RFR: 8238761: Asynchronous handshakes [v3]

[David Holmes]

On Fri, 18 Sep 2020 20:51:17 GMT, Daniel D. Daugherty <dcubed at openjdk.org> wrote:

>> Robbin Ehn has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Removed double check, fix comment, removed not needed function, updated logs
>
> src/hotspot/share/prims/whitebox.cpp line 2032:
> 
>> 2030:     void do_thread(Thread* th) {
>> 2031:       assert(th->is_Java_thread(), "sanity");
>> 2032:       JavaThread* jt = (JavaThread*)th;
> 
> Can whitebox.cpp code use the new as_Java_thread() call?

Yes it can. :)

> src/hotspot/share/runtime/handshake.hpp line 45:
> 
>> 43: // a single target/direct handshake or not, by the JavaThread that requested the
>> 44: // handshake or the VMThread respectively.
>> 45: class HandshakeClosure : public ThreadClosure, public CHeapObj<mtThread> {
> 
> Just to be clear. You haven't added support for a handshake that
> must only be executed by the target thread yet, right? That's
> future work, if I remember correctly...

AsyncHandshakeClosures have operations that must, by definition, be executed by the target thread (if they are executed
at all).

> src/hotspot/share/runtime/interfaceSupport.inline.hpp line 157:
> 
>> 155:
>> 156:     // Threads shouldn't block if they are in the middle of printing, but...
>> 157:     ttyLocker::break_tty_lock_for_safepoint(os::current_thread_id());
> 
> Can you explain why you had to add this?
> Did something show up in testing?

Yes please explain as this looks really bad.

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PR: https://git.openjdk.java.net/jdk/pull/151