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From "Jeff Stuckman (JIRA)" <j...@apache.org>
Subject [jira] Created: (DERBY-3961) Deadlock detection fails for InternalTransaction
Date Mon, 24 Nov 2008 05:07:44 GMT
Deadlock detection fails for InternalTransaction
------------------------------------------------

                 Key: DERBY-3961
                 URL: https://issues.apache.org/jira/browse/DERBY-3961
             Project: Derby
          Issue Type: Bug
    Affects Versions: 10.4.2.0
         Environment: Windows Vista
            Reporter: Jeff Stuckman


It is easy to cause a deadlock which is not detected by the deadlock detection algorithm.
The transactions fail due to a lock timeout , possibly because a transaction of type InternalTransaction
is part of the cycle.

Resolving issue DERBY-2991 will make it more difficult to cause such deadlocks, but it will
still be possible.

My test case creates two threads and executes the following statements until they deadlock
against each other:
UPDATE urls SET jobflag=? WHERE urlid=?	
SELECT urlid,url,expectation FROM urls WHERE site=?

The test eventually deadlocks with the following transaction and lock table contents:
XID     TYPE  MODE TABLENAME LOCKNAME  STATE TABLETYPE  LOCKCOUNT  INDEXNAME
2217109 ROW   S    URLS      (13,1)    GRANT T          1 FINDURLBYSITEANDJOB
2217114 ROW   X    URLS      (13,1)    WAIT  T          0 FINDURLBYSITEANDJOB
2217113 ROW   S    URLS      (15,1)    GRANT T          1 FINDURLBYSITEANDJOB
2217113 ROW   X    URLS      (3,132)   GRANT T          3          null
2217109 ROW   S    URLS      (3,132)   WAIT  T          0          null
2217109 TABLE IS   URLS      Tablelock GRANT T          2          null
2217113 TABLE IX   URLS      Tablelock GRANT T          4          null
2217114 TABLE IX   URLS      Tablelock GRANT T          1          null
2217113 ROW   S    URLS      (6,1)     GRANT T          1 SQL081111021116970

XID     GLOBAL_XID  USERNAME TYPE                 STATUS  FIRST_INSTANT SQL_TEXT
2217115 null        APP      UserTransaction      IDLE    null select * from SYSCS_DIAG.TRANSACTION_TABLE
2217114 null        APP      InternalTransaction  ACTIVE  null UPDATE urls SET jobflag=? WHERE
urlid=?
2217113 null        APP      UserTransaction      ACTIVE  (526,52925) UPDATE urls SET jobflag=?
WHERE urlid=?
2069160 null        null     SystemTransaction    IDLE    null          null
2217109 null        APP      UserTransaction      ACTIVE  null SELECT urlid,url,expectation
FROM urls WHERE site=?

Here is what I think is happening:
1. The SELECT statement begins to execute and the cursor is stepping through the result set.
The results are derived from index FINDURLBYSITEANDJOB as expected.
2. The UPDATE statement begins to execute. The row to be updated is the row immediately after
the SELECT statement's cursor. The row is locked and updated.
3. The UPDATE statement must perform index maintenance (tree rebalancing or similar?). This
apparently causes an InternalTransaction to be created. It then must lock the row that the
SELECT statement's cursor is currently occupying. It cannot do this, so the transaction waits.
4. The SELECT statement is ready to advance the cursor. However, it cannot advance the cursor
because the UPDATE statement has locked the next row. The transaction waits.
The result: Transaction 2217113 waits for the "nested transaction" 2217114 to complete. 2217114
waits for 2217109 to release its lock. 2217109 waits for 2217113 to release its lock. We have
a cycle and a deadlock. The transactions time out with "A lock could not be obtained within
the time requested", apparently because the dependency between transactions 2217113 and 2217114
is not detected.


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