[OpenJDK 2D-Dev] RFR: JDK-8263467: Incorrect double-checked locking in sun.java2d.CRenderer

[Aleksey Shipilev]

On Fri, 12 Mar 2021 03:19:12 GMT, Sergey Bylokhov <serb at openjdk.org> wrote:

>> SonarCloud reports multiple incorrect double-checked locking cases in `sun.java2d.CRenderer`. For example:
>> 
>>   Arc2D arcToShape;
>> 
>>   ...
>>             if (arcToShape == null) {
>>                 synchronized (this) {
>>                     if (arcToShape == null) {
>>                         arcToShape = new Arc2D.Float();
>>                     }
>>                 }
>>             }
>> 
>> `Arc2D` contains fields that are not `final`. `arcToShape` is not `volatile`. This makes it an incorrect DCL.
>> This code is used by Mac OS, do it would probably blow up some time later on M1.
>> 
>> This is the candidate fix that preserves the semantics. I am doing this fix blindly so far, without testing on real Mac OS. But maybe there is no need to do any of this, because the setter methods overwrite the contents of all these objects under their own sync. So, maybe we can just allocate those objects without DCL-backed caching and pass them in?
>
> I guess "Incorrect double-checked" is a too strict description of this code, it just tried to eliminate the creation of one object and do not care about the content of that object, since it is immediately overridden. 
> 
> I think It will be more clear to just eliminate this DCL since overhead and code complexity seems too much to just cache five objects.
> 
> Probably it was important in past(jdk6 and below) when this renderer was used for onscreen rendering. Currently, it is used for printing(if actually used at all).

Ok, I removed the "DCL" from that code and used the objects directly. AFAICS, synchronization is redundant on those local objects. What would be a good test for this change?

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PR: https://git.openjdk.java.net/jdk/pull/2948