RFR: 8197812: (ref) Data race in Finalizer

[Peter Levart]

Hi Martin,

On 02/14/18 02:30, Martin Buchholz wrote:
> Unlike most fixes to data races, this one should benefit performance.
>
> http://cr.openjdk.java.net/~martin/webrevs/jdk/Finalizer-data-race/
> https://bugs.openjdk.java.net/browse/JDK-8197812

Nice catch (for java-tsan tool)!

Although I think this race is benign. The code that synchronizes on 
'this' is the following:

   46     private boolean hasBeenFinalized() {
   47         return (next == this);
   48     }

And the only place where 'next' is set to 'this' is in remove() and 
remove is called from synchronized (this) block too. So 
hasBeenFinalized() may see any or no writes of 'next' performed out of 
synchronized (this), but they are never writes of value 'this', so the 
outcome is always 'false' in this case. The only write to 'next' with 
value of 'this' happens while synchronized (this) and synchronized 
(lock) at the same time so this write synchronizes with the read of 
'next' in hasBeenFinalized() as well as with all other writes of 'next'.

It may be that the intent was to refrain from using the shared 'lock' 
lock for the 2nd and subsequent calls to runFinalizer() and only use the 
more fine-grained 'this' lock in this case?

If someone was able to call runFinalizer() on the same instance in a 
loop he could prevent or slow-down normal processing of other 
Finalizer(s) if the shared 'lock' was always employed. What do you think?

Regards, Peter