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From "David Rees (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DBCP-44) [dbcp] Evictor thread in GenericObjectPool has potential for deadlock
Date Fri, 29 Feb 2008 04:00:51 GMT

    [ https://issues.apache.org/jira/browse/DBCP-44?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12573582#action_12573582
] 

David Rees commented on DBCP-44:
--------------------------------

It's funny, I've been using DBCP for years without having run across this issue once, and
now I've had two separate applications run across the same deadlock within a week.

I have another deadlock which is identical to the one above, except that the thread closing
the connection is a Servlet running in Tomcat, but the cause is the same.

The Evictor thread runs, locks the GenericObjectPool, enters validateConnection and then attempts
to lock a Poolable Connection.
Another thread Locks a PoolableConection, and attempts to lock the GenericObjectPool.

I've been analyzing the deadlock and it appears that the initial suggested workaround of setting
minIdle to 0. In my previous comment, the application did have a minIdle setting of 1. However,
in my latest deadlock, minIdle was set to zero.

However, in both cases, the testWhileIdle is to true which appears to cause the Evictor to
run.

So to really avoid this issue, it appears that you should set minIdle to 0 AND turn off testWhileIdle.
Or is minIdle really enough (and I am missing something?)

> [dbcp] Evictor thread in GenericObjectPool has potential for deadlock
> ---------------------------------------------------------------------
>
>                 Key: DBCP-44
>                 URL: https://issues.apache.org/jira/browse/DBCP-44
>             Project: Commons Dbcp
>          Issue Type: Bug
>    Affects Versions: 1.2.1
>         Environment: Operating System: All
> Platform: All
>            Reporter: Eric Layton
>             Fix For: 1.3
>
>         Attachments: DBCP-44.patch, deadlock.txt, deadlock_post_patch.txt, testConcurrency.java
>
>
> The GenericObjectPool Evictor thread can potentially cause a deadlock between
> its connection factory and java.sql.DriverManager.  The deadlock occurs when the
> Evictor thread is trying to make enough connections to bring the pool's idle
> connections up to what's specified in minIdle, at the same time a connection is
> being requested through DriverManager.getConnection().  See the attached stack
> trace dump:
> Found one Java-level deadlock:
> =============================
> "Thread-0":
>   waiting to lock monitor 0x0809a994 (object 0x698c2b48, a java.lang.Class),
>   which is held by "main"
> "main":
>   waiting to lock monitor 0x0809aad4 (object 0x65df7030, a
> org.apache.commons.dbcp.PoolableConnectionFactory),
>   which is held by "Thread-0"
>  
> Java stack information for the threads listed above:
> ===================================================
> "Thread-0":
>         at java.sql.DriverManager.getConnection(DriverManager.java:158)
>         - waiting to lock <0x698c2b48> (a java.lang.Class)
>         at
> org.apache.commons.dbcp.DriverManagerConnectionFactory.createConnection(DriverManagerConnectionFactory.java:48)
>         at
> org.apache.commons.dbcp.PoolableConnectionFactory.makeObject(PoolableConnectionFactory.java:290)
>         - locked <0x65df7030> (a org.apache.commons.dbcp.PoolableConnectionFactory)
>         at
> org.apache.commons.pool.impl.GenericObjectPool.addObject(GenericObjectPool.java:996)
>         at
> org.apache.commons.pool.impl.GenericObjectPool.ensureMinIdle(GenericObjectPool.java:978)
>         at
> org.apache.commons.pool.impl.GenericObjectPool.access$000(GenericObjectPool.java:124)
>         at
> org.apache.commons.pool.impl.GenericObjectPool$Evictor.run(GenericObjectPool.java:1090)
>         at java.lang.Thread.run(Thread.java:595)
> "main":
>         at
> org.apache.commons.dbcp.PoolableConnectionFactory.makeObject(PoolableConnectionFactory.java:290)
>         - waiting to lock <0x65df7030> (a
> org.apache.commons.dbcp.PoolableConnectionFactory)
>         at
> org.apache.commons.pool.impl.GenericObjectPool.borrowObject(GenericObjectPool.java:771)
