RFR: 8240588: _threadObj cannot be used on an exiting JavaThread

[Kim Barrett]

> On May 12, 2020, at 9:27 PM, David Holmes <david.holmes at oracle.com> wrote:
> 
> webrev: http://cr.openjdk.java.net/~dholmes/8240588/webrev.v3/
> bug: https://bugs.openjdk.java.net/browse/JDK-8240588

Looks good.

A couple comments for which I don't need a new webrev if you decide to
make the suggested changes.

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src/hotspot/share/runtime/threadSMR.cpp
1194   assert(Threads_lock->owned_by_self(), "invariant");
1202   assert(Threads_lock->owned_by_self(), "invariant");

assert_lock_strong(Threads_lock);

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src/hotspot/share/runtime/threadSMR.cpp
1207   for (Holder* current = _exiting_threads, **prev_next = &_exiting_threads;

This took me a while to parse.  I'd prefer a separate declaration of
prev_next before the for-loop.

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