RFR: 8257831: Suspend with handshakes [v4]

[Daniel D.Daugherty]

On Sat, 10 Apr 2021 07:38:37 GMT, Richard Reingruber <rrich at openjdk.org> wrote:

>> `HandshakeState::suspend()` is a synchronous handshake and adding the
>> handshake to the queue is lock free, but the execution of the synchronous
>> handshake itself requires a `HandshakeState::claim_handshake()` call which
>> does acquire the lock in question. We (the suspend requester) hold the lock
>> while the handshake is being processed so we either detect that
>> _handshakee->set_exiting() won the race (in the target thread) or we (the
>> suspend requester) win the race of setting the suspend flag so the target
>> thread can't exit yet.
>> 
>> Hopefully that helps explain this dance.
>
> Hi Dan,
> 
> thanks for picking up my question!
> 
>> `HandshakeState::suspend()` is a synchronous handshake and adding the
>> handshake to the queue is lock free, but the execution of the synchronous
>> handshake itself requires a `HandshakeState::claim_handshake()` call which
>> does acquire the lock in question.
> 
> My point would be that the attempt to execute the synchronous handshake for
> suspending a thread that is just about to call HandshakeState::thread_exit()
> cannot make progress (blocks) while the target thread is not safe
> (_thread_in_vm).
> 
> A synchronous handshake requires the target thread to be in a safe state for the
> requester to execute the handshake operation.  When executing
> HandshakeState::thread_exit() the suspendee is _thread_in_vm. And the requester
> will find it to be `_not_safe` when calling `possibly_can_process_handshake()`
> before calling `HandshakeState::claim_handshake()` or when calling
> `can_process_handshake()` afterwards. In both cases try_process() returns with
> failure _not_safe and the lock is not held.
> 
> ++
>  546	  if (!possibly_can_process_handshake()) {
>  547	    // JT is observed in an unsafe state, it must notice the handshake itself
>  548	    return HandshakeState::_not_safe;
>  549	  }
>  550	
>  551	  // Claim the mutex if there still an operation to be executed.
>  552	  if (!claim_handshake()) {
>  553	    return HandshakeState::_claim_failed;
>  554	  }
>  555	
>  556	  // If we own the mutex at this point and while owning the mutex we
>  557	  // can observe a safe state the thread cannot possibly continue without
>  558	  // getting caught by the mutex.
>  559	  if (!can_process_handshake()) {
>  560	    _lock.unlock();
>  561	    return HandshakeState::_not_safe;
>  562	  }
> 
> So isn't being unsafe sufficient to sync with suspend requests?

Interesting point that I didn't pick up from your previous comment.
Thanks for making it more clear for me. I need to mull on it for a bit.

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