>         at org.apache.commons.dbcp.PoolingDriver.connect(PoolingDriver.java:175)
>         at java.sql.DriverManager.getConnection(DriverManager.java:525)
>         - locked <0x698c2b48> (a java.lang.Class)
>         at java.sql.DriverManager.getConnection(DriverManager.java:193)
>         - locked <0x698c2b48> (a java.lang.Class)
>         at Deadlock.main(Deadlock.java:83)
>  
> Found 1 deadlock.
> The deadlock occurs when GenericObjectPool.addObject() calls synchronized method
> PoolableConnectionFactory.makeObject(), meanwhile static synchronized
> DriverManager.getConnection() is called.  makeObject() waits for the lock on
> DriverManager to be released, whereas DriverManager waits for the lock on the
> PoolableConnectionFactory instance to be released.
> The Java program below, based on ManualPoolingDriverExample.java provided with
> DBCP, duplicates the deadlock for me within several seconds of being run:
> import java.sql.*;
> import org.apache.commons.pool.*;
> import org.apache.commons.pool.impl.*;
> import org.apache.commons.dbcp.*;
>  
> /**
>  * Duplicate DBCP pool deadlocking.
>  *
>  * Compile with:
>  * /usr/java/jdk1.5.0/bin/javac
>  * -classpath commons-collections.jar:commons-dbcp-1.2.1.jar:commons-pool-1.2.jar
>  * Deadlock.java
>  *
>  * Run with:
>  * /usr/java/jdk1.5.0/bin/java
>  * -classpath
> commons-collections.jar:commons-dbcp-1.2.1.jar:commons-pool-1.2.jar:ojdbc14.jar:xerces.jar:.
>  * Deadlock
>  *
>  * Locks still occur when compiled and run with J2SDK 1.4.1_03.
>  */
> public class Deadlock {
>  
>         private static final int ACTIVE = 10;
>  
>         private static void init() {
>                 System.out.println("Loading drivers");
>  
>                 try {
>                     Class.forName("oracle.jdbc.driver.OracleDriver");
>                         Class.forName("org.apache.commons.dbcp.PoolingDriver");
>                 } catch (ClassNotFoundException e) {
>                     e.printStackTrace();
>                 }
>  
>                 System.out.println("Setting up pool");
>  
>                 try {
>                         GenericObjectPool.Config config = new
> GenericObjectPool.Config();
>                         config.maxActive = ACTIVE;
>                         config.minIdle = 2; // Idle limits are low to allow more
> possibility of locking.
>                         config.maxIdle = 4; // Locking only occurs when there
> are 0 idle connections in the pool.
>                         config.maxWait = 5000L;
>                         config.testOnBorrow = true;
>                         config.testOnReturn = false;
>                         config.testWhileIdle = true;
>                                 // Locking still occurs whether these tests are
> performed or not.
>                         config.whenExhaustedAction =
> GenericObjectPool.WHEN_EXHAUSTED_BLOCK;
>                                 // Locking still occurs regardless of the
> exhausted action.
>                         config.timeBetweenEvictionRunsMillis = 3000L; // The
> Evictor thread is involved in the lock, so run it quite often.
>                         config.minEvictableIdleTimeMillis = 6000L;
>                         config.numTestsPerEvictionRun = 3;
>  
>                         ObjectPool op = new GenericObjectPool(null, config);
>  
>                         ConnectionFactory cf = new
> DriverManagerConnectionFactory("jdbc:oracle:thin:@oracle8server:1521:sid",
> "username", "password");
>                         PoolableConnectionFactory pcf = new
> PoolableConnectionFactory(cf, op, null, "SELECT 1 FROM DUAL", false, true);
>                                 // Locking still occurs whether there is a
> validation query or not.
>  
>                         PoolingDriver pd =
> (PoolingDriver)DriverManager.getDriver("jdbc:apache:commons:dbcp:");
>                         pd.registerPool("PoolName", op);
>  
>                 } catch (SQLException e) {
>                     e.printStackTrace();
>                 }
>  
>                 System.out.println("Done");
>         }
>  
>  
>         public static void main(String[] args) {
>                 init();
>  
>                 Connection[] c = new Connection[ACTIVE];
>  
>                 try {
