RFR: 8189276: Make SuspendibleThreadSet and related code available to other GCs

[Roman Kennke]

> The SuspendibleThreadSet API for synchronizing any non-Java thread 
> with safepoints currently resides under gc/g1. It is very useful for 
> other GCs too (in particular, Shenandoah does use it too), so I wanted 
> to move it to a common location like gc/common. Then Kim Barrett 
> commented that it might actually be useful for other threads outside 
> GC land and to put it under runtime/. So I did:
>
> http://cr.openjdk.java.net/~rkennke/8189276/webrev.00/ 
> <http://cr.openjdk.java.net/%7Erkennke/8189276/webrev.00/>
>
> I also added a generic hook to call the STS from safepoint 
> sync/desync, which is consequently used by G1 now. In other words, the 
> CollectedHeap API that Erik Ö introduced is no longer used by G1. Only 
> CMS still uses that API because it has its own way to sync with 
> safepoints. I filed another bug 
> <https://bugs.openjdk.java.net/browse/JDK-8189364> for this. Although 
> I have my doubt it will ever be fixed. This seems to have been very 
> carefully evolved (to put it positive), and the risk of breaking it is 
> relatively high, and thus doesn't seem worth the struggle to make it 
> fit the STS.
>
> Issue:
> https://bugs.openjdk.java.net/browse/JDK-8189276
>
> What do you think? Ok to go in?
>
Replying to myself here.
I must admit that I am a bit reluctant to expose it to runtime/ unless 
there's a specific need for it. So maybe go back to the original plan to 
move it into gc/common and leave all the rest alone for now? What do 
others think?

Roman