on separate processors. This leads to a race condition where the wait flag may not appear to be set by waiting SPIDs.

The race condition is such that the SPID completing the DTC abort or commit operation successfully handles transactional duties but does not signal the waiters.



WORKAROUND
Use a simple T-SQL script to check sysprocesses for SPIDs with a waittype of 0x0201 and excessive wait times, for example > 30 seconds.

select * from sysprocesses where waittime > 30000 and waittype = 0x201 When located a simple KILL statement terminates the SPID and returns the worker thread for use by other SPIDs.

NOTE: KILL with the abort or commit option is not necessary when dealing with this issue.



STATUS
Microsoft has confirmed this to be a problem in SQL Server 7.0. This problem has been corrected in U.S. Service Pack 3 for Microsoft SQL Server 7.0. For more information about how to download and install the latest SQL Server service pack, click the following article number to view the article in the Microsoft Knowledge Base:

274799 How to obtain Service Pack 3 for Microsoft SQL Server 7.0 and Microsoft Data Engine (MSDE) 1.0

For more information, contact your primary support provider.



MORE INFORMATION
The condition requires two or more SPIDs to be enlisted in the same DTC transaction. It is extremely timing specific, requiring the worker threads associated with the DTC SPIDs, to run on separate CPUs at the same time. Furthermore, due to timing constraints, it is highly difficult to encounter the problem if the number of SPIDs enlisted in the same DTC transaction exceed maximum CPUs in use by SQL Server.

The SPIDs maintain the waittype of 0x0201 and the wait time continues to climb. The SPID is placed in a wait, 'sleeping', state by SQL Server and the worker thread is never woken.

When the SPID is in the wait state it should not hold locks or block other SPIDs on the SQL Server. It only monopolizes the assigned worker thread but takes no CPU cycles.

Keywords: kbbug kbfix kbqfe KB247871

-

[mailto:TECHNET@MICROSOFT.COM Send feedback to Microsoft]

© Microsoft Corporation. All rights reserved.