RFR: 8264682: MemProfiling does not own Heap_lock when using G1

[Ioi Lam]

On Mon, 5 Apr 2021 14:29:11 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> > Trivial fix for JDK-8264682.
> 
> No, not trivial at all. Dropping a GC-specific blob like this into shared code is highly questionable. I'm not sure how this issue should be addressed, but I don't think the proposed change is the right approach.

It looks like the following APIs can be reliably called without holding the Heap_lock. However, I am not sure if they would give the same result. I.e., can you get  `Universe::heap()->unused()` by calling `Universe::heap()->capacity() - Universe::heap()->unused()`.

JVM_ENTRY_NO_ENV(jlong, JVM_TotalMemory(void))
  size_t n = Universe::heap()->capacity();
  return convert_size_t_to_jlong(n);
JVM_END

JVM_ENTRY_NO_ENV(jlong, JVM_FreeMemory(void))
  size_t n = Universe::heap()->unused();
  return convert_size_t_to_jlong(n);
JVM_END

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PR: https://git.openjdk.java.net/jdk/pull/3340