RFR: JDK-8256844: Make NMT late-initializable

[Zhengyu Gu]

On Fri, 30 Jul 2021 16:28:54 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> src/hotspot/share/services/nmtPreInit.hpp line 202:
>> 
>>> 200:     assert((*aa) != NULL, "Entry not found: " PTR_FORMAT, p2i(p));
>>> 201:     NMTPreInitAllocation* a = (*aa);
>>> 202:     (*aa) = (*aa)->next;         // remove from its list
>> 
>> Could this be a race? There is no synchronization, read/write result could be arbitrary.
>
> The code is implicitly thread-safe because the hashmap is only modified in the pre-NMT-init phase. After NMT initialization, the table is read-only. During pre-NMT-init we are effectively single-threaded - at most two threads access the map, the thread loading the libjvm, and the thread calling CreateJavaVM, but not at the same time.
> 
> See also the asserts in the AllStatic class `NMTPreInit`.
> 
> (I should have described it more clearly, will add a comment.)

So, you are saying that there is no memory that is malloc'd pre-NMT-init phase and freed post-NMT-init phase?

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PR: https://git.openjdk.java.net/jdk/pull/4874