RFR: 8252049: Native memory leak in ciMethodData ctor [v3]

[Vladimir Ivanov]

> `ciMethodData` embeds `MethodData` to snapshot a state from the original `MethodData` instance.
> But `MethodData` embeds a `Mutex` which allocates a platform-specific implementation on C-heap.
> The `Mutex` is overwritten with `0`s, but the resources aren't deallocated, so the leak occurs.
> 
> Proposed fix is to run Mutex destructor right away. 
> 
> Initially, I thought about switching to `Mutex*`, but then I found that Coleen already tried that and observed a performance regression [1]. So, for now I chose the conservative approach.
> 
> In the longer term, I would consider replacing `MethodData::_extra_data_lock` with a lock-free scheme. Having a lock-per-MDO looks kind of excessive.
> 
> Testing:
> - [x] verified that no memory leak observed with the reported test
> - [x] tier1-4
> 
> [1] https://mail.openjdk.java.net/pipermail/hotspot-dev/2019-October/039783.html

Vladimir Ivanov has updated the pull request incrementally with one additional commit since the last revision:

  MDHeader shouldn't subclass StackObj

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1478/files
  - new: https://git.openjdk.java.net/jdk/pull/1478/files/011b26ca..af6b3d10

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1478&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1478&range=01-02

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1478.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1478/head:pull/1478

PR: https://git.openjdk.java.net/jdk/pull/1478