RFR: 8341819: LightweightSynchronizer::enter_for races with deflation [v2]

[Patricio Chilano Mateo]

On Thu, 10 Oct 2024 11:59:24 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

>> This is a regression from [JDK-8315884](https://bugs.openjdk.org/browse/JDK-8315884).
>> 
>> When using `+UseObjectMonitorTable` monitors are inflated in a locked state effectively blocking out deflation. `LightweightSynchronizer::enter_for` assumed this to be true. But when the `-UseObjectMonitorTable` path was added `// Do the old inflate and enter.` this is no longer true as it first inflates a monitor in an unlocked state and then tries to lock. We need to introduce a retry loop similar to what was used before [JDK-8315884](https://bugs.openjdk.org/browse/JDK-8315884).
>> 
>> I propose we add this retry loop for both cases, to decouple the `LightweightSynchronizer::enter_for` from how lock elimination is done. With a retry loop, the only requirements for using `LightweightSynchronizer::enter_for` is that the Object locked on cannot have been locked on by another thread, i.e. there is no contention, but makes no assumptions on the interaction with the deflation thread.
>> 
>> For `-UseObjectMonitorTable` 7bdbe114eb57fe7310f9664a434c4d9203e656fc should be enough, as it will assist the deflating thread with deflation, so the second call must succeed. 
>> 
>> However `+UseObjectMonitorTable` cannot do this so it must wait for the deflating thread to make progress. But as mentioned above, this would only happen if partial lock elimination is performed. E.g.
>> 
>> Object o = new Object();
>> synchronized(o) {
>>   o.wait(1);
>> }
>> synchronized(o) {
>>   deoptimize();
>> }
>> 
>> got transformed to 
>> 
>> Object o = new Object();
>> synchronized(o) {
>>   o.wait(1);
>> }
>> // synchronized(o) { Eliminated lock
>>   deoptimize();
>> // }
>> 
>> 
>> As far as I can tell, this does not happen. But I do not want to couple lock elimination decision with `LightweightSynchronizer::enter_for`. So I propose a retry loop instead of just the two calls.
>> After this change the only prerequisite for using `LightweightSynchronizer::enter_for` is that the object being synchronized can not have been reached by another JavaThread (except the deflating thread).  So there may never be contention, but there may be deflation.
>
> Axel Boldt-Christmas has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove @test id

Marked as reviewed by pchilanomate (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/21420#pullrequestreview-2360354550