RFR: 8257831: Suspend with handshakes [v3]

[Robbin Ehn]

On Wed, 7 Apr 2021 14:31:05 GMT, Richard Reingruber <rrich at openjdk.org> wrote:

>> I removed that line, not sure if the comment needs additional fixing.
>
> I think David requested updating of the comment for `JavaThread::java_suspend_self_with_safepoint_check()`.
> 
> I hope to delete the whole method soon and build on the new suspend mechanism, if Robbin is ok with that. For now I'd like to propose the following:
> 
> // Wait for another thread to perform object reallocation and relocking on behalf of
> // this thread.
> // Raw thread state transition to _thread_blocked and back again to the original
> // state before returning are performed. The current thread is required to
> // change to _thread_blocked in order to be seen to be safepoint/handshake safe
> // whilst suspended and only after becoming handshake safe, the other thread can
> // complete the handshake used to synchronize with this thread and then perform
> // the reallocation and relocking. We cannot use the thread state transition
> // helpers because we arrive here in various states and also because the helpers
> // indirectly call this method.  After leaving _thread_blocked we have to check
> // for safepoint/handshake, except if _thread_in_native. The thread is safe
> // without blocking then. Allowed states are enumerated in
> // SafepointSynchronize::block(). See also EscapeBarrier::sync_and_suspend_*()

Thanks!

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PR: https://git.openjdk.java.net/jdk/pull/3191