I talked with Doug, and we believe this is a problem.
We are investigating ways to fix it.
Bill
On Apr 4, 2004, at 5:18 AM, Doron Rajwan wrote:
> Assume the following:
>
> Thread1:
> Object o = new ...
> map.put("x", o);
>
> Thread2:
> Object value = map.get("x");
>
> As I understand, Thread2 must either get() null, or
> there is a happens-before edge between the put() in
> Thread1 and the get() in Thread2.
>
> To have such happens-before edge, Thread1 must write
> to volatile member 'count' ***before*** updating the
> 'table' member. However, as you can see below, the
> update of (an element of) 'table' is done before the
> write to 'count'.Thus, it is possible that Thread2
> will obtain a partially initialized object.
>
> Although the map itself is fine, the user objects
> passing through it are invalid.
>
> Is this true?
>
> thanks,
> Doron.
>
>
>
>
>
> V put(K key, int hash, V value, boolean
> onlyIfAbsent) {
> lock();
> try {
> int c = count;
> HashEntry[] tab = table;
> int index = hash & (tab.length - 1);
> HashEntry<K,V> first =
> (HashEntry<K,V>) tab[index];
>
> for (HashEntry<K,V> e = first; e !=
> null; e = (HashEntry<K,V>) e.next) {
> if (e.hash == hash &&
> key.equals(e.key)) {
> V oldValue = e.value;
> if (!onlyIfAbsent)
> e.value = value;
> ++modCount;
> count = c; // write-volatile
> return oldValue;
> }
> }
>
> tab[index] = new HashEntry<K,V>(hash,
> key, value, first);
> ++modCount;
> ++c;
> count = c; // write-volatile
> if (c > threshold)
> setTable(rehash(tab));
> return null;
> } finally {
> unlock();
> }
> }
>
>
> V get(Object key, int hash) {
> if (count != 0) { // read-volatile
> HashEntry[] tab = table;
> int index = hash & (tab.length - 1);
> HashEntry<K,V> e = (HashEntry<K,V>)
> tab[index];
> while (e != null) {
> if (e.hash == hash &&
> key.equals(e.key))
> return e.value;
> e = e.next;
> }
> }
> return null;
> }
>
>
> =====
> Doron Rajwan, mailto:doron@rajwan.org
>
> -------------------------------
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> http://www.cs.umd.edu/~pugh/java/memoryModel
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