Re: Question whether a problem with race conditions exists in this case

Eric Sosman <esosman@ieee-dot-org.invalid>
Wed, 14 Dec 2011 16:26:16 -0500
On 12/14/2011 12:07 PM, Saxo wrote:

I have a class Node as displayed below that holds a new value and the
previous value. Which one applies is determined by the value of
useNewValue. In my application there is a list of such nodes where for
each node the current value is replaced with a new one. The new values
should become effective for the entire list all>at once< through an
atomic change of useNewValue in every node. For that purpose, when the
new value is set, for every node in the list the same AtomicBoolean
instance is passed on to setNewValue(...), which is stored in
useNewValueParam. When the commit is done, the value of this instance
of AtomicBoolean is changed to true (this way the thread doing the
commit does not have to enter the synchronized block as all the other
threads calling aNode.get) and thus the new value of every node
becomes visible at once to every thread calling aNode.get().

public class Node {

    AtomicBoolean useNewValue = new AtomicBoolean(false);

     Why isn't this `private'? Is something else going on that
you haven't told us about?

     private Object newValue = null;
    private Object previousValue = null;
    private Object lock = new Object();

     What does `lock' buy you? Why not just synchronize on the
Node itself?

     public Object get() {
        synchronized(lock) {
            if(useNewValue.get()) // 1
                return newValue; // 2
            return previousValue; // 3

    public void setNewValue(Object newValue, AtomicBoolean
useNewValueParam) {
        synchronized(lock) {
                previousValue = this.newValue;
            this.newValue = newValue;
            // useNewValueParam is allways set to false when setNewValue is

     I don't see why that would matter.

             this.useNewValue = useNewValueParam;

At the same time there is never more than one thread iterating over
the node list calling setNewValue.

     So `synchronized' is just to ensure that the single setNewValue()
caller doesn't overlap any get() callers, is that right? (Nothing
wrong with that; I'm just trying to test my understanding of what
you're up to.)

This is made sure through
serialization of threads that want to iterate over the list calling
setNewValue. Serialization of threads is a bit crude, but not relevant
at the moment for the discussion of the problem described here.

My question is now whether this approach is free of race conditions or
starvation issues if implemented as described above. I have some
doubts whether everything is fine here as useNewValue is changed by
the commit thread by reference without entering synchronized(lock)
{ ... }. So is everything still fine if a context switch happens
between line 1 and 2 or between line 1 and 3? It would be for sure if
the commit thread entered the synchronized(lock) { ... } block, but it
does not (changing to the new values all at once wouldn't be possible

     When you change the shared AtomicBoolean, some threads executing
get() may have already observed the old value and may have decided
which value to return based on that now-outdated observation. So yes,
there's a race: It is possible for get() to return an old value after
the AtomicBoolean changes from true to false, or a new value after it
changes from false to true.

     What's the larger problem you're trying to solve?

Eric Sosman

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