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

[Roman Kennke]

Am 17.10.2017 um 23:10 schrieb 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?

Sorry, forgot to add something. One reason why we originally wanted to 
move it to gc/common (a new directory) instead of the existing gc/shared 
was that Erik H objected that gc/shared would be built into the minimal 
VM, and we probably wouldn't want that unless needed. However, moving it 
to runtime/ (without actual need), would achieve just that. So unless we 
can name an actual thread that would benefit from synchronizing using 
the STS, I suggest to leave the STS code in gc/common for now?

That would be:
http://cr.openjdk.java.net/~rkennke/8189276/webrev.01/ 
<http://cr.openjdk.java.net/%7Erkennke/8189276/webrev.01/>

Roman