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

[Axel Boldt-Christmas]

> 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

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/21420/files
  - new: https://git.openjdk.org/jdk/pull/21420/files/e16bfde1..bfc5a349

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21420&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21420&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/21420.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21420/head:pull/21420

PR: https://git.openjdk.org/jdk/pull/21420