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

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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) {
> 			if(this.useNewValue.get())
> 				previousValue = this.newValue;
> 			this.newValue = newValue;
> 			// useNewValueParam is allways set to false when setNewValue is
> called

     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
> otherwise).

     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
esosman@ieee-dot-org.invalid

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Thread

Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-14 09:07 -0800
  Re: Question whether a problem with race conditions exists in this case Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-12-14 17:53 +0000
    Re: Question whether a problem with race conditions exists in this case Lew <lewbloch@gmail.com> - 2011-12-14 10:44 -0800
      Re: Question whether a problem with race conditions exists in this case Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2011-12-14 10:54 -0800
      Re: Question whether a problem with race conditions exists in this case Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-12-14 21:15 +0000
        Re: Question whether a problem with race conditions exists in this case Tom Anderson <twic@urchin.earth.li> - 2011-12-15 14:58 +0000
          Re: Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-15 08:40 -0800
          Re: Question whether a problem with race conditions exists in this case Gene Wirchenko <genew@ocis.net> - 2011-12-15 11:55 -0800
            Re: Question whether a problem with race conditions exists in this case Tom Anderson <twic@urchin.earth.li> - 2011-12-16 14:06 +0000
              Re: Question whether a problem with race conditions exists in this case Gene Wirchenko <genew@ocis.net> - 2011-12-16 10:09 -0800
    Re: Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-14 11:47 -0800
  Re: Question whether a problem with race conditions exists in this case Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2011-12-14 10:53 -0800
    Re: Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-14 11:54 -0800
      Re: Question whether a problem with race conditions exists in this case Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2011-12-15 16:38 -0800
        Re: Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-15 23:01 -0800
          Re: Question whether a problem with race conditions exists in this case Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2011-12-16 09:34 -0800
            Re: Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-17 11:55 -0800
              Re: Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-19 05:33 -0800
  Re: Question whether a problem with race conditions exists in this case Lew <lewbloch@gmail.com> - 2011-12-14 11:04 -0800
    Re: Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-14 12:32 -0800
      Re: Question whether a problem with race conditions exists in this case markspace <-@.> - 2011-12-14 14:13 -0800
        Re: Question whether a problem with race conditions exists in this case Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-12-14 17:44 -0500
          Re: Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-14 14:50 -0800
            Re: Question whether a problem with race conditions exists in this case markspace <-@.> - 2011-12-14 15:26 -0800
      Re: Question whether a problem with race conditions exists in this case Lew <lewbloch@gmail.com> - 2011-12-15 01:34 -0800
    Re: Question whether a problem with race conditions exists in this case Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-12-14 21:38 +0000
  Re: Question whether a problem with race conditions exists in this case Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-12-14 16:26 -0500
    Re: Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-14 13:57 -0800
      Re: Question whether a problem with race conditions exists in this case Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-12-14 18:05 -0500
        Re: Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-14 23:25 -0800
          Re: Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-14 23:28 -0800
      Re: Question whether a problem with race conditions exists in this case Tom Anderson <twic@urchin.earth.li> - 2011-12-15 14:44 +0000
        Re: Question whether a problem with race conditions exists in this case Lew <lewbloch@gmail.com> - 2011-12-16 08:27 -0800
          Re: Question whether a problem with race conditions exists in this case Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2011-12-16 09:41 -0800
            Re: Question whether a problem with race conditions exists in this case Tom Anderson <twic@urchin.earth.li> - 2011-12-16 18:51 +0000
          Re: Question whether a problem with race conditions exists in this case Tom Anderson <twic@urchin.earth.li> - 2011-12-16 18:50 +0000
    Re: Question whether a problem with race conditions exists in this case Saxo <saxo123@gmx.de> - 2011-12-14 14:13 -0800
      Re: Question whether a problem with race conditions exists in this case Lew <lewbloch@gmail.com> - 2011-12-14 16:55 -0800

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