RFR: 8212748: ZGC: Add reentrant locking functionality

[Per Liden]

Hi Erik,

On 2018-10-25 15:44, Erik Österlund wrote:
> Hi,
> 
> ZGC needs a per-nmethod lock to be used for concurrent IC cleaning, 
> protecting misaligned oops from concurrently being patched by nmethod 
> entry barriers, and read using CompiledMethod::is_unloading(), with 
> interactions crossing JavaThreads and non-Java threads. This patch adds 
> that utility.
> 
> Webrev:
> http://cr.openjdk.java.net/~eosterlund/8212748/webrev.00/

I'd like to suggest that ZReentrantLock doens't inherit from ZLock, but 
that it instead has a ZLock. And that ZLocker is adjusted to take a T* 
instead of a ZLock*.

Also, I'm not sure I see the need for the reentrant_lock() function. 
Shouldn't we just have a lock counter and let lock/unlock do the right 
thing?

cheers,
Per

> 
> Bug:
> https://bugs.openjdk.java.net/browse/JDK-8212748
> 
> Thanks,
> /Erik