ObjectSynchronizer iterate only in-use monitors?

[Roman Kennke]

Am 16.05.2017 um 13:37 schrieb Carsten Varming:
> FYI. Last year I wrote a small patch[1] against JDK9 that enables
> monitor deflation to happen outside a safepoint. There was little
> interest then, so I never proposed it as a patch. The JDK9 code has
> moved on a little bit, so the patch probably doesn't apply cleanly,
> but the patch shows one way to complete the life of a monitor without
> going through a safepoint.

Wow, now that is interesting!

Can you give me a quick summary-description how you avoid racing with
other threads that might inflate the monitor? As far as I know, monitor
deflation (currently) requires that all Java threads stand still at a
safepoint.

Can it also be made to work with MonitorInUseLists?

Roman

>
> Carsten
>
> [1] http://cr.openjdk.java.net/~cvarming/monitor_deflate_conc/0/
> <http://cr.openjdk.java.net/%7Ecvarming/monitor_deflate_conc/0/>
>
> On Tue, May 16, 2017 at 6:32 AM, Roman Kennke <rkennke at redhat.com
> <mailto:rkennke at redhat.com>> wrote:
>
>     Am 16.05.2017 um 11:48 schrieb Robbin Ehn:
>     > Correction,
>     >
>     > On 05/16/2017 11:45 AM, Robbin Ehn wrote:
>     >> Yes, but since we are at a safepoint you should be able to use gc
>     >> worker in that case also?
>     >
>     > or the java thread. And scratch 'also' :)
>
>     This clearly requires co-operation from the GC.
>
>     Also, I noticed that some GCs cannot do deflation during root
>     scanning,
>     because they stow away mark words during marking, and temporarily
>     store
>     forwarding pointers into the mark words, and only restore mark words
>     after GC. This obviously conflicts with monitor deflation. Which means
>     we need some GC cooperation there too... I will figure something out.
>
>     Roman
>
>