RFR: 8252049: Native memory leak in ciMethodData ctor

[Coleen Phillimore]

On Fri, 27 Nov 2020 08:05:09 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

> `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

Changes requested by coleenp (Reviewer).

src/hotspot/share/oops/methodData.cpp line 1208:

> 1206: MethodData::MethodData(ciMethodData& data)
> 1207:   : _extra_data_lock(Mutex::leaf, "unused") {
> 1208:   _extra_data_lock.~Mutex(); // release allocated resources before zeroing

So if _extra_data_lock was a pointer to Mutex, then this statement would be simply "delete _extra_data_lock;" instead, but we'd still have this copy constructor, right?

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