Theads僵局问题

Question

Env:

  Wildfly 8.1
  Struts
  EJB 3.1
  Postgres 9

Facing a thread deadlock issue. Hoping to get some clarity.

Code:

• An action class calls two updates using an EJB

  bean.writeLockedUpdate(o1) #1 ... bean.writeLockedUpdate(o1) #2 

• EJB bean's writeLockedUpdate(Object) does plain JDBC update via JPA

Scenario:

• UserRequestThread1 starts and completed #1 and doing some work within action class
• UserRequestThread2 started and acquired write lock on the bean and is inside bean.writeLockedUpdate(o1).

Both user threads trying to modify same o1 object.

Thread dump snip for those two threads:

Thread1:

"default task-56" #558 prio=5 os_prio=0 tid=0x724a4400 nid=0x1674 waiting on condition [0x3d2ef000] java.lang.Thread.State: TIMED_WAITING (parking)
at org.jboss.as.ejb3.concurrency.EJBReadWriteLock$WriteLock.tryLock(EJBReadWriteLock.java:209)

at Bean$$$view22.writeLockedUpdate(Unknown Source)

Thread2:

"default task-43" #530 prio=5 os_prio=0 tid=0x6bf10800 nid=0x15aa runnable [0x299c8000] java.lang.Thread.State: RUNNABLE at java.net.SocketInputStream.socketRead0(Native Method)
at org.postgresql.core.VisibleBufferedInputStream.readMore(VisibleBufferedInputStream.java:143)

at Bean$$$view22.writeLockedUpdate(Unknown Source)

Possible explanation:

I believe here is what happening.

1. thread2 having acquired bean write lock, is wating for DB row lock
2. thread1 having acquired DB row lock, is waiting for bean write lock

Jconsole doesnt show any deadlock. Guessing its because the second lock being blocked is at DB level

Queation:

Is there a way to verify the above explanation ?

Thanks in advance.

Answer 1

Got the confirmation from both Java and postgresql DB.

1. The thread dump clearly shows that thread1 is waiting for the EJB write lock

2. Following query on postgres reveals the operations that are blocked

select bl.pid as blocked_pid, bl.mode as blocked_mode, a.usename as blocked_user, ka.query as blocking_statement, now() - ka.query_start as blocking_duration, kl.pid as blocking_pid, kl.mode as blocking_mode, ka.usename as blocking_user, a.query as blocked_statement, now() - a.query_start as blocked_duration from pg_catalog.pg_locks bl join pg_catalog.pg_stat_activity a on bl.pid = a.pid join pg_catalog.pg_locks kl join pg_catalog.pg_stat_activity ka on kl.pid = ka.pid on bl.transactionid = kl.transactionid and bl.pid != kl.pid where not bl.granted

In my case the above query returns one row that shows that update is blocked

Thanks, Rakesh