RFR (S) 8213092: Add more runtime locks for concurrent class unloading

[David Holmes]

Hi Coleen,

On 30/10/2018 1:45 PM, coleen.phillimore at oracle.com wrote:
> Summary: Add locks for calling CLDG::purge concurrently as well and for 
> calling SystemDictionary::do_unloading concurrently.
> 
> Ran linux-x64 tier1-6 through mach5 and hotspot/jtreg/runtime tests, 
> which include the module tests.
> 
> open webrev at http://cr.openjdk.java.net/~coleenp/8213092.01/webrev
> bug link https://bugs.openjdk.java.net/browse/JDK-8213092

So ... all the locks covered by an assert_locked_or_safepoint, or which 
are acquired by the VMThread at a safepoint, must never be held by a 
JavaThread if it could reach a safepoint whilst that lock is held - else 
we could deadlock. So can we check that with NoSafepointVerifiers?

Further, are these locks acquired by non-JavaThreads such that the 
VMThread may be delayed whilst a safepoint is active?

Thanks,
David

> Thanks,
> Coleen