Saturday, February 5, 2011

benign race condition (homegrown ConcurrentMutableInteger

Does the getInt() below need "synchronized"? I am not expert but I would go out on a limb and say no. It either gets the value before or the value after the increment, never a value inconsistent with reality. As Bill Lewis said in his pthread book about semaphore getCount(), whenever some thread wants to /use/ the getInt() return value, it should bear in mind the value could be stale, since other threads could change it any time.

The race between increment() and getInt() is probably benign. However, if increment() isn't synchronized, then you could get lost-updates -- inconsistent with reality. Note volatile field won't help.

// simpler than AtomicInteger, but not lockfree.
class ConcurrentMutableInteger implements Serializable{
 private int i = 0;
 public synchronized void increment() {this.i++;}
 public int getInt() {return this.i; }
}

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