[NETVIRT-39] Synchronization block on Mac String that is created in local scope

Thu, 19 Oct 2017 21:28:08 +0000

In ElanPacketInHandler.onPacketReceived():
byte[] srcMac = res.getSourceMACAddress();
// You create a new String here
String macAddress = NWUtil.toStringMacAddress(srcMac);

// You call setupMacFlows with the new created String
ElanUtils.setupMacFlows(…., macAddress)

In ElanUtils.setupMacFlows():
synchronized (macAddress) {
...
}

The macAddress String that the synchronized block is using as a mutex was created locally. If the same mac value will be used in a different place or by a different thread, it will have its own reference, thus not locking anything.

A naive solution will be to use String.intern() but this is a very bad practice, as illustrated here: http://stackoverflow.com/questions/461896/what-is-the-most-frequent-concurrency-issue-youve-encountered-in-java/463437#463437.

My suggested solution is to hold a map between macAddress to Object() and synchronized on the Object.
Map will be created like this:
CacheBuilder.newBuilder().weakValues()
To obtain the lock, you will use:
Object lock = Cache.asMap().putIfAbsent(mac, new Object());

Because we are using weakValues(), there is no need to remove the entries from the map, as they will be garbage collected if there is no live reference to them.

OpenDaylight JIRA

[NETVIRT-39] Synchronization block on Mac String that is created in local scope