jmx-dev [PATCH] JDK-7009998: JMX synchronization during connection restart is faulty

[Jaroslav Bachorik]

I am looking for a review and patch sponsor.

Webrev available at
http://cr.openjdk.java.net/~jbachorik/JDK-7009998/webrev.00

The issue is about a possible race condition in the
ClientCommunicatorAdmin when the reconnection process may be initiated
by more than one thread (eg. 3). The main reason is that the
re-connection routine logic is split into two synchronized blocks and it
relies on the state staying consistent when transiting from the one
synchronized block to the other.

The race condition is described by the reporter as:
"In reading the code there is a scenario where the synchronization does
the wrong thing if 3 threads attempt to go through the code at the same
time. Consider the code in
com.sun.jmx.remote.internal.ClientCommunicatorAdmin.restart, the first
thread will set the state to RE_CONNECTING and then leave the
sychronization block, the second thread will find the state to be
RE_CONNECTING and wait on the lock. When thread 1 finishes and sets the
state to CONNECTED, then thread 2 can leave the synchronization block -
but fails to set the state to RE_CONNECTING because that code is
incorrectly in the else branch. Thus thread 2 starts the reconnecting
and thread 3 wakes to find the state not RE_CONNECTING so it believes it
can safely start the reconnect and it also starts reconnecting. The bad
mode is discovered in the preReconnection method."

The fix is adding a return statement at the end of the first
synchronized block in case when the admin has been successfully
re-connected by the other thread.

Test in "test/com/sun/jmx/remote/CCAdminReconnectTest.java" tests the
fix. Changes in "make/netbeans/jmx/build.properties" are there for the
NetBeans project to recognize the newly added test.


Thanks,

-JB-