>                         printPoolStatus();
>  
>                         // Loop forever to create a high load.
>                         while (true) {
>                                 // Create a number of connections.
>                                 for (int i = 0; i < ACTIVE; ++i) {
>                                         c[i] =
> DriverManager.getConnection("jdbc:apache:commons:dbcp:PoolName");
>                                         printPoolStatus();
>                                 }
>                                 // Then immmediately drop them.
>                                 for (int i = 0; i < ACTIVE; ++i) {
>                                         try {
>                                                 if (c[i] != null) {
>                                                         c[i].close();
>                                                         printPoolStatus();
>                                                         c[i] = null;
>                                                 }
>                                         } catch (SQLException e) {
>                                                 e.printStackTrace();
>                                         }
>                                 }
>                         }
>  
>                 } catch (SQLException e) {
>                         e.printStackTrace();
>  
>                 } finally {
>                         // Close down any open connetions.
>                         for (int i = 0; i < ACTIVE; ++i) {
>                                 try {
>                                         if (c[i] != null) c[i].close();
>                                 } catch (SQLException e) { }
>                         }
>  
>                         System.out.println("Closing pool");
>                         try {
>                                 PoolingDriver pd =
> (PoolingDriver)DriverManager.getDriver("jdbc:apache:commons:dbcp:");
>                                 pd.closePool("PoolName");
>                         } catch (SQLException e) {
>                                 e.printStackTrace();
>                         }
>                         System.out.println("Pool closed");
>                 }
>  
>         }
>  
>  
>         /**
>          * Display pool status.  Locks still occur even if this method is never
> called.
>          */
>         private static void printPoolStatus() throws SQLException {
>                 PoolingDriver pd =
> (PoolingDriver)DriverManager.getDriver("jdbc:apache:commons:dbcp:");
>                 ObjectPool op = pd.getConnectionPool("PoolName");
>  
>                 System.out.println("Active / idle: " + op.getNumActive() + " / "
> + op.getNumIdle());
>         }
>  
> }
> The patch I initially suggest is as follows (sorry for not providing a diff):
> In org.apache.commons.dbcp.PoolableConnectionFactory.java we have:
>     synchronized public Object makeObject() throws Exception {
>         Connection conn = _connFactory.createConnection();
>         if(null != _stmtPoolFactory) {
>             KeyedObjectPool stmtpool = _stmtPoolFactory.createPool();
>             conn = new PoolingConnection(conn,stmtpool);
>             stmtpool.setFactory((PoolingConnection)conn);
>         }
>         return new PoolableConnection(conn,_pool,_config);
>     }
> I suggest changing that to this:
>     public Object makeObject() throws Exception {
>         Connection conn = _connFactory.createConnection();
>         synchronized (this) {
>             if(null != _stmtPoolFactory) {
>                 KeyedObjectPool stmtpool = _stmtPoolFactory.createPool();
>                 conn = new PoolingConnection(conn,stmtpool);
>                 stmtpool.setFactory((PoolingConnection)conn);
>             }
>             return new PoolableConnection(conn,_pool,_config);
>         }
>     }
> Note the move of the synchronized block from the entire method to within the
> method after the connection (obtained ultimately by DriverManager) is retrieved.
> I'm afraid I don't know enough about DBCP and pooling to know the full
> ramification of this change and other calls to makeObject().

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